Turbulence Structure in Decoupled Marine Stratocumulus: A Case Study from the ASTEX Field Experiment

Abstract

The average and turbulence structure of two marine stratocumulus layers, from the Atlantic Stratocumulus Transition Experiment, are analyzed. These layers ware in adjacent air masses with different histories: one cloud layer was in a clean air mass with a marine history and the other was in a continental air mass, which had a higher aerosol content. The air masses were brought together by synoptic-scale flow and are separated by a semiclear transition zone. The clouds were decoupled from the marine surface mixed layer in both air masses. In the transition zone, the marine mixed layer was deeper than that under either cloud layer. The total depth below the main inversion, including the cloud layer was, however, substantially greater than in the semiclear transition zone. The western clean cloud layer was more well mixed than the eastern aerosol-rich cloud layer, and the turbulence analysis shows that the western cloud layer complies to convective scaling. Buoyancy production of turbulence was also positive in the eastern cloud, but here shear production was larger than the buoyancy production by a factor of 4, and convective scaling fails. One cause of the stability differences may lie in differences in radiative forcing, both external and internal. The difference in external forcing is due to higher humidity aloft to the east, reducing the net cloud-top longwave cooling. The difference in internal forcing is due to differences in the cloud microphysics. The larger number of smaller drops in the eastern cloud, arising from the abundance of aerosol particles, increases both albedo and absorption, and thus solar heating. Increased solar heating balances the longwave cooling at the cloud top, warms the cloud interior, and decreases the depth of the net-cooled layer in the cloud. All these effects decrease the buoyancy in the eastern cloud layer in comparison to the western one.