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On the Arctic Boundary Layer: From Turbulence to Climate
Stockholm University, Faculty of Science, Department of Meteorology.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The boundary layer is the part of the atmosphere that is in direct contact with the ground via turbulent motion. At mid-latitudes the boundary layer is usually one or a few kilometers deep, while in the Arctic it is much more shallow, typically a few hundred meters or less. The reason is that here the absolute temperature increases in the lowest kilometer, making the boundary layer semi-permanently stably stratified. The exchange of heat, momentum and tracers between the atmosphere, ocean and ground under stable stratification is discussed from an observational, modeling and climate-change point of view. A compilation of six observational datasets, ordered by the Richardson number (rather than the widely used Monin-Obukhov length) reveals new information about turbulence in the very stably stratified regime. An essentially new turbulence closure model, based on the total turbulent energy concept and these observational datasets, is developed and tested against large-eddy simulations with promising results. The role of mesoscale motion in the exchange between the atmosphere and surface is investigated both for observations and in idealized model simulations. Finally, it is found that the stably stratified boundary layer is more sensitive to external surface forcing than its neutral and convective counterparts. It is speculated that this could be part of the explanation for the observed Arctic amplification of climate change.

Place, publisher, year, edition, pages
Stockholm: Meteorologiska institutionen (MISU) , 2007. , 165 p.
Keyword [en]
Atmospheric boundary layers, Turbulence, Stable stratification, Gravity waves, Mesoscale motion, Arctic climate
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences
Identifiers
URN: urn:nbn:se:su:diva-6585ISBN: 91-7155-373-8 (print)OAI: oai:DiVA.org:su-6585DiVA: diva2:196700
Public defence
2007-02-23, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 10:00
Opponent
Supervisors
Available from: 2007-02-01 Created: 2007-02-01Bibliographically approved
List of papers
1. Wave flow simulations over Arctic leads
Open this publication in new window or tab >>Wave flow simulations over Arctic leads
2005 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, Vol. 117, no 2, 259-273 p.Article in journal (Refereed) Published
Abstract [en]

We investigate the flow over Arctic leads using a mesoscale numerical model, typical of both summer and winter, under idealised conditions. We find that Arctic leads may be the source of standing atmospheric internal gravity waves during both seasons. The summertime wave may be compared with the wave generated by a small ridge, though with the phase reversed. The mechanism for exciting the wave is found to be the internal boundary layer developing due to horizontal variations in surface temperature and roughness length. During the more exploratory wintertime simulations, with substantial temperature difference between the lead and the ice surface, we find that secondary circulations and intermittent wave-breaking may occur. The effects of the lead appear far downstream.

Keyword
Arctic, Gravity waves, Ice leads, Intermittent and elevated turbulence, Wave-breaking
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-24078 (URN)10.1007/s10546-004-1427-2 (DOI)
Note
Part of urn:nbn:se:su:diva-6585Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2010-01-22Bibliographically approved
2. Observations of stably stratified shear-driven atmospheric turbulence at low and high Richardson numbers
Open this publication in new window or tab >>Observations of stably stratified shear-driven atmospheric turbulence at low and high Richardson numbers
2007 (English)In: Journal of the Atmospheric Sciences, ISSN 0022-4928, Vol. 64, no 2, 645-655 p.Article in journal (Refereed) Published
Abstract [en]

Stably stratified shear-driven turbulence is analyzed using the gradient Richardson number, Ri, as the stability parameter. The method overcomes the statistical problems associated with the widely used Monin–Obukhov stability parameter. The results of the Ri-based scaling confirm the presence of three regimes: the weakly and the very stable regimes and the transition in between them. In the weakly stable regime, fluxes scale in proportion with variance, while in the very stable regime, stress and scalar fluxes behave differently. At large Ri, the velocity field becomes highly anisotropic and the turbulent potential energy becomes approximately equal to half of the turbulent kinetic energy. It appears that even in the strongly stable regime, beyond what is known as the critical gradient Richardson number, turbulent motions are present.

National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-24079 (URN)10.1175/JAS3856.1 (DOI)000244276600023 ()
Note
Part of urn:nbn:se:su:diva-6585Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2010-01-22Bibliographically approved
3. A total turbulent energy closure model for neutrally and stably stratified atmospheric boundary layers
Open this publication in new window or tab >>A total turbulent energy closure model for neutrally and stably stratified atmospheric boundary layers
Show others...
2007 (English)In: Journal of the Atmospheric Sciences, ISSN 0022-4928, Vol. 64, no 11, 4113-4136 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a turbulence closure for neutral and stratified atmospheric conditions. The closure is based on the concept of the total turbulent energy. The total turbulent energy is the sum of the turbulent kinetic energy and turbulent potential energy, which is proportional to the potential temperature variance. The closure uses recent observational findings to take into account the mean flow stability. These observations indicate that turbulent transfer of heat and momentum behaves differently under very stable stratification. Whereas the turbulent heat flux tends toward zero beyond a certain stability limit, the turbulent stress stays finite. The suggested scheme avoids the problem of self-correlation. The latter is an improvement over the widely used Monin–Obukhov-based closures. Numerous large-eddy simulations, including a wide range of neutral and stably stratified cases, are used to estimate likely values of two free constants. In a benchmark case the new turbulence closure performs indistinguishably from independent large-eddy simulations.

National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-24080 (URN)10.1175/2007JAS2294.1 (DOI)000251283000024 ()
Note
Part of urn:nbn:se:su:diva-6585Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2010-01-22Bibliographically approved
4. Mesoscale variability in the summer Arctic boundary layer
Open this publication in new window or tab >>Mesoscale variability in the summer Arctic boundary layer
2009 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, Vol. 130, no 3, 1573-1472 p.Article in journal (Refereed) Published
Abstract [en]

Observations from the summer Arctic Ocean Experiment 2001 (AOE-2001) are analysed with a focus on the interactions between mesoscale and boundary-layer dynamics. Wavelet analyses of surface-pressure variations show daylong periods with different characteristics, some featuring episodes of pronounced high-frequency surface-pressure variability, here hypothesized to be caused by trapped gravity waves. These episodes are accompanied by enhanced boundary-layer turbulence and an enhanced spectral gap, but with only minor influence on the surface stress. During these episodes, mesoscale phenomena were often encountered and usually identified as front-like features in the boundary layer, with a peak in drizzle followed by changing temperature. These phenomena resemble synoptic fronts, though they are generally shallow, shorter-lasting, have no signs of frontal clouds, and do not imply a change in air mass. Based on this analysis, we hypothesize that the root cause of the episodes with high-frequency surface-pressure variance are shallow, mesoscale fronts moving across the pack ice. They may be formed due to local-to-regional horizontal contrasts, for example, between air with different lifetimes over the Arctic or with perturbations in the cloud field causing differential cooling of the boundary layer. Thermal contrasts sharpen as the air is transported with the mean flow. The propagating mesoscale fronts excite gravity waves, which affect the boundary-layer turbulence and also seem to favour entrainment of free tropospheric air into the boundary layer.

Keyword
Arctic, Mesoscale
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-24081 (URN)10.1007/s10546-009-9354-x (DOI)000263418500005 ()
Note
Part of urn:nbn:se:su:diva-6585Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2010-01-18Bibliographically approved
5. Sensitivity of the dry stable boundary layer to external surface forcing
Open this publication in new window or tab >>Sensitivity of the dry stable boundary layer to external surface forcing
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-24082 (URN)
Note
Part of urn:nbn:se:su:diva-6585Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2010-01-13Bibliographically approved

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