Effect of Sea Surface Temperature and its Rate to Change on Boundary Layer Structure

A wide variety of cloud regimes was observed during ASTEX. What determines the cloud regimes in the subtropical marine boundary layer? As a column of boundary layer air moves equatorward with the low-level flow, it moves over progressively warmer sea surfaces. Does the boundary layer remain in quasi-equilibrium with the changing sea surface temperature (SST)? We are performing a series of numerical simulations using a two dimensional (2-D) cloud ensemble model (CEM) to investigate this. In the first set of simulations, we find the equilibrium structure of the boundary layer as a function of SST for prescribed surface wind speed, subsidence profile and upper tropospheric thermodynamic structure. In subsequent sets we find the boundary layer structure as a function of SST as it is subjected to a changing SST for various rates of SST change. These rates of SST change correspond to different trajectories of the boundary layer column.

The CEM explicitly calculates cumulus scale motions and includes third moment turbulence closure, a turbulent scale condensation scheme, and an advanced radiation code. We have used the CEM to simulate the trade wind cumulus boundary layer observed during BOMEX (Barbados Oceanographic and Meteorological Experiment). The agreement between the simulated and observed profiles of the apparent heat source and the apparent moisture sink is quite good. We also found that the sensitivity of the simulated convective (cloud-scale plus turbulent) fluxes, the kinetic energy, kinetic energy dissipation rate, and cloud amount to turbulent length scale changes is low. These results suggest that the CEM is well-suited for simulating the various cloud regimes observed in the subtropical marine boundary layer.