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

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, DQ. 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 of DQ.

Large-Eddy Simulations of EUCLIPSE–GASS Lagrangian Stratocumulus-to-Cumulus Transitions: Mean State, Turbulence, and Decoupling

“…The LES results point to a reduction of the amount of subtropical marine low clouds in a warmer climate (Blossey et al 2013; Van der Dussen et al 2015;Bretherton 2015). The study by Zhang et al (2013), and follow-up studies by Dal Gesso et al (2014) andDal Gesso et al (2015) report a wide scatter in the change of the steady-state subtropical low-cloud amount in the SCM results.…”

“…For the ASTEX case, the large-scale divergence gradually decreases with time, and the observed weakening of the wind velocities is taken into account by a time-varying geostrophic forcing (Van der Dussen et al 2013). For the composite cases, the large-scale divergence and the geostrophic forcing are constant in time, where the geostrophic winds are the same as the initial profiles of the horizontal wind velocity components shown in Fig.…”

“…The setup of the fourth SCT case is described in detail by Van der Dussen et al (2013). This case is based on observations collected during the first ASTEX Lagrangian experiment Bretherton et al 1995;De Roode and Duynkerke 1997) and largescale forcing conditions as obtained from ERA-Interim.…”

“…The SCT has been the subject of several observational (e.g., Albrecht et al 1995;Bretherton et al 1995;De Roode and Duynkerke 1997;Sandu et al 2010) and modeling studies (e.g., Krueger et al 1995;Sandu and Stevens 2011;Van der Dussen et al 2013). Chung et al (2012) studied a series of steady-state LESs in the SCT regime, which can be interpreted as an Eulerian view of the transition.…”

“…This stratification prevents surface-driven thermals from reaching the stratocumulus cloud, except if they become saturated. In that case, latent heat release due to condensation of water allows the plumes to rise as positively buoyant cumulus clouds, which may penetrate the stratocumulus cloud layer to inject it with moisture from below (Wang and Lenschow 1995;Miller and Albrecht 1995;De Roode and Duynkerke 1996; Van der Dussen et al 2014). Meanwhile, the stratocumulus gradually thins if entrainment of relatively warm and dry freetropospheric air dominates the longwave radiative cooling at cloud top and the moisture supply from below.…”