DIURNAL CLOUD AND THERMODYNAMIC VARIATIONS IN THE STRATOCUMULUS TRANSITION REGIME - A CASE-STUDY USING IN-SITU AND REMOTE SENSORS

Radiosonde, in situ, and surface-based remote sensor data from the Atl antic Stratocumulus Transition Experiment are used to study the diurna l cycle of cloud and thermodynamic structure. A cloud layer and decoup led subcloud layer separated by a stable transition layer, often obser ved in the vicinity of cumulus cloud base, characterizes the thermodyn amic structure during the study period. The mode of cloud structure is cumulus with bases below decoupled stratus. Data are presented that s upport the hypothesis that diurnal variations in cumulus development a re modulated by the stability in the transition layer. The frequency o f cumulus convection decreases during the afternoon, but mesoscale reg ions of vigorous cumulus with cloud tops overshooting the base of the trade inversion and increased surface drizzle rates are present during the late afternoon and early evening, when the transition layer is th e most stable. It is postulated that mesoscale organization may be req uired to accumulate enough water vapor in the subcloud layer to produc e the convective available potential energy needed for developing cumu lus to overcome transition layer stability. The mesoscale regions appe ar to fit the description of cyclic cumulus convection proposed in a p revious study, and this theory is expanded to account for diurnal vari ations in the stability of the transition layer. The occurrence of the se mesoscale clusters of vigorous convection makes it difficult to det ermine if the latent heat flux in the cloud layer has actually decreas ed in the late afternoon and early evening, when the transition layer is the most stable.Liquid water structure was examined and no pronounc ed diurnal signal was found. Results showed that clouds thicker than a pproximately 450 m tended to have subadiabatic integrated liquid water contents, presumably due to evaporation of drizzle in the subcloud la yer, removal of liquid water at the surface, and the evaporation of cl oud water at cloud top. A significant fraction of clouds less than 450 m thick produced liquid water contents that were greater than adiabat ic, and there may be a physical mechanism that could produce such valu es in this cloud system (i.e., lateral detrainment of cloud water from convective elements mixing with existing liquid water in decoupled st ratus or with liquid water detrained by nearby convective elements). U nfortunately, instrument limitations may have also produced these grea ter-than-adiabatic values and the extent of instrument artifacts in th ese results is unclear.