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Paths to improving atmospheric models across scales: The importance of the unresolved scales
Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
2018 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Correct representation of physical processes, the parametrizations, and their interaction with the resolved circulation is crucial for the performance of numerical models. Here, focus is put on understanding model biases and developing tools to alleviate existing biases. Atmospheric blocking can divert the typical atmospheric flow for several days up to weeks and thereby impacts the mean climate of the region experiencing blocking. Models typically underestimate the frequency of atmospheric blocking. Based on results from the global climate model EC-Earth, it is found that the atmospheric model resolution is not strongly influencing the representation of atmospheric blocking once the grid reaches about 80 km grid length in the horizontal. Updating several physical parametrizations, and thereby the model version, is the largest contributor to advancements in simulating atmospheric blocking. The importance of the topography for the large-scale atmospheric flow is further investigated with the reanalysis ERA-Interim by applying a simplified theoretical analysis. It is found that the idealized topographic forcing theory can explain some part of the observed large-scale properties of the flow, though the method does mainly produce relative results. The explained part of the large-scale structure is increased during periods of northwesterly flow and when the flow impinges the mountain ridge almost orthogonally.

Small-scale processes acting in air masses transported from midlatitudes to the Arctic are also discussed. Numerical models often struggle with representing the stable conditions in the Arctic and tend to underestimate the downward longwave impact during cloudy conditions. A comparison of single-column models (SCMs) indicates that most models can capture the bimodal longwave distribution which develops from alternating cloudy and clear-sky conditions. SCMs are often used for model development as they allow to decouple the parametrized physical processes from the large-scale environment and enable many parameter sensitivity tests. A new tool is presented which can be used for the development of physical parametrizations in marine and polar conditions. It combines one-dimensional models of the atmosphere and ocean, including sea-ice, into a coupled atmosphere-ocean SCM (AOSCM). The presented setup constitutes an advantage compared to SCMs of one component because the coupling is directly modelled and the interaction between the respective boundary layers does not dependent on prescribed boundary conditions.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Meteorology, Stockholm University , 2018. , s. 34
Nyckelord [en]
global climate models, atmospheric blocking, topographic forcing, physical parametrizations, single-column model, atmosphere-ocean single-column model, air-sea interaction
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
URN: urn:nbn:se:su:diva-155716ISBN: 978-91-7797-306-5 (tryckt)ISBN: 978-91-7797-307-2 (digital)OAI: oai:DiVA.org:su-155716DiVA, id: diva2:1201866
Disputation
2018-06-15, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Forskningsfinansiär
Swedish e‐Science Research Center
Anmärkning

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Manuscript.

Tillgänglig från: 2018-05-23 Skapad: 2018-04-26 Senast uppdaterad: 2018-05-21Bibliografiskt granskad
Delarbeten
1. Resolution, physics and atmosphere–ocean interaction – How do they influence climate model representation of Euro-Atlantic atmospheric blocking?
Öppna denna publikation i ny flik eller fönster >>Resolution, physics and atmosphere–ocean interaction – How do they influence climate model representation of Euro-Atlantic atmospheric blocking?
2017 (Engelska)Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 69, nr 1, artikel-id 1406252Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Atmospheric blocking events are known to locally explain a large part of climate variability. However, despite their relevance, many current climate models still struggle to represent the observed blocking statistics. In this study, simulations of the global climate model EC-Earth are analysed with respect to atmospheric blocking. Seventeen simulations map the uncertainty space defined by the three-model characteristics: atmospheric resolution, physical parameterization and complexity of atmosphere–ocean interaction, namely an atmosphere coupled to an ocean model or forced by surface data. Representation of the real-world statistics is obtained from reanalyses ERA-20C, JRA-55 and ERA-Interim which agree on Northern Hemisphere blocking characteristics. Blocking events are detected on a central blocking latitude which is individually determined for each simulation. The frequency of blocking events tends to be underestimated relative to ERA-Interim over the Atlantic and western Eurasia in winter and overestimated during spring months. However, only few model setups show statistically significant differences compared to ERA-Interim which can be explained by the large inter-annual variability of blocking. Results indicate slightly larger biases relative to ERA-Interim in coupled than in atmosphere-only models but differences between the two are not statistically significant. Although some resolution dependence is present in spring, the signal is weak and only statistically significant if the physical parameterizations of the model are improved simultaneously. Winter blocking is relatively more sensitive to physical parameterizations, and this signal is robust in both atmosphere-only and coupled simulations, although stronger in the latter. Overall, the model can capture blocking frequency well despite biases in representing the mean state of geopotential height over this area. Blocking signatures of geopotential height are represented more similar to ERA-Interim and only weak sensitivities to model characteristics remain.

Nyckelord
atmospheric blocking, EC-Earth, model bias, physical parameterizations, atmospheric blocking composites, parameter sensitivity study
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-150462 (URN)10.1080/16000870.2017.1406252 (DOI)000422996500001 ()
Tillgänglig från: 2017-12-19 Skapad: 2017-12-19 Senast uppdaterad: 2018-04-26Bibliografiskt granskad
2. Diagnosing topographic forcing in an atmospheric dataset: the case of the North American Cordillera
Öppna denna publikation i ny flik eller fönster >>Diagnosing topographic forcing in an atmospheric dataset: the case of the North American Cordillera
Visa övriga...
(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-155664 (URN)
Tillgänglig från: 2018-04-25 Skapad: 2018-04-25 Senast uppdaterad: 2018-04-26Bibliografiskt granskad
3. Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice: the Larcform 1 single column model intercomparison
Öppna denna publikation i ny flik eller fönster >>Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice: the Larcform 1 single column model intercomparison
Visa övriga...
2016 (Engelska)Ingår i: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 8, nr 3, s. 1345-1357Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Weather and climate models struggle to represent lower tropospheric temperature and moisture profiles and surface fluxes in Arctic winter, partly because they lack or misrepresent physical processes that are specific to high latitudes. Observations have revealed two preferred states of the Arctic winter boundary layer. In the cloudy state, cloud liquid water limits surface radiative cooling, and temperature inversions are weak and elevated. In the radiatively clear state, strong surface radiative cooling leads to the build-up of surface-based temperature inversions. Many large-scale models lack the cloudy state, and some substantially underestimate inversion strength in the clear state. Here, the transformation from a moist to a cold dry air mass is modeled using an idealized Lagrangian perspective. The trajectory includes both boundary layer states, and the single-column experiment is the first Lagrangian Arctic air formation experiment (Larcform 1) organized within GEWEX GASS (Global atmospheric system studies). The intercomparison reproduces the typical biases of large-scale models: some models lack the cloudy state of the boundary layer due to the representation of mixed-phase microphysics or to the interaction between micro- and macrophysics. In some models, high emissivities of ice clouds or the lack of an insulating snow layer prevent the build-up of surface-based inversions in the radiatively clear state. Models substantially disagree on the amount of cloud liquid water in the cloudy state and on turbulent heat fluxes under clear skies. Observations of air mass transformations including both boundary layer states would allow for a tighter constraint of model behavior.

Nyckelord
Arctic, boundary-layer, mixed-phase clouds, models, intercomparison, inversion
Nationell ämneskategori
Geovetenskap och miljövetenskap
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-137540 (URN)10.1002/2016MS000630 (DOI)000387793500017 ()
Tillgänglig från: 2017-01-13 Skapad: 2017-01-09 Senast uppdaterad: 2018-04-26Bibliografiskt granskad
4. An EC-Earth coupled atmosphere-ocean single-column model (AOSCM) for studying coupled marine and polar processes
Öppna denna publikation i ny flik eller fönster >>An EC-Earth coupled atmosphere-ocean single-column model (AOSCM) for studying coupled marine and polar processes
Visa övriga...
(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Nationell ämneskategori
Meteorologi och atmosfärforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-155543 (URN)
Forskningsfinansiär
EU, Horisont 2020, 727862
Tillgänglig från: 2018-04-24 Skapad: 2018-04-24 Senast uppdaterad: 2019-01-07Bibliografiskt granskad

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