VERTICAL MASS FLUX CALCULATIONS IN HAWAIIAN TRADE CUMULUS CLOUDS FROMDUAL-DOPPLER RADAR

Two ground-based Doppler radars and an instrumented aircraft provided a means for computing the vertical mass flux in trade wind cumulus clo uds that formed east of the island of Hawaii during the Hawaiian Rainb and Project of 1990. This study compares the mass fluxes of small isol ated cells with larger groups of clouds and rainbands. Because of exce llent sensitivity, the 5.5-cm wavelength radars were capable of detect ing Bragg backscatter, which extended the measurements to include some precipitation-free air within and surrounding the clouds. The shape o f the vertical profile of vertical mass flux within shallow cumulus cl ouds and cloud groups varied considerably over the cloud's life cycle but was comparatively independent of cloud size. The early stages of c onvection displayed a mass flux profile that resembled those produced by buoyancy sorting and entraining plume models, but the mature and la ter stages were considerably more affected by precipitation-driven dow ndrafts and included mean downward mass fluxes. The vertical mass Bur profiles predicted by a three-dimensional LES model of an isolated cum ulus cloud showed the same evolutionary phases as the observations.