Inferences of weekly cycles in summertime rainfall

In several continental regions a weekly cycle of air pollution aerosols has been observed. It is usually characterized by concentration minima on weekends (Saturday and Sunday) and maxima on weekdays (Tuesday-Friday). Several studies have associated varying aerosol concentrations with precipitation production and attempted to determine whether or not there is a corresponding weekly cycle of precipitation. Results to date have been mixed. Here we examine a 12 year national composited radar data set for evidence of weekly precipitation cycles during the warm season (June-August). Various statistical quantities are calculated and subjected to "bootstrap" testing in order to assess significance. In many parts of the United States, warm season precipitation is relatively infrequent, with a few extreme events contributing to a large percentage of the total 12 year rainfall. For this reason, the statistics are often difficult to interpret. The general area east of the Mississippi River and north of 37°N contains regions where 25%-50% daily rainfall increases are inferred for weekdays (Tuesday-Friday) relative to weekends. The statistics suggest that western Pennsylvania is the largest and most likely contiguous region to have a weekly cycle. Parts of northern Florida and southeastern coastal areas infer a reverse-phase cycle, with less rainfall during the week than on weekends. Spot checks of surface rain gauge data confirm the phase of these radar-observed anomalies in both Pennsylvania and Florida. While there are indications of a weekly cycle in other locations of the United States, the degree of confidence is considerably lower. There is a strong statistical inference of weekday rainfall maxima over a net 8% of the area examined, which is approximately twice the area of cities. Future examination of lofted aerosol content, related condensation/ice nuclei spectra, and knowledge of the convective dynamical regime are needed in order to assess how anthropogenic aerosols may affect rainfall at urban and regional scales. If radar is the primary method of observation, it is also necessary to examine the effects of variable aerosol content on the parametric relationship between rainfall rate and radar reflectivity factor. Polarimetric radar observations could also serve to verify microphysical-dynamical hypotheses regarding precipitation production.