Observations and numerical simulations of the diurnal cycle of the EUROCS stratocumulus case

Abstract

As part of the European Project on Cloud Systems in Climate Models, the diurnal cycle of stratocumulus has been simulated with Large-Eddy Simulation (LES) models and Single Column Models (SCMs). The models were initialized and compared with observations collected in marine stratocumulus in July 1987 during the First International Satellite Cloud Climatology Project Regional Experiment. The results of the six LES models are found to be in a fair agreement with the observations. They all capture the distinct diurnal variation in the cloud liquid-water path, the turbulence profiles and clearly show a decoupled boundary layer during daytime and a vertically well-mixed boundary layer during the night. Entrainment of relatively dry and warm air from just above the inversion into the boundary layer is the major process modifying the thermodynamic structure of the boundary layer during the night. The differences that arise in the liquid-water path evolution can therefore be attributed mainly to differences in the entrainment rate. The mean entrainment rates computed from the LES model results are 0.58 +/- 0.08 cm s(-1) and 0.36 +/- 0.03 cm s(-1) for the night-time and daytime periods, respectively. If the horizontal domain size in a LES model is enlarged, mesoscale fluctuations develop. This leads to a broader liquid-water path distribution and a reduction of the cloud albedo. To assess the quality of the representation of stratocumulus in general-circulation models, results from ten SCMs are compared with observations and LES results. The SCM latent and sensible heat fluxes at the surface agree fairly well with the LES results. Many of the SCMs predict a liquid-water path which is much too low, a cloud cover smaller than unity, and cloud tops that are lower than the observations and the LES results. This results in a much larger amount of downwelling short-wave radiation absorbed at the sea surface. Improvement of entrainment parametrizations is needed for a better representation of stratocumulus in SCMs. Observations and LES results of entrainment rates for different stratocumulus cases are compared. The observed entrainment rates in Atlantic stratocumulus clouds during the Atlantic Stratocumulus Transition Experiment (ASTEX) are larger than for the ones over the Pacific Ocean off the coast of California. Results from LES models corroborate these findings. The differences in the entrainment rate can likely be attributed to the smaller inversion jumps of the liquid-water potential temperature for the ASTEX stratocumulus case

A continuous spectral aerosol-droplet microphysics model

“…The model is initialized with the liquid water potential temperature (θ l ) and total water mixing ratio (q tot ) shown in Fig. 3 based on the First International Satellite CLoud Climatology Project Regional Experiment (FIRE I) from Duynkerke et al (2004) and as incorporated in Chen et al (2011). Soundings from FIRE I have been used in numerous modeling studies of marine stratocumulus (e.g., Hill et al, 2008Hill et al, , 2009Chen et al, 2011).…”

“…These models have been developed for applications in both warm and mixed-phase clouds (e.g., Tzivion et al, 1987Tzivion et al, , 1989Feingold et al, 1988;Reisin et al, 1996;Stevens et al, 1996;Geresdi, 1998;Geresdi and Rasmussen, 2005;Rasmussen et al, 1987;Khain and Lynn, 2009;Fan et al, 2009;Xue et al, 2010;Lebo and Seinfeld, 2011). Instead of tracking a single number concentration and mass for each type of cloud particle, the bin microphysics schemes discretize the distributions into size bins that span the smallest cloud droplets to large rain/graupel/snow/hail particles.…”

Formulation of the Dutch Atmospheric Large-Eddy Simulation (DALES) and overview of its applications

“…The GABLS focuses on the clear boundary layer, mainly on stable and transitional situations (Holtslag, 2006;Beare et al, 2006;Basu et al, 2008). The GCSS looks at different aspects of boundary layer clouds, mainly shallow cumulus and stratocumulus clouds (Bretherton et al, 1999a,b;Duynkerke et al, 1999Duynkerke et al, , 2004Brown et al, 2002;Siebesma et al, 2003;Stevens et al, 2001Stevens et al, , 2005Ackerman et al, 2009;van Zanten et al, 2010). Other useful intercomparison studies on the clear convective boundary layer were performed by Nieuwstadt and de Valk (1987) and by Fedorovich et al (2004b).…”

Evaluating the diurnal cycle in cloud top temperature from SEVIRI

“…Over large areas of the ocean there is a small diurnal cycle in CTT, caused by stratiform cloud (Yang and Slingo, 2001;Wood, 2002;Duynkerke et al, 2004). During the night, longwave radiative cooling at the cloud top drives turbulent mixing, creating a deeper cloud layer.…”

“…During the night, longwave radiative cooling at the cloud top drives turbulent mixing, creating a deeper cloud layer. In the daytime, solar heating of the cloud top causes the cloud layer to become stably stratified, cutting off the transport of heat and moisture from the surface, creating a thinner cloud layer (Duynkerke et al, 2004).…”