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.
Highlights
[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
The results suggested that the entrainment rate in the large-eddy simulation (LES) models was on average about 50% greater than the values derived from the aircraft observations
[24] The three snapshots of the cloud fields as obtained from the Dutch Atmospheric LES (DALES) model presented in Figure 3 show the clear evolution during the stratocumulus transition
Summary
[2] Large portions of the Earth’s oceans are covered by fields of stratocumulus clouds [Wood, 2012]. [5] During the first Lagrangian of the Atlantic Stratocumulus Transition Experiment (ASTEX), aircraft observations of a transition from a solid stratocumulus-topped boundary layer to a boundary layer dominated by shallow cumulus clouds penetrating a thin veil of broken stratocumulus were collected [Albrecht et al, 1995; Bretherton and Pincus, 1995; Bretherton et al, 1995; De Roode and Duynkerke, 1997] These observations were used by Bretherton et al [1999] to set up a model intercomparison case in order to study the representation of the entire transition with single-column model (SCM) versions of numerical weather prediction and climate models as well as with two-dimensional turbulence models. The cloud droplet number density Nc is assumed to be constant at 100 cm
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