AN OBSERVATIONAL STUDY OF THE ROLE OF PENETRATING CUMULUS IN A MARINESTRATOCUMULUS-TOPPED BOUNDARY-LAYER

Isolated cumuli penetrating through marine stratocumulus clouds were d ocumented during the Atlantic Stratocumulus Transition Experiment. Thi s paper aims at understanding the role of the penetrating cumulus in r egulating stratocumulus and boundary-layer structure through analysis of data from the NCAR Electra aircraft. When penetrating cumulus cloud s are present, the boundary layer is generally decoupled from the near -surface air except in the cumulus region. Therefore, air in the cumul us region includes air entrained at the cloud top, as well as air modi fied by surface processes. In the stratocumulus region, however, entra ined inversion air and moist surface air are confined to separate laye rs. As a result, large horizontal variations are found in scalars, suc h as ozone and water vapor. Turbulence statistics and conditional samp ling of entrainment events in the cumulus and stratocumulus regions in dicate that stronger entrainment may occur at the cumulus top compared to the surrounding stratocumulus. This analysis is, however, complica ted by insufficient sampling of cloud-top jump conditions in both regi ons. Convergent flow in the lower boundary layer and compensating dive rging how in the upper boundary layer were identified along the flight track. This flow field, together with the vertical coupling of surfac e air with the cloud layer in the cumulus region, helps to transport m oisture upwards from the sea surface and disperse it to the surroundin g stratocumulus sheet, thus helping to maintain the stratocumulus cove r.