Estimates of Characteristic Times for Precipitation Scavenging

Abstract

We address the problem of using climatological data to estimate residence times in the atmosphere of particles subjected to precipitation scavenging. Basic parameters are the scavenging coefficient λ, the rainfall intensity R and the length of dry and precipitation periods T̃_d and T̃_p. Previous models used to estimate residence times are reviewed and compared with “real” data. It is found that some of the earlier models substantially underestimate the average residence times, particularly for particles with a high value of the scavenging coefficient. An approximate model valid for long-lived particles with a relatively small scavenging coefficient [λ≲(5 h)⁻¹ at R = 1 mm h⁻¹] is formulated and tested. Daily rainfall values are adequate as input in this model but several years of data are required in order to wore at reasonable estimates of the residence time. The fundamental difficulty associated with all the models discussed is that a proper application requires Lagrangian rainfall data, i.e., information about the weather experienced by the particle as it is carried along with the winds, whereas virtually all data available from meteorological observations are obtained at fixed locations (Eulerian data).