Relationship between the Scavenging Coefficient for Pollutants in Precipitation and the Radar Reflectivity Factor. Part I: Derivation

Abstract

The relation between the scavenging coefficient ? (s?1) for air pollutants in precipitation and the radar reflectivity factor Z (mm6 m?3) is based on the fact that they are both functions of the hydrometeor size distribution. In this paper, which combines the fields of air pollution physics, cloud physics, and radar meteorology, ??Z relationships are derived analytically for below-cloud gaseous and particulate pollutants and, with certain restrictions, for pollutants incorporated into cloud droplets. For the types of precipitation and pollutant considered, it can be shown that ? ≈ aZb, where the coefficient a has an order of magnitude of 10?7?10?6 for submicron aerosol particles, 10?6?10?5 for highly soluble gases, and 10?5 for pollutants in cloud droplets. In stratiform rain the exponent b ranges between about 0.4 and 0.6, so that an increase of Z by a factor of 10 approximately corresponds to a twofold to fourfold increase in ?. In snowfall, mainly due to the diversity of solid hydrometeors, the value of b may vary more considerably but probably is somewhat smaller than in rain. Because weather radar estimates the spatial distribution of Z essentially in real time, ??Z relationships can be used to monitor and nowcast those areas most significantly exposed to wet deposition.