TURBULENCE STRUCTURE IN DECOUPLED MARINE STRATOCUMULUS - A CASE-STUDYFROM THE ASTEX FIELD EXPERIMENT

The average and turbulence structure of two marine stratocumulus layer s, from the Atlantic Stratocumulus Transition Experiment, are analyzed . These layers were in adjacent air masses with different histories: o ne cloud layer was in a clean air mass with a marine history and the o ther was in a continental air mass, which had a higher aerosol content . The air masses were brought together by synoptic-scale Row and are s eparated by a semiclear transition zone. The clouds were decoupled fro m 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 l ayer was, however, substantially greater than in the semiclear transit ion zone. The western clean cloud layer was more well mixed than the e astern aerosol-rich cloud layer, and the turbulence analysis shows tha t the western cloud layer complies to convective scaling. Buoyancy pro duction 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 differe nces may lie in differences in radiative forcing, both external and in ternal. The difference in external forcing is due to higher humidity a loft to the east, reducing the net cloud-top longwave cooling. The dif ference in internal forcing is due to differences in the cloud microph ysics. The larger number of smaller drops in the eastern cloud, arisin g from the abundance of aerosol particles, increases both albedo and a bsorption, and thus solar heating. Increased solar heating balances th e longwave cooling at the cloud top, warms the cloud interior, and dec reases the depth of the net-cooled layer in the cloud. All these effec ts decrease the buoyancy in the eastern cloud layer in comparison to t he western one.