The GASS/EUCLIPSE model intercomparison of the stratocumulus transition as observed during ASTEX: LES results

Abstract

Large-eddy simulations of a Lagrangian transition from a vertically well-mixed stratocumulus-topped boundary layer to a situation in which shallow cumuli penetrate an overlying layer of thin and broken stratocumulus are compared with aircraft observations collected during the Atlantic Stratocumulus Transition Experiment. Despite the complexity of the case and the long simulation period of 40 h, the six participating state-of-the-art models skillfully and consistently represent the observed gradual deepening of the boundary layer, a negative buoyancy flux at the top of the subcloud layer and the development of a double-peaked vertical velocity variance profile. The moisture flux from the subcloud to the stratocumulus cloud layer by cumulus convection exhibits a distinct diurnal cycle. During the night the moisture flux at the stratocumulus cloud base exceeds the surface evaporation flux, causing a net drying of the subcloud layer, and vice versa during daytime. The spread in the liquid water path (LWP) among the models is rather large during the first 12 h. From additional sensitivity experiments it is demonstrated that this spread is mainly attributable to differences in the parameterized precipitation rate. The LWP differences are limited through a feedback mechanism in which enhanced drizzle fluxes result in lower entrainment rates and subsequently a reduced drying at cloud top. The spread is furthermore reduced during the day as cloud layers with a greater LWP absorb more solar radiation and hence evaporate more.

Turbulent Transport in the Gray Zone: A Large Eddy Model Intercomparison Study of the CONSTRAIN Cold Air Outbreak Case

“…To address the sensitivity of the cloud evolution to the prescribed N c value, some additional simulations were performed with values for N c in the range of observed values between 10 and 100 cm −3 . The resulting changes in the cloud effective radius and the cloud optical depth were computed from the mean volume radius of the droplets and the assumption of lognormal cloud droplet size distribution following Van der Dussen et al (). The contour plots for the LWP as obtained from DALES are shown in Figure .…”

“…Large eddy simulation (LES) models are well capable of capturing the observed SCT in the subtropics (De Roode et al, ; Van der Dussen et al, ). In addition, Honnert et al (), Dorrestijn et al (), and Shin and Hong () demonstrated that the partitioning of the subgrid and resolved turbulent transports as a function of horizontal grid spacing can be well diagnosed from turbulence resolving LES models.…”

A refined statistical cloud closure using double-Gaussian probability density functions

“…The setup of the LES study for the ASTEX case is similar to that proposed by the Euclipse ASTEX Lagrangian model 2 Rain in cumulus over the ocean 3 Atmospheric radiation measurement 4 Dynamics and chemistry of marine stratocumulus 5 Atlantic stratocumulus transition experiment 6 GEWEX (Global Energy and Water Experiment) Cloud system studies intercomparison case (van der Dussen et al, 2013). The initial profiles are identical to the first GCSS ASTEX "A209" modelling intercomparison case and the model is forced by time-varying sea surface temperature and divergence taken from Bretherton et al (1999).…”

“…The initial data and the large-scale forcing for the standard RICO simulations are based on the precipitating shallow cumulus case that was constructed by the GCSS boundary layer working group and described by van Zanten et al (2011). A modified moister version, which differs from the standard setup only by a moister initial profile, was first used by Stevens and Seifert (2008), to which we refer for a detailed setup of the case.…”

“…distribution which lead to the use of, for example, doubleGaussian (Lewellen and Yoh, 1993;Larson et al, 2001a), Gamma (Bougeault, 1982b) or Beta (Tompkins, 2002) distributions. Perraud et al (2011) tested several of this distributions against model data and found that the double-Gaussian distribution gives best results.…”