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  • 1. Achtert, P.
    et al.
    Brooks, I. M.
    Brooks, B. J.
    Moat, B. I.
    Prytherch, J.
    Persson, P. O. G.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Measurement of wind profiles by motion-stabilised ship-borne Doppler lidar2015In: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 8, no 11, p. 4993-5007Article in journal (Refereed)
    Abstract [en]

    Three months of Doppler lidar wind measurements were obtained during the Arctic Cloud Summer Experiment on the icebreakerOden during the summer of 2014. Such ship-borne Doppler measurements require active stabilisation to remove the effects of ship motion. We demonstrate that the combination of a commercial Doppler lidar with a custom-made motion-stabilisation platform enables the retrieval of wind profiles in the Arctic atmospheric boundary layer during both cruising and ice-breaking with statistical uncertainties comparable to land-based measurements. This held true particularly within the atmospheric boundary layer even though the overall aerosol load was very low. Motion stabilisation was successful for high wind speeds in open water and the resulting wave conditions. It allows for the retrieval of vertical winds with a random error below 0.2 m s−1. The comparison of lidar-measured wind and radio soundings gives a mean bias of 0.3 m s−1 (2°) and a mean standard deviation of 1.1 m s−1 (12°) for wind speed (wind direction). The agreement for wind direction degrades with height. The combination of a motion-stabilised platform with a low-maintenance autonomous Doppler lidar has the potential to enable continuous long-term high-resolution ship-based wind profile measurements over the oceans.

  • 2. Balsley, Ben
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    On the scale-dependence of the gradient Richardson number in the residual layer2008In: Boundary-Layer Meteorology, Vol. 127, p. 57-72Article in journal (Refereed)
    Abstract [en]

    We present results of a technique for examining the scale-dependence of the gradient Richardson number, Ri, in the nighttime residual layer. The technique makes use of a series of high-resolution, in situ, vertical profiles of wind speed and potential temperature obtained during CASES-99 in south-eastern Kansas, U.S.A. in October 1999. These profiles extended from the surface, through the nighttime stable boundary layer, and well into the residual layer. Analyses of the vertical gradients of both wind speed, potential temperature and turbulence profiles over a wide range of vertical scale sizes are used to estimate profiles of the local Ri and turbulence structure as a function of scale size. The utility of the technique lies both with the extensive height range of the residual layer as well as with the fact that the sub-metre resolution of the raw profiles enables a metre-by-metre ‘sliding’ average of the scale-dependent Richardson number values over hundreds of metres vertically. The results presented here show that small-scale turbulence is a ubiquitous and omnipresent feature of the residual layer, and that the region is dynamic and highly variable, exhibiting persistent turbulent structure on vertical scales of a few tens of metres or less. Furthermore, these scales are comparable to the scales over which the Ri is less than or equal to the critical value of Ric of 0.25, although turbulence is also shown to exist in regions with significantly larger Ri values, an observation at least consistent with the concept of hysteresis in turbulence generation and maintenance. Insofar as the important scale sizes are comparable to or smaller than the resolution of current models, it follows that, in order to resolve the observed details of small Ri values and the concomitant turbulence generation, future models need to be capable of significantly higher resolutions.

  • 3. Balsley, Ben
    et al.
    Svensson, Gunilla
    Stockholm University.
    Tjernström, Michael
    Stockholm University.
    On the scale-dependence of the gradient Richardson number in the residual layer2007In: Boundary-Layer Meteorology, Vol. 127, p. 57-72Article in journal (Refereed)
  • 4. Birch, C. E.
    et al.
    Brooks, I. M.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Shupe, M. D.
    Mauritsen, T.
    Sedlar, J.
    Lock, A. P.
    Earnshaw, P.
    Persson, P. O. G.
    Milton, S. F.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies2012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, no 7, p. 3419-3435Article in journal (Refereed)
    Abstract [en]

    Observations made during late summer in the central Arctic Ocean, as part of the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and vertical temperature structure in the Met Office Unified Model (MetUM). The observation period can be split into 5 regimes; the first two regimes had a large number of frontal systems, which were associated with deep cloud. During the remainder of the campaign a layer of low-level cloud occurred, typical of central Arctic summer conditions, along with two periods of greatly reduced cloud cover. The short-range operational NWP forecasts could not accurately reproduce the observed variations in near-surface temperature. A major source of this error was found to be the temperature-dependant surface albedo parameterisation scheme. The model reproduced the low-level cloud layer, though it was too thin, too shallow, and in a boundary-layer that was too frequently well-mixed. The model was also unable to reproduce the observed periods of reduced cloud cover, which were associated with very low cloud condensation nuclei (CCN) concentrations (< 1 cm(-3)). As with most global NWP models, the MetUM does not have a prognostic aerosol/cloud scheme but uses a constant CCN concentration of 100 cm(-3) over all marine environments. It is therefore unable to represent the low CCN number concentrations and the rapid variations in concentration frequently observed in the central Arctic during late summer. Experiments with a single-column model configuration of the MetUM show that reducing model CCN number concentrations to observed values reduces the amount of cloud, increases the near-surface stability, and improves the representation of both the surface radiation fluxes and the surface temperature. The model is shown to be sensitive to CCN only when number concentrations are less than 10-20 cm(-3).

  • 5.
    Birch, Cathryn
    et al.
    University of Leeds.
    Brooks, Ian
    University of Leeds.
    Michael, Tjernström
    Stockholm University, Faculty of Science, Department of Meteorology .
    Milton, Sean
    UK Met Office.
    Earnshaw,
    UK Met Office.
    Persson, Ola
    NOAA/ESRL-PSD.
    Söderberg, Stefan
    WeatherTech Scandinavia.
    The performance of a global and mesoscale model over the central Arctic Ocean during the summer melt season2009In: Journal of Geophysical Research : Atmospheres, Vol. 114, p. D13104-Article in journal (Refereed)
    Abstract [en]

    Measurements of turbulent fluxes, clouds, radiation, and profiles of meanmeteorological parameters, obtained over an ice floe in the central Arctic Ocean during theArctic Ocean Experiment 2001, are used to evaluate the performance of U.K. Met OfficeUnified Model (MetUM) and Coupled Ocean/Atmosphere Mesoscale Prediction System(COAMPS) in the lower atmosphere during late summer. Both the latest version of theMetUM and the version operational in 2001 are used in the comparison to gain aninsight as to whether updates to the model have improved its performance over the Arcticregion. As with previous model evaluations over the Arctic, the pressure, humidity,and wind fields are satisfactorily represented in all three models. The older version of theMetUM underpredicts the occurrence of low-level Arctic clouds, and the liquid and icecloud water partitioning is inaccurate compared to observations made during SHEBA.In the newer version, simulated ice and liquid water paths are improved, but theoccurrence of low-level clouds are overpredicted. Both versions overestimate the amountof radiative heat absorbed at the surface, leading to a significant feedback of errorsinvolving the surface albedo, which causes a large positive bias the surface temperature.Cloud forcing in COAMPS produces similar biases in the downwelling shortwave andlongwave radiation fluxes to those produced by UM(G25). The surface albedoparameterization is, however, more realistic, and thus, the total heat flux and surfacetemperature are more accurate for the majority of the observation period.

  • 6. Bocquet, Florence
    et al.
    Balsley, Ben
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Comparing Estimates of Turbulence Based on Near-Surface Measurements in the Nocturnal Stable Boundary Layer2011In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 138, no 1, p. 43-60Article in journal (Refereed)
    Abstract [en]

    Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (epsilon) are reasonably well correlated with concurrent measurements of vertical velocity variance (sigma(2)(w)) obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in the first 100m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent background turbulence exists even during the most stable atmospheric conditions, where e can exhibit values as low as 10(-7) m(2) s(-3). We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and e to provide a proxy measurement for sigma(2)(w) at altitudes higher than tower heights.

  • 7. Brooks, Ian M.
    et al.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Persson, P. Ola G.
    Shupe, Matthew D.
    Atkinson, Rebecca A.
    Canut, Guylaine
    Birch, Cathryn E.
    Mauritsen, Thorsten
    Sedlar, Joseph
    Brooks, Barbara J.
    The Turbulent Structure of the Arctic Summer Boundary Layer During The Arctic Summer Cloud-Ocean Study2017In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 122, no 18, p. 9685-9704Article in journal (Refereed)
    Abstract [en]

    The mostly ice covered Arctic Ocean is dominated by low-level liquid-or mixed-phase clouds. Turbulence within stratocumulus is primarily driven by cloud top cooling that induces convective instability. Using a suite of in situ and remote sensing instruments we characterize turbulent mixing in Arctic stratocumulus, and for the first time we estimate profiles of the gradient Richardson number at relatively high resolution in both time (10 min) and altitude (10 m). It is found that the mixing occurs both within the cloud, as expected, and by wind shear instability near the surface. About 75% of the time these two layers are separated by a stably stratified inversion at 100-200 m altitude. Exceptions are associated with low cloud bases that allow the cloud-driven turbulence to reach the surface. The results imply that turbulent coupling between the surface and the cloud is sporadic or intermittent.

    Plain Language Summary: The lower atmosphere over the summertime Arctic Ocean often consists of two well-mixed layers-a surface mixed layer and a cloud mixed layer-that are separated by a weak decoupling layer at about 100 to 300 m above the surface. In these cases, the cloud cannot interact directly with the surface. Large-scale forecast and climate models consistently fail to reproduce this observed structure and may thus fail to correctly reproduce the cloud properties and the amount of energy absorbed by or emitted from the surface as solar and infrared radiation. This contributes to errors in reproducing changes in sea ice concentration over time. Here we use measurements made in the central Arctic to study the processes controlling whether or not the cloud is coupled to the surface. The effect of wind at the surface is found not to be a controlling factor. The depth of the cloud mixed layer is critical, but the multiple processes influencing it cannot be separated using the data available here. However, cooling at cloud top by infrared radiation is key, as is the extension of cloud into the temperature inversion-a unique feature of Arctic clouds.

  • 8. Chang, R. Y. -W
    et al.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graus, M.
    Mueller, M.
    Paatero, J.
    Burkhart, J. F.
    Stohl, A.
    Orr, L. H.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hayden, K.
    Li, S. -M
    Hansel, A.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leaitch, W. R.
    Abbatt, J. P. D.
    Aerosol composition and sources in the central Arctic Ocean during ASCOS2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 20, p. 10619-10636Article in journal (Refereed)
    Abstract [en]

    Measurements of submicron aerosol chemical composition were made over the central Arctic Ocean from 5 August to 8 September 2008 as a part of the Arctic Summer Cloud Ocean Study (ASCOS) using an aerosol mass spectrometer (AMS). The median levels of sulphate and organics for the entire study were 0.051 and 0.055 mu gm(-3), respectively. Positive matrix factorisation was performed on the entire mass spectral time series and this enabled marine biogenic and continental sources of particles to be separated. These factors accounted for 33% and 36% of the sampled ambient aerosol mass, respectively, and they were both predominantly composed of sulphate, with 47% of the sulphate apportioned to marine biogenic sources and 48% to continental sources, by mass. Within the marine biogenic factor, the ratio of methane sulphonate to sulphate was 0.25+/-0.02, consistent with values reported in the literature. The organic component of the continental factor was more oxidised than that of the marine biogenic factor, suggesting that it had a longer photochemical lifetime than the organics in the marine biogenic factor. The remaining ambient aerosol mass was apportioned to an organic-rich factor that could have arisen from a combination of marine and continental sources. In particular, given that the factor does not correlate with common tracers of continental influence, we cannot rule out that the organic factor arises from a primary marine source.

  • 9. de Boer, G.
    et al.
    Shupe, M. D.
    Caldwell, P. M.
    Bauer, S. E.
    Persson, O.
    Boyle, J. S.
    Kelley, M.
    Klein, S. A.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Near-surface meteorology during the Arctic Summer Cloud Ocean Study (ASCOS): evaluation of reanalyses and global climate models2014In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, no 1, p. 427-445Article in journal (Refereed)
    Abstract [en]

    Atmospheric measurements from the Arctic Summer Cloud Ocean Study (ASCOS) are used to evaluate the performance of three atmospheric reanalyses (European Centre for Medium Range Weather Forecasting (ECMWF)-Interim reanalysis, National Center for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) reanalysis, and NCEP-DOE (Department of Energy) reanalysis) and two global climate models (CAM5 (Community Atmosphere Model 5) and NASA GISS (Goddard Institute for Space Studies) ModelE2) in simulation of the high Arctic environment. Quantities analyzed include near surface meteorological variables such as temperature, pressure, humidity and winds, surface-based estimates of cloud and precipitation properties, the surface energy budget, and lower atmospheric temperature structure. In general, the models perform well in simulating large-scale dynamical quantities such as pressure and winds. Near-surface temperature and lower atmospheric stability, along with surface energy budget terms, are not as well represented due largely to errors in simulation of cloud occurrence, phase and altitude. Additionally, a development version of CAMS, which features improved handling of cloud macro physics, has demonstrated to improve simulation of cloud properties and liquid water amount. The ASCOS period additionally provides an excellent example of the benefits gained by evaluating individual budget terms, rather than simply evaluating the net end product, with large compensating errors between individual surface energy budget terms that result in the best net energy budget.

  • 10. Devasthale, A.
    et al.
    Sedlar, J.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Characteristics of water-vapour inversions observed over the Arctic by Atmospheric Infrared Sounder (AIRS) and radiosondes2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 18, p. 9813-9823Article in journal (Refereed)
    Abstract [en]

    An accurate characterization of the vertical structure of the Arctic atmosphere is useful in climate change and attribution studies as well as for the climate modelling community to improve projections of future climate over this highly sensitive region. Here, we investigate one of the dominant features of the vertical structure of the Arctic atmosphere, i.e. water-vapour inversions, using eight years of Atmospheric Infrared Sounder data (2002-2010) and radiosounding profiles released from the two Arctic locations (North Slope of Alaska at Barrow and during SHEBA). We quantify the characteristics of clear-sky water vapour inversions in terms of their frequency of occurrence, strength and height covering the entire Arctic for the first time. We found that the frequency of occurrence of water-vapour inversions is highest during winter and lowest during summer. The inversion strength is, however, higher during summer. The observed peaks in the median inversion-layer heights are higher during the winter half of the year, at around 850 hPa over most of the Arctic Ocean, Siberia and the Canadian Archipelago, while being around 925 hPa during most of the summer half of the year over the Arctic Ocean. The radiosounding profiles agree with the frequency, location and strength of water-vapour inversions in the Pacific sector of the Arctic. In addition, the radiosoundings indicate that multiple inversions are the norm with relatively few cases without inversions. The amount of precipitable water within the water-vapour inversion structures is estimated and we find a distinct, two-mode contribution to the total column precipitable water. These results suggest that water-vapour inversions are a significant source to the column thermodynamics, especially during the colder winter and spring seasons. We argue that these inversions are a robust metric to test the reproducibility of thermodynamics within climate models. An accurate statistical representation of water-vapour inversions in models would mean that the large-scale coupling of moisture transport, precipitation, temperature and water-vapour vertical structure and radiation are essentially captured well in such models.

  • 11. Devasthale, A.
    et al.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caian, M.
    Thomas, M. A.
    Kahn, B. H.
    Fetzer, E. J.
    Influence of the arctic oscillation on the vertical distribution of clouds as observed by the a train constellation of satellites2012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, no 21, p. 10535-10544Article in journal (Refereed)
    Abstract [en]

    The main purpose of this study is to investigate the influence of the Arctic Oscillation (AO), the dominant mode of natural variability over the northerly high latitudes, on the spatial (horizontal and vertical) distribution of clouds in the Arctic. To that end, we use a suite of sensors on-board NASA's A-Train satellites that provide accurate observations of the distribution of clouds along with information on atmospheric thermodynamics. Data from three independent sensors are used (AQUA-AIRS, CALIOP-CALIPSO and CPR-CloudSat) covering two time periods (winter half years, November through March, of 2002-2011 and 2006-2011, respectively) along with data from the ERA-Interim reanalysis. We show that the zonal vertical distribution of cloud fraction anomalies averaged over 67-82 degrees N to a first approximation follows a dipole structure (referred to as Greenland cloud dipole anomaly, GCDA), such that during the positive phase of the AO, positive and negative cloud anomalies are observed eastwards and westward of Greenland respectively, while the opposite is true for the negative phase of AO. By investigating the concurrent meteorological conditions (temperature, humidity and winds), we show that differences in the meridional energy and moisture transport during the positive and negative phases of the AO and the associated thermodynamics are responsible for the conditions that are conducive for the formation of this dipole structure. All three satellite sensors broadly observe this large-scale GCDA despite differences in their sensitivities, spatio-temporal and vertical resolutions, and the available lengths of data records, indicating the robustness of the results. The present study also provides a compelling case to carry out process-based evaluation of global and regional climate models.

  • 12. Devasthale, Abhay
    et al.
    Sedlar, Joseph
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kahn, Brian H.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Fetzer, Eric J.
    Tian, Baijun
    Teixeira, Joao
    Pagano, Thomas S.
    A DECADE OF SPACEBORNE OBSERVATIONS OF THE ARCTIC ATMOSPHERE: Novel Insights from NASA's AIRS Instrument2016In: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 97, no 11, p. 2163-2176Article in journal (Refereed)
    Abstract [en]

    Arctic sea ice is declining rapidly and its annual ice extent minima reached record lows twice during the last decade. Large environmental and socioeconomic implications related to sea ice reduction in a warming world necessitate realistic simulations of the Arctic climate system, not least to formulate relevant environmental policies on an international scale. However, despite considerable progress in the last few decades, future climate projections from numerical models still exhibit the largest uncertainties over the polar regions. The lack of sufficient observations of essential climate variables is partly to blame for the poor representation of key atmospheric processes, and their coupling to the surface, in climate models. Observations from the hyper spectral Atmospheric Infrared Sounder (AIRS) instrument on board National Aeronautics and Space Administration (NASA)'s Aqua satellite are contributing toward improved understanding of the vertical structure of the atmosphere over the poles since 2002, including the lower troposphere. This part of the atmosphere is especially important in the Arctic, as it directly impacts sea ice and its short-term variability. Although in situ measurements provide invaluable ground truth, they are spatially and temporally inhomogeneous and sporadic over the Arctic. A growing number of studies are exploiting AIRS data to investigate the thermodynamic structure of the Arctic atmosphere, with applications ranging from understanding processes to deriving climatologies; all of which are also useful to test and improve parameterizations in climate models. As the AIRS data record now extends more than a decade, a select few of many such noteworthy applications of AIRS data over this challenging and rapidly changing landscape are highlighted here.

  • 13. Devasthale, Abhay
    et al.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Karl-Goran
    Thomas, Manu Anna
    Jones, Colin
    Sedlar, Joseph
    Stockholm University, Faculty of Science, Department of Meteorology .
    Omar, Ali H.
    The vertical distribution of thin features over the Arctic analysed from CALIPSO observations: Part 1: Optically thin clouds2011In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 63, no 1, p. 77-85Article in journal (Refereed)
    Abstract [en]

    Clouds play a crucial role in the Arctic climate system. Therefore, it is essential to accurately and reliably quantify and understand cloud properties over the Arctic. It is also important to monitor and attribute changes in Arctic clouds. Here, we exploit the capability of the CALIPSO-CALIOP instrument and provide comprehensive statistics of tropospheric thin clouds, otherwise extremely difficult to monitor from passive satellite sensors. We use 4 yr of data (June 2006-May 2010) over the circumpolar Arctic, here defined as 67-82 degrees N, and characterize probability density functions of cloud base and top heights, geometrical thickness and zonal distribution of such cloud layers, separately for water and ice phases, and discuss seasonal variability of these properties. When computed for the entire study area, probability density functions of cloud base and top heights and geometrical thickness peak at 200-400, 1000-2000 and 400-800 m, respectively, for thin water clouds, while for ice clouds they peak at 6-8, 7-9 and 400-1000 m, respectively. In general, liquid clouds were often identified below 2 km during all seasons, whereas ice clouds were sensed throughout the majority of the upper troposphere and also, but to a smaller extent, below 2 km for all seasons.

  • 14. Graversen, Rune G.
    et al.
    Mauritsen, Thorsten
    Drijfhout, Sybren
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mårtensson, Sebastian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Warm winds from the Pacific caused extensive Arctic sea-ice melt in summer 20072011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 36, no 11-12, p. 2103-2112Article in journal (Refereed)
    Abstract [en]

    During summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. The role of the atmospheric energy transport in this extreme melt event is explored using the state-of-the-art ERA-Interim reanalysis data. We find that in summer 2007 there was an anomalous atmospheric flow of warm and humid air into the region that suffered severe melt. This anomaly was larger than during any other year in the data (1989-2008). Convergence of the atmospheric energy transport over this area led to positive anomalies of the downward longwave radiation and turbulent fluxes. In the region that experienced unusual ice melt, the net anomaly of the surface fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. When the ocean successively became ice-free, the surface-albedo decreased causing additional absorption of shortwave radiation, despite the fact that the downwelling solar radiation was smaller than average. We argue that the positive anomalies of net downward longwave radiation and turbulent fluxes played a key role in initiating the 2007 extreme ice melt, whereas the shortwave-radiation changes acted as an amplifying feedback mechanism in response to the melt.

  • 15.
    Graversen, Rune Grand
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology.
    Atmospheric mass-budget inconsistency in the ERA-40 reanalysis2007In: Quarterly Journal of the Royal Meteorological society, ISSN 0035-9009, Vol. 133, p. 673-680Article in journal (Refereed)
  • 16.
    Graversen, Rune Grand
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology.
    Reply: Communications arising2008In: Nature, Vol. 455, p. E4-E5Article in journal (Refereed)
  • 17.
    Held, A.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brooks, I. M.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the potential contribution of open lead particle emissions to the = ntral Arctic aerosol concentration2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 7, p. 3093-3105Article in journal (Refereed)
    Abstract [en]

    We present direct eddy covariance measurements of aerosol number fluxes, dominated by sub-50 nm particles, at the edge of an ice floe drifting in the central Arctic Ocean. The measurements were made during the ice-breaker borne ASCOS (Arctic Summer Cloud Ocean Study) expedition in August 2008 between 2 degrees-10 degrees W longitude and 87 degrees-87.5 degrees N latitude. The median aerosol transfer velocities over different surface types (open water leads, ice ridges, snow and ice surfaces) ranged from 0.27 to 0.68 mm s(-1) during deposition-dominated episodes. Emission periods were observed more frequently over the open lead, while the snow behaved primarily as a deposition surface. Directly measured aerosol fluxes were compared with particle deposition parameterizations in order to estimate the emission flux from the observed net aerosol flux. Finally, the contribution of the open lead particle source to atmospheric variations in particle number concentration was evaluated and compared with the observed temporal evolution of particle number. The direct emission of aerosol particles from the open lead can explain only 5-10% of the observed particle number variation in the mixing layer close to the surface.

  • 18. Held, A.
    et al.
    Orsini, D. A.
    Vaattovaara, P.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Near-surface profiles of aerosol number concentration and temperature over the Arctic Ocean2011In: Atmospheric Measurement Techniques, ISSN 1867-1381, Vol. 4, no 8, p. 1603-1616Article in journal (Refereed)
    Abstract [en]

    Temperature and particle number concentration profiles were measured at small height intervals above open and frozen leads and snow surfaces in the central Arctic. The device used was a gradient pole designed to investigate potential particle sources over the central Arctic Ocean. The collected data were fitted according to basic logarithmic flux-profile relationships to calculate the sensible heat flux and particle deposition velocity. Independent measurements by the eddy covariance technique were conducted at the same location. General agreement was observed between the two methods when logarithmic profiles could be fitted to the gradient pole data. In general, snow surfaces behaved as weak particle sinks with a maximum deposition velocity upsilon(d) = 1.3 mm s(-1) measured with the gradient pole. The lead surface behaved as a weak particle source before freeze-up with an upward flux F(c) = 5.7 x 10(4) particles m(-2) s(-1), and as a relatively strong heat source after freeze-up, with an upward maximum sensible heat flux H = 13.1 W m(-2). Over the frozen lead, however, we were unable to resolve any significant aerosol profiles.

  • 19. Holtslag, A. A. M.
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Baas, P.
    Basu, S.
    Beare, B.
    Beljaars, A. C. M.
    Bosveld, F. C.
    Cuxart, J.
    Lindvall, Jenny
    Stockholm University, Faculty of Science, Department of Meteorology .
    Steeneveld, G. J.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Van de Wiel, B. J. H.
    STABLE ATMOSPHERIC BOUNDARY LAYERS AND DIURNAL CYCLES: Challenges for Weather and Climate Models2013In: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 94, no 11, p. 1691-1706Article in journal (Refereed)
    Abstract [en]

    The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.

  • 20. Igel, Adele L.
    et al.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Savre, Julien
    Sedlar, Joseph
    Stockholm University, Faculty of Science, Department of Meteorology .
    The free troposphere as a potential source of arctic boundary layer aerosol particles2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 13, p. 7053-7060Article in journal (Refereed)
    Abstract [en]

    This study investigates aerosol particle transport from the free troposphere to the boundary layer in the summertime high Arctic. Observations from the Arctic Summer Cloud Ocean Study field campaign show several occurrences of high aerosol particle concentrations above the boundary layer top. Large-eddy simulations suggest that when these enhanced aerosol concentrations are present, they can be an important source of aerosol particles for the boundary layer. Most particles are transported to the boundary layer by entrainment. However, it is found that mixed-phase stratocumulus clouds, which often extend into the inversion layer, also can mediate the transport of particles into the boundary layer by activation at cloud top and evaporation below cloud base. Finally, the simulations also suggest that aerosol properties at the surface sometimes may not be good indicators of aerosol properties in the cloud layer.

  • 21.
    Johansson, Erik
    et al.
    Swedish Meteorological and Hydrological Institute (SMHI), Sweden.
    Devasthale, A.
    L'Ecuyer, T.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    The vertical structure of cloud radiative heating over the Indian subcontinent during summer monsoon2015In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 15, no 20, p. 11557-11570Article in journal (Refereed)
    Abstract [en]

    Clouds forming during the summer monsoon over the Indian subcontinent affect its evolution through their radiative impact as well as the release of latent heat. While the latter is previously studied to some extent, comparatively little is known about the radiative impact of different cloud types and the vertical structure of their radiative heating/cooling effects. Therefore, the main aim of this study is to partly fill this knowledge gap by investigating and documenting the vertical distributions of the different cloud types associated with the Indian monsoon and their radiative heating/cooling using the active radar and lidar sensors on-board CloudSat and CALIPSO. The intraseasonal evolution of clouds from May to October is also investigated to understand pre-to-post monsoon transitioning of their radiative heating/cooling effects. The vertical structure of cloud radiative heating (CRH) follows the northward migration and retreat of the monsoon from May to October. Throughout this time period, stratiform clouds radiatively warm the middle troposphere and cool the upper troposphere by more than +/- 0.2 K day(-1) (after weighing by cloud fraction), with the largest impacts observed in June, July and August. During these months, the fraction of high thin cloud remains high in the tropical tropopause layer (TTL). Deep convective towers cause considerable radiative warming in the middle and upper troposphere, but strongly cool the base and inside of the TTL. This cooling is stronger during active (-1.23 K day(-1)) monsoon periods compared to break periods (-0.36 K day(-1)). The contrasting radiative warming effect of high clouds in the TTL is twice as largeduring active periods than in break periods. These results highlight the increasing importance of CRH with altitude, especially in the TTL. Stratiform (made up of alto- and nimbostratus clouds) and deep convection clouds radiatively cool the surface by approximately -100 and -400Wm(-2) respectively while warming the atmosphere radiatively by about 40 to 150Wm(-2). While the cooling at the surface induced by deep convection and stratiform clouds is largest during active periods of monsoon, the importance of stratiform clouds further increases during break periods. The contrasting CREs (cloud radiative effects) in the atmosphere and at surface, and during active and break periods, should have direct implications for the monsoonal circulation.

  • 22.
    Johansson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute (SMHI), Sweden.
    Devasthale, Abhay
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    L'Ecuyer, Tristan
    How Does Cloud Overlap Affect the Radiative Heating in the Tropical Upper Troposphere/Lower Stratosphere?2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 10, p. 5623-5631Article in journal (Refereed)
    Abstract [en]

    Characterizing two-layer cloud systems has historically been difficult. These systems have a strong radiative impact on the composition of and the processes in the upper troposphere-lower stratosphere (UTLS). Using 4 years of combined spaceborne lidar and radar observations, the radiative impact of two-layer cloud systems in the tropical UTLS is characterized, and its sensitivity to the properties of top- and bottom-layer clouds is further quantified. Under these overlapping cloud conditions, the bottom-layer clouds can fully suppress the radiative heating caused by high clouds in the UTLS, by inducing strong longwave cooling. If the vertical separation between the layers is <4 km, the radiative heating of the high cloud changes sign from positive to negative. Furthermore, the radiative effect at the top of the atmosphere is investigated, and it is found that the characteristic net warming by cirrus with ice water path <50 g/m(2) is suppressed in the two-layered system.

  • 23.
    Johansson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute (SMHI), Sweden.
    Devasthale, Abhay
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    L'Ecuyer, Tristan
    Response of the lower troposphere to moisture intrusions into the Arctic2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 5, p. 2527-2536Article in journal (Refereed)
    Abstract [en]

    Water vapor intrusions (WVIs) explain a significant fraction of total moisture transport and its variability in the Arctic. WVIs can precondition the Arctic atmosphere for accelerated melting of sea ice through effects on surface longwave radiation. Using data from the NASA's A-Train convoy of satellites to estimate the response of the lower troposphere to WVIs into the Arctic, we show that WVIs are associated with a surface warming of up to 5.3K (3.3K) in winter and 2.3K (1.6K) in summer, when averaged over the entire Arctic Ocean. The intrusions also lead to additional cloud radiative heating of up to 0.15K/d via up to 30% increased cloudiness in the vertical and also cause a weakening of the stability in the lower troposphere. The lower tropospheric and surface warming during winter and spring highlights the importance of understanding contribution of preconditioning to accelerated ice melt in the Arctic.

  • 24.
    Johansson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Devasthale, Abhay
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    L'Ecuyer, Tristan
    Wyser, Klaus
    Vertical cloud radiative heating in the tropics: Confronting the EC-Earth model with satellite observationsManuscript (preprint) (Other academic)
    Abstract [en]

    Understanding the coupling of clouds to large-scale circulation is one of the grand challenges faced by the global climate community. In this context, realistically simulating the vertical structure of cloud radiative heating/cooling (CRH) is a key premise to understand these couplings using Earth system models. Here, we evaluate CRH in two versions of the European Community Earth System Model (EC-Earth) using retrievals derived from the combined radar and lidar data from the CloudSat and CALIPSO satellites. One model version is used with two different horizontal resolutions (high resolution and its standard counterpart, i.e. EC-Earth3P-HR and EC-Earth3P). The other model version, EC-Earth3, is the EC-Earth version used for the AMIP experiment for CMIP6 experiments. The study focuses on the tropical region and the vertical structure of CRH and cloud properties, as such an evaluation has not yet been carried out for EC-Earth. We begin by evaluating the large-scale intra-seasonal variability in CRH in the different model versions, followed by an investigation of the changes in CRH during different phases of the El Nino Southern Oscillation (ENSO), a process that dominates the interannual climate variability in the tropics.

    All versions of EC-Earth evaluated here generally capture both the intra-seasonal and meridional variability in CRH over the convectively active and stratocumulus regions, and the CRH during the positive and negative phases of ENSO. However, two key differences between all model simulations and satellite retrievals emerge. First, the magnitude of CRH over the convectively active zones is up to twice as large in the models compared to the satellite data. Further dissection of net CRH into its shortwave and longwave components reveals noticeable differences in their vertical structure. The shortwave component of the radiative heating is overestimated by all model versions in the lowermost troposphere and underestimated in the middle troposphere. These over- and underestimations of shortwave heating are partly compensated by an overestimation of longwave cooling in the lowermost troposphere and heating in the middle troposphere. The observed biases in CRH can be traced back to disagreements in the cloud amount and cloud water content. We observe no noticeable improvement in the simulation of CRH by purely increasing the horizontal resolution in the model.

  • 25.
    Kapsch, Marie-Luise
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune G.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Economou, Theodoros
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    The importance of spring atmospheric conditions for predictions of the Arctic summer sea ice extent2014In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 41, no 14, p. 5288-5296Article in journal (Refereed)
    Abstract [en]

    Recent studies have shown that atmospheric processes in spring play an important role for the initiation of the summer ice melt and therefore may strongly influence the September sea ice concentration (SSIC). Here a simple statistical regression model based on only atmospheric spring parameters is applied in order to predict the SSIC over the major part of the Arctic Ocean. By using spring anomalies of downwelling longwave radiation or atmospheric water vapor as predictor variables, correlation coefficients between observed and predicted SSIC of up to 0.5 are found. These skills of seasonal SSIC predictions are similar to those obtained using more complex dynamical forecast systems, despite the fact that the simple model applied here takes neither information of the sea ice state, oceanic conditions nor feedback mechanisms during summer into account. The results indicate that a realistic representation of spring atmospheric conditions in the prediction system plays an important role for the predictive skills of a model system.

  • 26.
    Kapsch, Marie-Luise
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune Grand
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Springtime atmospheric energy transport and the control of Arctic summer sea-ice extent2013In: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 3, no 8, p. 744-748Article in journal (Refereed)
    Abstract [en]

    The summer sea-ice extent in the Arctic has decreased in recent decades, a feature that has become one of the most distinct signals of the continuing climate change. However, the interannual variability is large—the ice extent by the end of the summer varies by several million square kilometres from year to year. The underlying processes driving this year-to-year variability are not well understood. Here we demonstrate that the greenhouse effect associated with clouds and water vapour in spring is crucial for the development of the sea ice during the subsequent months. In years where the end-of-summer sea-ice extent is well below normal, a significantly enhanced transport of humid air is evident during spring into the region where the ice retreat is encountered. This enhanced transport of humid air leads to an anomalous convergence of humidity, and to an increase of the cloudiness. The increase of the cloudiness and humidity results in an enhancement of the greenhouse effect. As a result, downward long-wave radiation at the surface is larger than usual in spring, which enhances the ice melt. In addition, the increase of clouds causes an increase of the reflection of incoming solar radiation. This leads to the counterintuitive effect: for years with little sea ice in September, the downwelling short-wave radiation at the surface is smaller than usual. That is, the downwelling short-wave radiation is not responsible for the initiation of the ice anomaly but acts as an amplifying feedback once the melt is started.

  • 27.
    Kapsch, Marie-Luise
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune Grand
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bintanja, Richard
    The Effect of Downwelling Longwave and Shortwave Radiation on Arctic Summer Sea Ice2016In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 29, no 3, p. 1143-1159Article in journal (Refereed)
    Abstract [en]

    The Arctic summer sea ice has diminished fast in recent decades. A strong year-to-year variability on top of this trend indicates that sea ice is sensitive to short-term climate fluctuations. Previous studies show that anomalous atmospheric conditions over the Arctic during spring and summer affect ice melt and the September sea-ice extent (SIE). These conditions are characterized by clouds, humidity and heat anomalies which all affect shortwave (SWD) and longwave (LWD) radiation to the surface. In general, positive LWD anomalies are associated with cloudy and humid conditions, whereas positive anomalies of SWD appear under clear-sky conditions. Here we investigate the effect of realistic anomalies of LWD and SWD on summer sea ice, by performing experiments with the Community Earth System Model. The SWD and LWD anomalies are studied separately and in combination for different seasons. It is found that positive LWD anomalies in spring and early summer have significant impact on the September SIE, whereas winter anomalies show only little effect. Positive anomalies in spring and early summer initiate an earlier melt onset, hereby triggering several feedback mechanisms that amplify melt during the succeeding months. Realistic positive SWD anomalies appear only important if they occur after the melt has started and the albedo is significantly reduced relative to winter conditions. Simulations where both positive LWD and negative SWD anomalies are implemented simultaneously, mimicking cloudy conditions, reveal that clouds during spring have a significant impact on summer sea ice while summer clouds have almost no effect.

  • 28.
    Kapsch, Marie-Luise
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Max-Planck Institute for Meteorology, Germany.
    Skific, Natasa
    Graversen, Rune G.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Francis, Jennifer A.
    Summers with low Arctic sea ice linked to persistence of spring atmospheric circulation patterns2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 3-4, p. 2497-2512Article in journal (Refereed)
    Abstract [en]

    The declining trend of Arctic September sea ice constitutes a significant change in the Arctic climate system. Large year-to-year variations are superimposed on this sea-ice trend, with the largest variability observed in the eastern Arctic Ocean. Knowledge of the processes important for this variability may lead to an improved understanding of seasonal and long-term changes. Previous studies suggest that transport of heat and moisture into the Arctic during spring enhances downward surface longwave radiation, thereby controlling the annual melt onset, setting the stage for the September ice minimum. In agreement with these studies, we find that years with a low September sea-ice concentration (SIC) are characterized by more persistent periods in spring with enhanced energy flux to the surface in forms of net longwave radiation plus turbulent fluxes, compared to years with a high SIC. Two main atmospheric circulation patterns related to these episodes are identified: one resembles the so-called Arctic dipole anomaly that promotes transport of heat and moisture from the North Pacific, whereas the other is characterized by negative geopotential height anomalies over the Arctic, favoring cyclonic flow from Siberia and the Kara Sea into the eastern Arctic Ocean. However, differences between years with low and high September SIC appear not to be due to different spring circulation patterns; instead it is the persistence and intensity of processes associated with these patterns that distinguish the two groups of anomalous years: Years with low September SIC feature episodes that are consistently stronger and more persistent than years with high SIC.

  • 29.
    Kleman, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology .
    Destouni, Georgia
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Rubbat förtroende för forskarna2010In: Svenska Dagbladet, ISSN 1101-2412, no 25 majArticle in journal (Other (popular science, discussion, etc.))
  • 30. Koenigk, Torben
    et al.
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, RuneGrand
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Willén, Ulrika
    Wyser, Klaus
    Arctic climate change in 21st century CMIP5 simulations with EC-Earth2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 40, no 11-12Article in journal (Refereed)
    Abstract [en]

    The Arctic climate change is analyzed in anensemble of future projection simulations performed withthe global coupled climate model EC-Earth2.3. EC-Earthsimulates the twentieth century Arctic climate relativelywell but the Arctic is about 2 K too cold and the sea icethickness and extent are overestimated. In the twenty-firstcentury, the results show a continuation and strengtheningof the Arctic trends observed over the recent decades,which leads to a dramatically changed Arctic climate,especially in the high emission scenario RCP8.5. Theannually averaged Arctic mean near-surface temperatureincreases by 12 K in RCP8.5, with largest warming in theBarents Sea region. The warming is most pronounced inwinter and autumn and in the lower atmosphere. The Arcticwinter temperature inversion is reduced in all scenarios anddisappears in RCP8.5. The Arctic becomes ice free inSeptember in all RCP8.5 simulations after a rapid reductionevent without recovery around year 2060. Taking intoaccount the overestimation of ice in the twentieth century,our model results indicate a likely ice-free Arctic inSeptember around 2040. Sea ice reductions are most pronouncedin the Barents Sea in all RCPs, which lead to themost dramatic changes in this region. Here, surface heatfluxes are strongly enhanced and the cloudiness is substantiallydecreased. The meridional heat flux into theArctic is reduced in the atmosphere but increases in theocean. This oceanic increase is dominated by an enhancedheat flux into the Barents Sea, which strongly contributes tothe large sea ice reduction and surface-air warming in thisregion. Increased precipitation and river runoff lead to morefreshwater input into the Arctic Ocean. However, most ofthe additional freshwater is stored in the Arctic Ocean whilethe total Arctic freshwater export only slightly increases.

  • 31.
    Kupiszewski, Pjotr
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Paul Scherrer Institute, Switzerland .
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sjögren, S.
    Sedlar, J.
    Graus, M.
    Mueller, M.
    Brooks, B.
    Swietlicki, E.
    Norris, S.
    Hansel, A.
    Vertical profiling of aerosol particles and trace gases over the central Arctic Ocean during summer2013In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, no 24, p. 12405-12431Article in journal (Refereed)
    Abstract [en]

    Unique measurements of vertical size-resolved aerosol particle concentrations, trace gas concentrations and meteorological data were obtained during the Arctic Summer Cloud Ocean Study (ASCOS, www.ascos.se), an International Polar Year project aimed at establishing the processes responsible for formation and evolution of low-level clouds over the high Arctic summer pack ice. The experiment was conducted from on board the Swedish icebreaker Oden, and provided both ship-and helicopter-based measurements. This study focuses on the vertical helicopter profiles and onboard measurements obtained during a three-week period when Oden was anchored to a drifting ice floe, and sheds light on the characteristics of Arctic aerosol particles and their distribution throughout the lower atmosphere. Distinct differences in aerosol particle characteristics within defined atmospheric layers are identified. Within the lowermost couple hundred metres, transport from the marginal ice zone (MIZ), condensational growth and cloud processing develop the aerosol population. During two of the four representative periods defined in this study, such influence is shown. At altitudes above about 1 km, long-range transport occurs frequently. However, only infrequently does large-scale subsidence descend such air masses to become entrained into the mixed layer in the high Arctic, and there-fore long-range transport plumes are unlikely to directly influence low-level stratiform cloud formation. Nonetheless, such plumes can influence the radiative balance of the planetary boundary layer (PBL) by influencing formation and evolution of higher clouds, as well as through precipitation transport of particles downwards. New particle formation was occasionally observed, particularly in the near-surface layer. We hypothesize that the origin of these ultrafine particles could be in biological processes, both primary and secondary, within the open leads between the pack ice and/or along the MIZ. In general, local sources, in combination with upstream boundary-layer transport of precursor gases from the MIZ, are considered to constitute the origin of cloud condensation nuclei (CCN) particles and thus be of importance for the formation of interior Arctic low-level clouds during summer, and subsequently, through cloud influences, for the melting and freezing of sea ice.

  • 32. Loewe, Katharina
    et al.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Paukert, Marco
    Sedlar, Joseph
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hoose, Corinna
    Modelling micro- and macrophysical contributors to the dissipation of an Arctic mixed-phase cloud during the Arctic Summer Cloud Ocean Study (ASCOS)2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 11, p. 6693-6704Article in journal (Refereed)
    Abstract [en]

    The Arctic climate is changing; temperature changes in the Arctic are greater than at midlatitudes, and changing atmospheric conditions influence Arctic mixed-phase clouds, which are important for the Arctic surface energy budget. These low-level clouds are frequently observed across the Arctic. They impact the turbulent and radiative heating of the open water, snow, and sea-ice-covered surfaces and influence the boundary layer structure. Therefore the processes that affect mixed-phase cloud life cycles are extremely important, yet relatively poorly understood. In this study, we present sensitivity studies using semi-idealized large eddy simulations (LESs) to identify processes contributing to the dissipation of Arctic mixed-phase clouds. We found that one potential main contributor to the dissipation of an observed Arctic mixed-phase cloud, during the Arctic Summer Cloud Ocean Study (ASCOS) field campaign, was a low cloud droplet number concentration (CDNC) of about 2 cm(-3). Introducing a high ice crystal concentration of 10 L-1 also resulted in cloud dissipation, but such high ice crystal concentrations were deemed unlikely for the present case. Sensitivity studies simulating the advection of dry air above the boundary layer inversion, as well as a modest increase in ice crystal concentration of 1 L-1, did not lead to cloud dissipation. As a requirement for small droplet numbers, pristine aerosol conditions in the Arctic environment are therefore considered an important factor determining the lifetime of Arctic mixed-phase clouds.

  • 33.
    Lundén, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wisthaler, Armin
    Institut fur Ionenphysik and Angewandte Physik, Universität Innsbruck.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hansel, Armin
    Institut fur Ionenphysik and Angewandte Physik, Universität Innsbruck.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    The vertical distribution of atmospheric DMS in the high Arctic summer2010In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 62, no 3, p. 160-171Article in journal (Refereed)
    Abstract [en]

    The vertical structure of gas-phase dimethyl sulfide, DMS(g), in the high Arctic atmosphere is investigated during a summer season. The model results show that the near-surface DMS(g) concentration over open ocean is very variable both in time and space, depending on the local atmospheric conditions. Profiles over ocean have typically highest concentration near the surface and decrease exponentially with height. Over the pack-ice, the concentrations are typically lower and the vertical structure changes as the air is advected northward. Modeled DMS(g) maxima above the local boundary layer were present in about 3\% of the profiles found over the pack-ice. These maxima were found in association to frontal zones. Our results also show that DMS(g) can be mixed downward by turbulence into the local boundary layer and act as a local near--surface DMS(g) source over the pack-ice and may hence influence the growth of cloud condensation nuclei and cloud formation in the boundary layer. Profile observations are presented in support to the model results. They show that significant DMS(g) concentrations exist in the Arctic atmosphere at altitudes not to be expected when only considering vertical mixing in the boundary layer.

     

  • 34. Mauritsen, T.
    et al.
    Sedlar, J.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leck, C.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Martin, M.
    Shupe, M.
    Sjögren, S.
    Sierau, B.
    Persson, P.O.G.
    Brooks, I.M.
    Swietlicki, E.
    Aerosols indirectly warm the Arctic2010Manuscript (preprint) (Other academic)
    Abstract [en]

    On average, airborne aerosol particles cool the Earth's surface directly by absorbing and scattering sunlight and indirectly by influencing cloud reflectivity, life time, thickness or extent. Here we show that over the central Arctic Ocean, where there is frequently a lack of aerosol particles upon which clouds may form, a small increase in aerosol loading may enhance cloudiness thereby likely causing a climatologically significant warming at the ice-covered Arctic surface. Under these low concentration conditions cloud droplets grow to drizzle sizes and fall, even in the absence of collisions and coalescence, thereby diminishing cloud water. Evidence from a case study suggests that interactions between aerosol, clouds and precipitation could be responsible for attaining the observed low aerosol concentrations.

  • 35. Mauritsen, T.
    et al.
    Sedlar, Joseph
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Martin, M.
    Shupe, M.
    Sjogren, S.
    Sierau, B.
    Persson, P. O. G.
    Brooks, I. M.
    Swietlicki, E.
    An Arctic CCN-limited cloud-aerosol regime2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 1, p. 165-173Article in journal (Refereed)
    Abstract [en]

    On average, airborne aerosol particles cool the Earth's surface directly by absorbing and scattering sunlight and indirectly by influencing cloud reflectivity, life time, thickness or extent. Here we show that over the central Arctic Ocean, where there is frequently a lack of aerosol particles upon which clouds may form, a small increase in aerosol loading may enhance cloudiness thereby likely causing a climatologically significant warming at the ice-covered Arctic surface. Under these low concentration conditions cloud droplets grow to drizzle sizes and fall, even in the absence of collisions and coalescence, thereby diminishing cloud water. Evidence from a case study suggests that interactions between aerosol, clouds and precipitation could be responsible for attaining the observed low aerosol concentrations.

  • 36. Naakka, T.
    et al.
    Nygård, T.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Vihma, T.
    Pirazzini, R.
    Brooks, M.
    The Impact of Radiosounding Observations on Numerical Weather Prediction Analyses in the Arctic2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 14, p. 8527-8535Article in journal (Refereed)
    Abstract [en]

    The radiosounding network in the Arctic, despite being sparse, is a crucial part of the atmospheric observing system for weather prediction and reanalysis. The spatial coverage of the network was evaluated using a numerical weather prediction model, comparing radiosonde observations from Arctic land stations and expeditions in the central Arctic Ocean with operational analyses and background fields (12-hr forecasts) from European Centre for Medium-Range Weather Forecasts for January 2016 to September 2018. The results show that the impact of radiosonde observations on analyses has large geographical variation. In data-sparse areas, such as the central Arctic Ocean, high-quality radiosonde observations substantially improve the analyses, while satellite observations are not able to compensate for the large spatial gap in the radiosounding network. In areas where the network is reasonably dense, the quality of background field is more related to how radiosonde observations are utilized in the assimilation and to the quality of those observations.

  • 37. Norris, S. J.
    et al.
    Brooks, I. M.
    de Leeuw, G.
    Sirevaag, A.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brooks, B. J.
    Birch, C. E.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Measurements of bubble size spectra within leads in the Arctic summer pack ice2011In: Ocean Science, ISSN 1812-0784, E-ISSN 1812-0792, Vol. 7, no 1, p. 129-139Article in journal (Refereed)
    Abstract [en]

    The first measurements of bubble size spectra within the near-surface waters of open leads in the central Arctic pack ice were obtained during the Arctic Summer Cloud-Ocean Study (ASCOS) in August 2008 at 8787.6 degrees N, 1-11 degrees W. A significant number of small bubbles (30-100 mu m diameter) were present, with concentration decreasing rapidly with size from 100-560 mu m; no bubbles larger than 560 mu m were observed. The bubbles were present both during periods of low wind speed (U < 6 m s(-1)) and when ice covered the surface of the lead. The low wind and short open-water fetch precludes production of bubbles by wave breaking suggesting that the bubbles are generated by processes below the surface. When the surface water was open to the atmosphere bubble concentrations increased with increasing heat loss to the atmosphere. The presence of substantial numbers of bubbles is significant because the bursting of bubbles at the surface provides a mechanism for the generation of aerosol and the ejection of biological material from the ocean into the atmosphere. Such a transfer has previously been proposed as a potential climate feedback linking marine biology and Arctic cloud properties.

  • 38. Overland, James
    et al.
    Turner, John
    Francis, Jennifer
    Gillett, Nathan
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    The Arctic and Antarctic: Two faces of climate change2008In: EOS: Transactions, Vol. 89, no 19, p. 177-178Article in journal (Refereed)
  • 39. Pithan, Felix
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Chechin, Dmitry
    Cronin, Timothy W.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Neggers, Roel
    Shupe, Matthew D.
    Solomon, Amy
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wendisch, Manfred
    Role of air-mass transformations in exchange between the Arctic and mid-latitudes2018In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 11, no 11, p. 805-812Article, review/survey (Refereed)
    Abstract [en]

    Pulses of warm and moist air from lower latitudes provide energy to the Arctic and form its main energy source outside of the summer months. These pulses can cause substantial surface warming and trigger ice melt. Air-mass transport in the opposite direction, away from the Arctic, leads to cold-air outbreaks. The outbreaks are often associated with cold extremes over continents, and extreme surface heat fluxes and occasional polar lows over oceans. Air masses advected across the strong Arctic-to-mid-latitude temperature gradient are rapidly transformed into colder and dryer or warmer and moister air masses by clouds, radiative and turbulent processes, particularly in the boundary layer. Phase changes from liquid to ice within boundary-layer clouds are critical in these air-mass transformations. The presence of liquid water determines the radiative effects of these clouds, whereas the presence of ice is crucial for subsequent cloud decay or dissipation, processes that are poorly represented in weather and climate models. We argue that a better understanding of how air masses are transformed on their way into and out of the Arctic is essential for improved prediction of weather and climate in the Arctic and mid-latitudes. Observational and modelling exercises should take an air-mass-following Lagrangian approach to attain these goals.

  • 40. Prenni, Anthony J.
    et al.
    Harrington, Jerry Y.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    DeMott, Paul J.
    Avramov, Alexander
    Long, Charles N.
    Kreidenweis, Sonia M.
    Olsson, Peter Q.
    Verlinde, Johannes
    Can Ice-Nucleating Aerosols Affect Arctic Seasonal Climate?2007In: Bulletin of the American Meteorological Society, ISSN 1520-0477, Vol. 88, no 4, p. 541-550Article in journal (Refereed)
  • 41.
    Prytherch, John
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Leeds, United Kingdom.
    Brooks, Ian
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thornton, Brett
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Salisbury, Dominic
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Anderson, Leif
    Geibel, Marc C.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Air-sea CO2and CH4 gas transfer velocity in Arctic sea-ice regions fromeddy covariance flux measurements onboard Icebreaker Oden2017In: Geophysical Research Abstracts, ISSN 1029-7006, E-ISSN 1607-7962, Vol. 19, article id 697Article in journal (Refereed)
    Abstract [en]

    The Arctic Ocean is an important sink for atmospheric CO2, and there is ongoing debate on whether seafloor seeps in the Arctic are a large source of CH4 to the atmosphere. The impact of warming waters, decreasing sea-ice extent and expanding marginal ice zones on Arctic air-sea gas exchange depends on the rate of gas transfer in the presence of sea ice. Sea ice acts as a near-impermeable lid to air-sea gas exchange, but is also hypothesised to enhance gas transfer rates through physical processes such as increased surface-ocean turbulence from ice-water shear and ice-edge form drag. The dependence of the gas transfer rate on sea-ice concentration remains uncertain due to a lack of in situ measurements. Here we present the first direct estimates of gas transfer rate in a wide range of Arctic sea-ice conditions. The estimates were derived from eddy covariance CO2 and CH4 fluxes, measured from the Swedish Icebreaker Oden during two expeditions: the 3-month duration Arctic Clouds in Summer Experiment (ACSE) in 2014, a component of the Swedish-Russian-US Arctic Ocean Investigation on Climate-Cryosphere-Carbon Interactions (SWERUS-C3) in the eastern Arctic Ocean shelf region; and the Arctic Ocean 2016 expedition to the high latitude Arctic Ocean. Initial CO2 results from ACSE showed that the gas transfer rate has a near-linear dependence on sea-ice concentration, and that some previous indirect measurements and modelling estimates overestimate gas transfer rates in sea-ice regions. This supports a linear sea-ice scaling approach for assessments of polar ocean carbon fluxes. Air-sea gas transfer model assumptions (e.g. Schmidt number dependence) will be examined using simultaneous CO2 and CH4 measurements, and observations in different ice conditions (e.g. summer melt, autumn freeze up, central Arctic and marginal ice zones) will be compared.

  • 42.
    Prytherch, John
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Leeds, UK.
    Brooks, Ian M.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thornton, Brett F.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Salisbury, Dominic J.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Anderson, Leif G.
    Geibel, Marc C.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Direct determination of the air-sea CO2 gas transfer velocity in Arctic sea ice regions2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 8, p. 3770-3778Article in journal (Refereed)
    Abstract [en]

    The Arctic Ocean is an important sink for atmospheric CO2. The impact of decreasing sea ice extent and expanding marginal ice zones on Arctic air-sea CO2 exchange depends on the rate of gas transfer in the presence of sea ice. Sea ice acts to limit air-sea gas exchange by reducing contact between air and water but is also hypothesized to enhance gas transfer rates across surrounding open-water surfaces through physical processes such as increased surface-ocean turbulence from ice-water shear and ice-edge form drag. Here we present the first direct determination of the CO2 air-sea gas transfer velocity in a wide range of Arctic sea ice conditions. We show that the gas transfer velocity increases near linearly with decreasing sea ice concentration. We also show that previous modeling approaches overestimate gas transfer rates in sea ice regions.

  • 43.
    Ranjha, Raza
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Semedo, Alvaro
    Global distribution and seasonal variability of coastal low-level jets derived from ERA-Interim reanalysis2013In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 65, p. 20412-Article in journal (Refereed)
    Abstract [en]

    A low-level wind maximum is often found over the oceans near many coasts around the world. These Coastal Low-Level Jets (CLLJs) play an important role in the coastal weather and have significant impacts on regional climate and ecology as well as on a number of human activities. The presence of CLLJs is related to various local circumstances such as land-sea temperature contrasts, upwelling, coastal terrain, orientation of the coast, etc., but also to the large-scale atmospheric dynamics. This makes studies of CLLJs not only interesting but also challenging.

    In the present study, based on ERA-Interim reanalysis data, the global distribution, spatio-temporal structure, and the seasonal variability of CLLJs are documented. Seasonal data from 1980 to 2011 are used to identify areas where CLLJs are frequently found in the lowest 2 km, following criteria based on the vertical profiles of wind speed and temperature. The results are analyzed to highlight the fundamental aspects and distinctive features of the CLLJs across the globe, including their occurrence rate, jet height, wind-speed maximum and horizontal extent.

    Global maps of CLLJs are constructed for the summer and winter seasons. The west coasts of North America, the Iberian Peninsula, north-western Africa and the south-eastern coast of the Arabian Peninsula make up the northern-hemispheric CLLJ regions, while the west coasts of South America, Australia and southern Africa comprise the south-hemispheric equivalents. The existence and characteristics of CLLJs along the southern coast of Oman and the western coast of the Iberian Peninsula regions are also discussed, not fully envisaged before in the context of CLLJs. The highest occurrence of CLLJs is found during the summer in both hemispheres, and the coast of Oman has the globally highest CLLJ frequency, with also the highest maximum wind speeds. The most commonly found CLLJ has a maximum wind speed between 9 and 15 m s-1, and occurs at heights between 500 and 700 m a.s.l.

  • 44.
    Ranjha, Raza
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Lund University, Sweden.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Semedo, Alvaro
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Cardoso, Rita M.
    Structure and Variability of the Oman Coastal Low-Level Jet2015In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 67, article id 25285Article in journal (Refereed)
    Abstract [en]

    During the boreal summer strong southwesterly, coast parallel low-level winds prevail off the coast of Oman and over the Arabian Sea. The offshore large-scale structure and variability of these coastal winds has been a topic of numerous studies because of their strong connection to the south-Asian summer monsoon. However, low-level coastal jets along the coast of Oman have not been studied in detail, especially on a mesoscale level, despite their globally high frequency of occurrence. In the current study, regional atmospheric modeling has been utilized in an attempt to resolve the mesoscale structure, spatial variability and temporal characteristics of the Oman coastal low-level jet (CLLJ). The limited area model COAMPS® has been used at 6-km resolution for a five month period from May through September, during 2009. The model output compares favorably with the seasonal climatology for the coast of Oman.

    In this study, analysis of high resolution model fields reveals the mesoscale structure of the Oman CLLJ, clearly distinguishing it from the large-scale southwesterly monsoon circulation farther offshore over the Arabian Sea. The Oman CLLJ is closer to the coast and spread northeastwards along the coast of Oman. Although the synoptic forcing that preconditions the Oman CLLJ is different from other areas of CLLJ occurrences, nevertheless, the jet is still driven primarily by the coastal baroclinicity due to sharp land-sea thermal contrasts. Within the study period, July exhibits the highest CLLJ frequency with ~80% occurrences and also manifests highest winds around 27 m s-1.

  • 45.
    Ranjha, Raza
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Semedo, Alvaro
    Modelling coastal low-level wind-jets: does horizontal resolution matter?2016In: Meteorology and atmospheric physics (Print), ISSN 0177-7971, E-ISSN 1436-5065, Vol. 128, no 2, p. 263-278Article in journal (Refereed)
    Abstract [en]

    Atmospheric flows in coastal regions are impacted by land-sea temperature contrasts, complex terrain, shape of the coastline, among many things. Along the west coast of central North America, winds in the boundary layer are mainly from north or northwest, roughly parallel to the coastline. Frequently, the coastal low-level wind field is characterized by a sharp wind maximum along the coast in the lowest kilometer. This feature, commonly referred to as a Coastal Low-Level Jet (CLLJ), has significant impact on the climatology of the coastal region, influencing aviation, shipping, fishing and a number of other human activities along the littoral zone, and better understanding and forecasting of this is vital.

    The purpose of this study is to elucidate the issue of proper resolution in a mesoscale numerical model to describe the physics of a CLLJ, and its impact on the upper ocean. The COAMPS® model is used for a summer event to determine the realism of the model results compared to observations, from an area of supercritical flow adjustment between Pt. Sur and Pt. Conception, California. Simulations at different model horizontal resolutions, from 54 km to 2 km are performed. While the model produces realistic results with increasing details at higher resolution, the results do not converge even at a resolution of only few kilometers and an objective analysis of model errors do not show an increased skill with increasing resolution. New methods may thus have to be developed to evaluate models at very high resolution. Based on all available information, a compromise resolution appears to be at least 6 km.

  • 46.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Elagib, Nadir Ahmed
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Characterization of the Sahelian-Sudan rainfall based on observations and regional climate models2018In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 202, p. 205-218Article in journal (Refereed)
    Abstract [en]

    The African Sahel region is known to be highly vulnerable to climate variability and change. We analyze rainfall in the Sahelian Sudan in terms of distribution of rain-days and amounts, and examine whether regional climate models can capture these rainfall features. Three regional models namely, Regional Model (REMO), Rossby Center Atmospheric Model (RCA) and Regional Climate Model (RegCM4), are evaluated against gridded observations (Climate Research Unit, Tropical Rainfall Measuring Mission, and ERA-interim reanalysis) and rain gauge data from six arid and semi-arid weather stations across Sahelian Sudan over the period 1989 to 2008. Most of the observed rain-days are characterized by weak (0.1-1.0 mm/day) to moderate ( > 1.0-10.0 mm/day) rainfall, with average frequencies of 18.5% and 48.0% of the total annual rain-days, respectively. Although very strong rainfall events ( > 30.0 mm/day) occur rarely, they account for a large fraction of the total annual rainfall (28-42% across the stations). The performance of the models varies both spatially and temporally. RegCM4 most closely reproduces the observed annual rainfall cycle, especially for the more arid locations, but all of the three models fail to capture the strong rainfall events and hence underestimate its contribution to the total annual number of rain days and rainfall amount. However, excessive moderate rainfall compensates this underestimation in the models in an annual average sense. The present study uncovers some of the models' limitations in skillfully reproducing the observed climate over dry regions, will aid model users in recognizing the uncertainties in the model output and will help climate and hydrological modeling communities in improving models.

  • 47.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Climate impact of deforestation over South Sudan in a regional climate model2013In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 33, no 10, p. 2362-2375Article in journal (Refereed)
    Abstract [en]

    This study examines the sensitivity of climate to changes in vegetation cover and land use in South Sudan. The focus lies on the effect of deforestation on precipitation and surface temperature especially during the rainy season. Sensitivity experiments are performed with the third version of the Abdus Salam International Centre for Theoretical Physics Regional Climate Model (RegCM3) where the present forest and vegetation cover south of 10 degrees N in Sudan are replaced by either grass or, as an extreme case, desert. The model experiments were conducted for a time period of almost 21 years, from January 1989 to August 2009, and were preceded by a control experiment to ascertain the fidelity of the model simulations. The experiments indicate that the vegetation changes affect precipitation and surface temperature in both Southern and Central Sudan significantly although the land cover changes were imposed only in the south. The precipitation during the rainy season (June through September) was reduced in the perturbed region by about 0.1-2.1 mm d(-1) for the desert scenario and by 0.1-0.9 mm d(-)1 for the grass scenario. The surface temperature increases by about 1.2 and 2.4 degrees C in the grass and desert scenario, respectively. The precipitation reduction is thus not only local but also extends to Central Sudan and neighbouring regions. The study demonstrates significant dependency for Southern and Central Sudan precipitation on the land use in Southern Sudan and indicates that the deforestation has both local and non-local regional climatic effects.

  • 48.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sources of Sahelian-Sudan moisture: Insights from a moisture-tracing atmospheric model2016In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 121, no 13, p. 7819-7832Article in journal (Refereed)
    Abstract [en]

    The summer rainfall across Sahelian-Sudan is one of the main sources of water for agriculture, human, and animal needs. However, the rainfall is characterized by large interannual variability, which has attracted extensive scientific efforts to understand it. This study attempts to identify the source regions that contribute to the Sahelian-Sudan moisture budget during July through September. We have used an atmospheric general circulation model with an embedded moisture-tracing module (Community Atmosphere Model version 3), forced by observed (1979-2013) sea-surface temperatures. The result suggests that about 40% of the moisture comes with the moisture flow associated with the seasonal migration of the Intertropical Convergence Zone (ITCZ) and originates from Guinea Coast, central Africa, and the Western Sahel. The Mediterranean Sea, Arabian Peninsula, and South Indian Ocean regions account for 10.2%, 8.1%, and 6.4%, respectively. Local evaporation and the rest of the globe supply the region with 20.3% and 13.2%, respectively. We also compared the result from this study to a previous analysis that used the Lagrangian model FLEXPART forced by ERA-Interim. The two approaches differ when comparing individual regions, but are in better agreement when neighboring regions of similar atmospheric flow features are grouped together. Interannual variability with the rainfall over the region is highly correlated with contributions from regions that are associated with the ITCZ movement, which is in turn linked to the Atlantic Multidecadal Oscillation. Our result is expected to provide insights for the effort on seasonal forecasting of the rainy season over Sahelian Sudan.

  • 49.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lagrangian tracing of Sahelian Sudan moisture sources2015In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, no 14, p. 6793-6808Article in journal (Refereed)
    Abstract [en]

    The Sahelian Sudan is an arid to semiarid region that depends on the seasonal rainfall as the main source of water, but its rainfall has large interannual variability. Such dry regions usually have their main moisture sources elsewhere; thus, the rainfall variability is directly related to the moisture transport. This study seeks to identify source regions of water vapor for Sahelian Sudan during the monsoon period, from July to September. We have used the Lagrangian trajectory model FLEXPART driven by ERA-Interim reanalysis for the time period 1998 to 2008. The results show that most of the air masses that reach this region during the monsoon period have their major origins over the Arabian Peninsula, Central Africa, or are associated with the tropical easterly jet. Flow associated with Intertropical Convergence Zone contributes almost half of the total precipitated water; most of it comes from Central Africa. This suggests that moisture recycling is the major contributor, compared to Oceanic sources. The flows from the northeast (Arabian Peninsula and north Asia) and east (Horn of Africa and north Indian Ocean) contribute about one third of the precipitated water. The rest of precipitated water comes from the Mediterranean, subtropical Atlantic, and western Sahel, all with smaller contribution. Our results also indicate that different subregions of Sahelian Sudan have different moisture sources. Such result needs to be taken into account in seasonal forecasting practices.

  • 50.
    Savre, Julien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Large-eddy simulations of an Arctic mixed-phase stratiform cloud observed during ISDAC: sensitivity to moisture aloft, surface fluxes and large-scale forcing2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 689, p. 1177-1190Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulation (LES) is used to examine the complex interactions between cloud properties and boundary-layer structure in Arctic low-level mixed-phase clouds using idealised conditions based on the Indirect and Semi-Direct Aerosol Campaign (ISDAC, April 2008). The persistence of steady mixed-phase conditions depends mostly on a balance between ice vertical redistribution and ice growth by vapour deposition in such a way that ice crystals cannot accumulate within the cloud layer to consume the available liquid water. An external source of water vapour is necessary to balance the net sink of total water in the cloud layer. Two main local sources of moisture are present: the initial moist surface layer and the free troposphere. In the studied case, the surface layer is found to be the dominant source of vapour to the cloud, the temperature inversion preventing significant entrainment from above. In most of the cases, the simulated boundary layer becomes rapidly well-mixed despite the stabilising effect of ice sublimation and latent cooling close to the surface. The minor effect of near-surface latent cooling on stability is connected to the initially moist surface layer limiting ice sublimation. Water vapour supply in the sub-cloud layer, resulting from entrainment of moisture from aloft, reduces ice sublimation above the surface layer and contributes to the maintenance of some degree of boundary-layer decoupling. In contrast, moisture surface fluxes reduce sublimation in the surface layer and accelerate cloud-surface coupling. Overall, the persistence of cloud-surface decoupling remains mostly driven by large-scale heat and moisture advection.

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