DISTRIBUTIONS OF SURFACE-LAYER BUOYANCY VERSUS LIFTING CONDENSATION LEVEL OVER A HETEROGENEOUS LAND-SURFACE

Onset and coverage of small cumulus clouds depend on the relative abun dance of surface-layer air parcels possessing favorable buoyancy and m oisture-two variables that are coupled through the surface energy budg et, This abundance is described using a joint frequency distribution ( JFD) as a function of virtual potential temperature theta(nu) and heig ht of the lifting condensation level z(LCL) It is shown analytically t hat the shape and spread of this JFD depends on the ranges of Bowen ra tios and solar forcings (albedoes, cloud shading, etc.) that exist wit hin a domain of heterogeneous land use. To sample the character of suc h JFDs in the red atmosphere, a case study is presented using turbulen ce data gathered by aircraft flying in the surface layer of southwest France. This case study includes 4 days of clear skies during the Hydr ologic Atmospheric Pilot Experiment (HAPEX) of 1986. The full flight t rack during HAPEX overflew a wide range of land use including evergree n forest, corn, vineyards, pastures, and irrigated fields over varied topography. The JFDs from these full tracks are found to be quite comp lex, being frequently multimodal with a convoluted perimeter. However, when a full track is broken into segments, each over a subdomain of q uasi-homogeneous land use, the resulting segment JFDs are mono-modal w ith simpler topology. Such a characterization of JFDs provides guidanc e toward eventual subgrid cumulus parameterization in large-scale fore cast models, with associated impacts in aviation forecasting, pollutan t venting and chemical reactions, vertical dispersion and turbulence m odulation, and radiation balance in climate-change models.