Trace substances in rain water: concentration variations during convective rains, and their interpretation

The temporal changes of contaminant concentrations in rain water samples collected sequentially at a fixed point are dependent upon both the rain scavenging and drop growth processes and the advective effects of the motion field upon the scalar field of concentration variations in the rain cloud. The observations that rainfall intensity and contaminant concentrations are usually negatively correlated, but occasionally for short periods are positively correlated, are explained. Periods of positive correlation appear mainly because of a predominating advective effect, whereas, all drop growth and scavenging processes operate to produce the negative correlation.In the massive storms characterized by Browning (1964) as “SR-storms” and Newton (1966) as “persistent” an extraordinarily well-organized downdraft is inferred. The strong evaporative effect of this downdraft upon rain in contact with it produces an atypical variation of concentrations in relation to rainfall rates. An example of this is presented.