Abstract
The temporal histories of 702 convective echoes measured in the St. Louis area during the 1973 summer were studied to discern potential effects on echo behavior of the urban influences and those effects resulting from the merger of two or more echoes. The 190 echoes that merged grew faster (50%), became taller (52%), and lasted longer (122%) than non-merger echoes. The average echo top growth 10 min after a merger was 1500 m, and on any given day 80% of the heights of merger echoes at a given stage of echo life were higher than those of non-merger echoes. The 137 echoes that crossed the urban area were longer lasting (119%), faster growing (61%), taller (30%), and more merged (44% vs 23%) than the non-urban echoes. The 61 urban echoes that subsequently merged over or beyond St. Louis were demonstrably longer lasting (110 vs 44 min) and taller (4800 m at 10 min after entry into the urban area and 5900 m at urban exit) than 76 urban echoes that did not merge. The urban echo that merged was also measurably different than the rural merged echo. The average urban merged echo lasted 51% longer, grew 100% faster, and achieved a height 20 min after merger that was 133% higher. The urban area apparently affected nearly half (44%) of the echoes over it leading to larger, more vigorous, and longer lasting storms that always merged with one or more other storms. This dynamic process leads to more rain, short-duration rainstorms and hailstorms in and east of St. Louis.
