Scavenging of Urban Pollutants by Thunderstorm Rainfall: Numerical Experimentation

Abstract

Rainfall collected for the squall line case of 21 August 1972 in the St. Louis area has been analyzed for certain pollutants known to be present in urban air. To aid in the interpretation of these measurements a two-dimensional time-dependent squall line model is utilized to approximate the internal air flow and rainwater distribution within the storm. At a particular time when a typical mature circulation is present, a steady state is assumed and a pollutant plume with dimensions and concentrations characteristic of the St. Louis urban plume is allowed to interact with the model thunderstorm circulation. The study focuses upon the effect of internal storm motion upon precipitation scavenging and treats microphysical processes in a relatively simple manner. Two extreme situations are considered: 1) the pollutant does not interact with the water substance in the storm, but is merely redistributed by the storm circulation; and 2) all pollutant enters into cloud-water immediately upon entering the cloud boundary. In the second case, scavenging of pollutants by precipitation is calculated along with the deposition of pollutant on the ground in rainfall. The deposited amounts are compared with the limited number of measurements from the same storm. The trend and order of magnitude of deposition arrived at in the model compare favorably with the observations. Areas where additional observations are greatly needed are specified and desirable directions for model development are discussed.