A computational study of the modification of raindrop size distributions in subcloud downdrafts

Abstract

A computational study is made of the variation of steady-state raindrop-size distributions (0.004 cm [less than or equal to] radius [less than or equal to] 0.40 cm) in adiabatic subcloud downdrafts of constant magnitude. The cloud base drop-size distribution is assumed to be Marshall-Palmer. The microphysical processes progressively introduced are evaporation, collision-coalescence, aerodynamic and collisional breakup of drops. Collision-coalescence and breakup are treated through a stochastic model. Thermodynamic and hydrometeoric variables of temperature, relative humidity, total liquid water, rainfall rate and radar reflectivity are computed as functions of height below cloud base along with the drop-size distributions. It is found that evaporation tends to deplete, as expected, the smaller members of the droplet population. The slope of the Marshall-Palmer distribution as determined by the distribution of larger size drops remains virtually unaffected by evaporation. When collision-coalescence is included, the depletion of the smaller drops is enhanced by the larger drops sweeping out the smaller ones. Aerodynamic breakup when added to evaporation and collision-coalescence has little effect on the resulting drop-size distribution except for very high initial precipitation rates. When collisional breakup is added to the previously mentioned microphysical processes, the effect of aerodynamic breakup is virtually eliminated because collisional breakup quickly depletes the concentration of drops with radii greater than 0.15 cm. Computed raindrop-size distributions agree quite well with measured maritime raindrop spectra reported in the literature, the agreement being better for the higher precipitation rates measured. Other results of the study, as anticipated, are (i) that the downdraft remains unsaturated in the presence of precipitation, the degree of subsaturation increasing with the strength of the downdraft, and (ii) that the temperature in the downdraft lies between the moist-adiabatic and the dry-adiabatic lapse rates of temperature.