A Composite Mesoscale Cumulonimbus Budget

Abstract

Composite maps at levels from 950 to 150 mb of relative wind field (Vτ), mixing ratio (r), equivalent potential temperature (?E), and temperature perturbation (T?) from the growth and decay phases of a mean mesoscale cumulonimbus system (systems used had a maximum radar echo area >400 km2) were constructed using radar and one rawinsonde (experiment VIMHEX) for days having a similar synoptic-scale wind field. Echo area and track were measured from radar film, and relative winds calculated by subtracting a mean echo velocity; positions of radiosonde data points relative to the echo as center were computed, scaled by an echo radius, and plotted with echo motion vectors aligned along one coordinate axis. Mass flows into the mean system at all levels give vertical mass transports for growth and decay phases, and net mass balance. The net convergence of r closely balances a mean surface rainfall per echo, and the net enthalpy source by the cumulonimbus system. Fluxes of ?E, into and out of the system for 5K ranges confirm energy conservation, and give updraft, downdraft transports. The vertical structure of net mass r and ?E fluxes are presented. The mesoscale results are related to the large-scale modification of the mean atmosphere, using a theoretical cumulonimbus model. The large-scale vertical motion is computed as a residual from the temperature and water vapor budgets. Suitably averaged, the synoptic-scale mass transport is similar but not identical to the (life-cycle mean) cumulonimbus vertical mass transport. It is concluded that parametric models of cumulonimbus convection in terms of mass transport are quite realistic for these data above the lowest 150 mb, where the effects of horizontal variations between updraft and downdraft are dominant. The precise relationship between synoptic-scale controls and cumulonimbus-scale mass transport remains unclear.