Influence of Shallow Cumuli on Subcloud Turbulence Fluxes Analyzed from Aircraft Data

Abstract

The purpose of this study is to improve understanding of shallow cumuli in the planetary boundary layer (PBL) by quantitatively analysing subcloud turbulence variables. Aircraft turbulence data for three flights from the 1986 Hydrologic?Atmospheric Pilot Experiment project over southwest France is extensively analyzed in terms of both cumulus regime and eddy dimensions. The turbulent part of any variable has been divided into three spatial frequency bands to estimate the scale-dependent effects of cumuli on the total turbulent fluxes and energy at several levels within the subcloud layer. Case 1 (21 May) had strong active (positively buoyant) cumuli that were deep and large in diameter; additionally, there were weak active cumuli that were shallow and small in diameter. Case 2 (9 May) had weak active cumuli with a smaller vertical depth, and case 3 (13 June) had only forced (negatively buoyant) cumuli. The National Center of Atmospheric Research King Air aircraft made gust probe measurements along several horizontal flight legs at different times and altitudes within the subcloud layer, and a few vertical sounding legs provided continuous vertical profiles within the entire boundary layer. The results show that water vapor fluxes, buoyancy fluxes, and vertical motion near cloud base were enhanced by active cumuli influence on larger turbulence scales. In contrast, results for weak active and forced cumulus cases show subcloud turbulence characteristics very similar to those observed in a dry convective boundary layer. By enhancing the larger-scale turbulence and fluxes near cloud base, the active cumuli contribute to cooling and drying of the upper portion of the subcloud layer. These large cumuli were associated with more vigorous convective plumes of medium scale but otherwise did not seem to influence the average surface turbulence of smaller scales.