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Evaluation of near surface parameters in the two versions of the atmospheric model in cesm1 using flux station observations
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
2013 (English)In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 26, no 1, 26-44 p.Article in journal (Refereed) Published
Abstract [en]

This paper describes the performance of the Community Atmosphere Model (CAM) versions 4 and 5 in simulating near-surface parameters. CAM is the atmospheric component of the Community Earth System Model (CESM). Most of the parameterizations in the two versions are substantially different, and that is also true for the boundary layer scheme: CAM4 employs a nonlocal K-profile scheme, whereas CAM5 uses a turbulent kinetic energy (TKE) scheme. The evaluation focuses on the diurnal cycle and global observational and reanalysis datasets are used together with multiyear observations from 35 flux tower sites, providing high-frequency measurements in a range of different climate zones. It is found that both model versions capture the timing of the diurnal cycle but considerably overestimate the diurnal amplitude of net radiation, temperature, wind, and turbulent heat fluxes. The seasonal temperature range at mid-and high latitudes is also overestimated with too warm summer temperatures and too cold winter temperatures. The diagnosed boundary layer is deeper in CAM5 over ocean in regions with low-level marine clouds as a result of the turbulence generated by cloud-top cooling. Elsewhere, the boundary layer is in general shallower in CAM5. The two model versions differ substantially in their representation of near-surface wind speeds over land. The low-level wind speed in CAM5 is about half as strong as in CAM4, and the difference is even larger in areas where the subgrid-scale terrain is significant. The reason is the turbulent mountain stress parameterization, only applied in CAM5, which acts to increase the surface stress and thereby reduce the wind speed.

Place, publisher, year, edition, pages
2013. Vol. 26, no 1, 26-44 p.
National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:su:diva-88313DOI: 10.1175/JCLI-D-12-00020.1ISI: 000313712100002OAI: oai:DiVA.org:su-88313DiVA: diva2:611859
Note

AuthorCount:3;

Available from: 2013-03-19 Created: 2013-03-12 Last updated: 2017-12-06Bibliographically approved
In thesis
1. The Representation of Atmospheric Boundary Layer Processes in Global Climate Models
Open this publication in new window or tab >>The Representation of Atmospheric Boundary Layer Processes in Global Climate Models
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The atmospheric boundary layer is the lowest part of the atmosphere, which is in direct contact with the surface. It is here, in this turbulent layer, that the exchange of heat, moisture and momentum between the surface and the atmosphere takes place. This thesis examines how well the boundary layer is described in global climate models with particular focus on the representation of the diurnal cycle. Two versions of the Community Atmosphere Model (CAM) that employ different turbulence parameterizations are evaluated in the same model framework. It is found that both overestimate the amplitude of the diurnal cycles of near-surface variables compared to observations from flux tower sites. The 10-m wind is much lower in CAM5 than in CAM4 due to the Turbulent Mountain Stress (TMS) parameterization of subgrid orography in CAM5. Additionally, the diurnal temperature range (DTR) is studied in a large set of models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The model discrepancies are large both in simulations of the present day and in projections of the future. A correlative approach is used to assess which parameters are important for the model differences in DTR. No single parameter is found to be responsible, but clouds play an important role in all seasons and so do the evaporative fraction in summer. The diurnal cycles of these CMIP5 models are also evaluated against flux tower observations. The diurnal cycle of temperature is well captured, while most variables show a large inter-model spread. A subset of the models are analyzed deeper regarding their vertical boundary layer structure for a flux site in Oklahoma. A substantial warm summer bias is revealed in the models. Finally, the impact of TMS and changes in the vertical diffusion in CAM5 is studied. It is found that, although the inclusion of TMS leads to an improved large-scale circulation, the wind turning becomes too strong, which adds to the overestimation of the ageostrophic flow in the boundary layer. Instead, increasing the diffusivity in stable conditions tends to both degrade the large-scale circulation and cause an underestimated wind turning in the boundary layer.

Place, publisher, year, edition, pages
Department of Meteorology, Stockholm University, 2014. 45 p.
Keyword
The atmospheric boundary layer, Global climate models, Diurnal cycle, Flux towers, Diurnal temperature range
National Category
Natural Sciences Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-103053 (URN)978-91-7447-918-8 (ISBN)
Public defence
2014-06-05, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2014-05-14 Created: 2014-04-29 Last updated: 2014-05-05Bibliographically approved

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