Inadvertent Rain Modification as Indicated by Surface Raincells

Abstract

Rainfall data from a large dense network are being used to study inadvertent rainfall modification in the St. Louis area. Surface raincells are delineated and then analyzed to determine the character of any urban-induced changes in precipitation. Results for comparisons of 605 potential effect cells and 870 non-effect cells from the summer of 1971?72 provide strong evidence that cell characteristics have been sizeably altered by the local urban-industrial environment. For cells occurring in the urban-industrial zone of St. Louis, the average rainfall volume was 176% greater than for cells in the control sample. For cells occurring in the separate industrial region of Wood River, the average volume was 262% greater than the cells in the control sample. The results show that the primary change in St. Louis cells is total rain area, and this and other results suggest that this relates to dynamic effects induced by the urban heat island. The primary change in Wood River cells is in rain intensity, and this and other results suggest that this relates to microphysical effects from the industrial aerosols and additions of moisture into the atmosphere, particularly in dry summers. Importantly the primary causes of observed rain increases in St. Louis and Wood River appear to differ, and additional data must be collected and analyzed to enlarge on the interesting two-summer results.