A Technique for Computing Vertical Transports by Precipitating Cumuli

Abstract

A method is described for calculating cumulus-scale vertical transports of mass, sensible heat and hori-zontal momentum from detailed measurements of precipitation. The basic premises are that the amount of lifting in a cumulus cell is related to the precipitation which it produces and that the temperature excess and entrainment are reflected in its vertical development. The amount of cellular precipitation may be obtained from quantitative radar data or high-resolution rain gauge records, the cell depths from radar and radiosonde data. The mass of air transported upward within the cells is computed from the conservation of water given the amount of cellular precipitation. A relationship between measured cellular rain and condensate within cumulus cells is based on available empirical information. Entrainment rate and the shape of the mass transport curve as a function of height are specified in a manner consistent with physical considerations, experimental evidence, and one-dimensional dynamic models of cumulus cells. For the transport of sensible heat the temperature excess within the cells is computed from conservation equations for water and vertical momentum. For the transport of horizontal momentum, the difference in horizontal wind speed is calculated from the conservation of horizontal momentum and the drag force exerted by the environment on the air in the updrafts. Uncertainties in the computed transports are obtained by estimating limiting values for the assumptions involved.