An LES model study of the influence of the free tropospheric thermodynamic conditions on the stratocumulus response to a climate perturbation

Journal of Advances in Modeling Earth Systems

J. J. van der Dussen²  S. R. de Roode²  S. Dal Gesso¹ 

[1] Royal Netherlands Meteorological Institute, De Bilt, Netherlands;Department of Geoscience and Remote Sensing, Delft University of Technology, Delft, Netherlands

关键词: stratocumulus;    steady states;    free tropospheric humidity;    climate response;    lower tropospheric stability;    large‐eddy simulation;   

DOI  :  10.1002/2014MS000380

来源: Wiley

【 摘 要 】

Abstract

Twenty-five large-eddy simulations are performed to study how free tropospheric thermodynamic conditions control equilibrium state solutions of stratocumulus-topped marine boundary layers. In particular, we systematically vary the lower tropospheric stability (LTS) and a similar measure for the bulk humidity difference between the 700 hPa level and the surface,ΔQ. For all simulations, a completely overcast boundary layer is obtained in which the turbulence is mainly driven by cloud top radiative cooling. The steady state liquid water path (LWP) is rather insensitive to the LTS, but increases significantly and almost linearly with the free tropospheric humidity. In a second suite of runs, the response of the stratocumulus layer to an idealized global warming scenario is assessed by applying a uniform warming of 2 K to the initial temperature profile including the sea surface while the initial relative humidity profile is kept identical to the control case. The warming of the sea surface acts to increase the latent heat flux, which invigorates turbulence in the boundary layer. The steady state inversion height therefore increases, despite the competing effect of a more humid free troposphere that increases the downwelling radiative flux and hence tends to decrease the entrainment rate. The stratocumulus layer nevertheless thins for all free tropospheric conditions as cloud base rises more than cloud top. This implies a positive stratocumulus cloud-climate feedback for this scenario as thinner clouds reflect less shortwave radiation back to space. The cloud thinning response to the climate perturbation is found to be mostly controlled by the change ofΔQ.

【 授权许可】

CC BY-NC-ND   
© 2015. The Authors.

Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.