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Use of Pseudoadiabatic Adjustment of Turbulence for a Simplified Nighttime Stratocumulus Case: A One-Dimensional Study

Robert SiggDepartment of Meteorology, Stockholm University, Stockholm, Sweden

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Abstract

A turbulence–condensation parameterization scheme for a stratocumulus-topped planetary boundary layer that may be applied in mesoscale numerical weather prediction models is presented in this paper. Due to the condensation closure, conservative variables such as liquid water potential temperature and total water mixing ratio are not suitable to use and an approach referred to as pseudoadiabatic adjustment of turbulent fluxes (within the cloud) is suggested as a replacement.

This approach is investigated and compared with large eddy simulation (LES) results for a simplified stratocumulus case driven mainly by cloud-top radiative cooling. It is shown that the results for two vertical resolutions, 25 m and 100 m, are similar to LES data and both a maximum buoyancy flux at the cloud top and a quasi-linear total water flux are produced. The discrepancies between the runs with 100- and 25-m resolution are mainly related to the entrainment process, which is much weaker in the former run. A diurnal cycle is also performed and it is concluded that the presented parameterization can simulate the cloud characteristics during the day including the formation of a stable layer beneath the cloud.

Corresponding author address: Dr. Robert Sigg, Department of Meteorology, Stockholm University, S-10691 Stockholm, Sweden.

Email: robert.sigg@misu.su.se

Abstract

A turbulence–condensation parameterization scheme for a stratocumulus-topped planetary boundary layer that may be applied in mesoscale numerical weather prediction models is presented in this paper. Due to the condensation closure, conservative variables such as liquid water potential temperature and total water mixing ratio are not suitable to use and an approach referred to as pseudoadiabatic adjustment of turbulent fluxes (within the cloud) is suggested as a replacement.

This approach is investigated and compared with large eddy simulation (LES) results for a simplified stratocumulus case driven mainly by cloud-top radiative cooling. It is shown that the results for two vertical resolutions, 25 m and 100 m, are similar to LES data and both a maximum buoyancy flux at the cloud top and a quasi-linear total water flux are produced. The discrepancies between the runs with 100- and 25-m resolution are mainly related to the entrainment process, which is much weaker in the former run. A diurnal cycle is also performed and it is concluded that the presented parameterization can simulate the cloud characteristics during the day including the formation of a stable layer beneath the cloud.

Corresponding author address: Dr. Robert Sigg, Department of Meteorology, Stockholm University, S-10691 Stockholm, Sweden.

Email: robert.sigg@misu.su.se

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