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

Twenty-five large-eddy simulations are performed to study how free tropospheric thermody-
namic 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,
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. For all simulations, a completely over-
cast 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 condi-
tions 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 of
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