Abstract
This study presents a series of steady-state large-eddy simulations (LESs) to study the stratocumulus to shallow cumulus cloud transition. To represent the different stages of what can be interpreted as an Eulerian view of the transition, each simulation is assigned a unique sea surface temperature (SST) and run until statistically steady. The LES runs are identical in every other aspect. These idealized boundary-driven steady-state LESs allow for a simple parametric assessment of cloud-controlling factors in isolation from initial conditions and time-lag effects inherent in the Lagrangian view of the transition. The analysis of the thermodynamic energy budget reveals that, as the cloud regime transitions from stratocumulus to shallow cumulus, changes in the cloud radiative cooling term are balanced by changes in the subsidence warming term. This leads to a linear regression between the cloud fraction (CF) and an integral that scales, to a first-order approximation, as the lower-tropospheric stability (LTS). The study also considers the response of the boundary layer to a step change in SST that triggers the transition from stratocumulus to shallow cumulus. An examination of the time-lag conditional average centered on events when cumulus thermals are penetrating the stratocumulus deck suggests that the net effect of cumulus thermals in the transition is not to dry the stratocumulus deck but rather to moisten it. It is shown that the Gaussian probability density function (pdf) model of Sommeria and Deardorff describes the evolution of CF well during this step-change transition, suggesting that the systematic decrease in cloud cover is essentially associated with the mean drying of the air just below the cloud top.
