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

Abstract

[1] 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.

Observational Boundary Layer Energy and Water Budgets of the Stratocumulus-to-Cumulus Transition

“…Our budget analysis provides a holistic view of the subtropical marine boundary layer in the climatological mean, which should be useful for validating and improving global models, large-eddy simulations (e.g., Chung et al 2012;van der Dussen et al 2013), parameterizations, intercomparisons such as the Cloud Feedback Model Intercomparison Project (CFMIP)/Global Energy and Water Cycle Experiment (GEWEX) Atmospheric System Studies (GASS) Intercomparison of LargeEddy and Single-Column Models (CGILS; Zhang et al 2013), and in particular attempts to unify the parameterizations of BL cloud regimes (e.g., Golaz et al 2002;Suselj et al 2013).…”

Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice: the Larcform 1 single column model intercomparison

“…This setup allows models to freely develop their own state and biases over several days while retaining the simplicity of a single-column setup and has successfully been used in other intercomparison studies [Bretherton et al, 1999;van der Dussen et al, 2013]. As our frame of reference follows the moving air mass, advective tendencies are set to zero even though an observer at a ground station (i.e., in a Eulerian framework) would report substantial advection of heat and moisture as the modeled column is advected into an area that was previously dominated by cooler and dryer air.…”

Influences of Subsidence and Free‐Tropospheric Conditions on the Nocturnal Growth of Nonclassical Marine Stratocumulus

“…Using Research Flight 5 (RF05) of the Physics of Stratocumulus Top (POST; e.g., Gerber et al, 2010Gerber et al, , 2013 field campaign as a starting point, we systematically change the conditions toward CTEI. Previously studied nonclassical cases include POST RF13 (Gerber et al, 2010;Kopec et al, 2016;Malinowski et al, 2013;Pedersen et al, 2018) and flights A209 and RF06 (e.g., Chlond & Wolkau, 2000;Duynkerke et al, 1999;van der Dussen et al, 2013) of the Atlantic Stratocumulus Transition Experiment (ASTEX; Albrecht et al, 1995). However, those studies were focused on different problems than those addressed here.…”