Numerical Simulations and a Conceptual Model of the Stratocumulus to Trade Cumulus Transition

A two-dimensional eddy-resolving model is used to study the transition from the stratocumulus topped boundary layer to the trade cumulus boundary layer. The 10-day simulations use an idealized Lagrangian trajectory representative of summertime climatological conditions in the subtropical northeastern Pacific. The sea surface temperature is increased steadily at 1.5 K day−1, reflecting the southwestward advection of the subtropical marine boundary layer by the trade winds, while the free tropospheric temperature remains unchanged. Results from simulations with both a fixed diurnally averaged shortwave radiative forcing and a diurnally varying shortwave forcing are presented. A two-stage model for the boundary layer evolution consistent with these simulations is proposed. In the first stage, decoupling is induced by increased latent heat fluxes in the deepening boundary layer. After decoupling, cloud cover remains high, but the cloudiness regime changes from a single stratocumulus layer to sporadic cumulus that detrain into stratocumulus clouds. In the second stage, farther SST increase causes the cumuli to become deeper and more vigorous, penetrating farther into the inversion and entraining more and more dry above-inversion air. This evaporates liquid water in cumulus updrafts before they detrain, causing the eventual dissipation of the overlying stratocumulus. Diurnal variations of insolation lead to a large daytime reduction in stratocumulus cloud amount, but they have little impact on the systematic evolution of boundary layer structure and cloud. The simulated cloudiness changes are not consistent with existing criteria for cloud-top entrainment instability.