NUMERICAL SIMULATIONS AND A CONCEPTUAL-MODEL OF THE STRATOCUMULUS TO TRADE CUMULUS TRANSITION

A two-dimensional eddy-reserving model is used to study the transition from the stratocumulus topped boundary layer to the trade cumulus bou ndary layer. The 10-day simulations use an idealized Lagrangian trajec tory representative of summertime climatological conditions in the sub tropical northeastern Pacific. The sea surface temperature is increase d steadily at 1.5 K day(-1), reflecting the southwestward advection of the subtropical marine boundary layer by the trade winds, while the f ree tropospheric temperature remains unchanged. Results from simulatio ns with both a fixed diurnally averaged shortwave radiative forcing an d 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 late nt heat fluxes in the deepening boundary layer. After decoupling, clou d cover remains high, but the cloudiness regime changes from a single stratocumulus layer to sporadic cumulus that detrain into stratocumulu s clouds. In the second stage, farther SST increase causes the cumuli to become deeper and more vigorous, penetrating farther into the inver sion and entraining more and more dry above-inversion air. This evapor ates liquid water in cumulus updrafts before they detrain, causing the eventual dissipation of the overlying stratocumulus. Diurnal variatio ns of insolation lead to a large daytime reduction in stratocumulus cl oud 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 in stability.