Tuesday, 24 January 2017
Using K-means clustering and Self Organizing Maps, tropopause heights on extreme precipitation days in the US Northeast are separated into 6 basic patterns, and an analysis of the extreme precipitation associated with each pattern is conducted. Extreme precipitation is identified as the top 1% of wet-day precipitation at 35 Northeast stations. For each set of pattern days, the key ingredients and underlying dynamics that lead to extreme precipitation are examined. Meteorological ingredients examined include 500 hPa geopotential heights, MSLP, vertical velocity, precipitable water and integrated moisture transport, low-level moisture convergence, Q-vectors, and storm tracks and fronts. The tropopause patterns include two types of summertime upper-level ridges, a wintertime deep Eastern US trough, a shallower summertime version of this trough, and two versions of cold-season East-Central US troughs. A large fraction of extreme precipitation occurs with the two ridge patterns. Overall these patterns are consistent with thermal boundaries or stalled fronts, and convective activity due to surface heating and ample moisture supply. Although some extremes occur along the coast (where moisture transport from the southwest is stronger on extreme days), more extremes occur inland where low-level moisture convergence is strong, and moisture delivered from the Great Lakes in the presence of warm temperatures creates localized instability. The wintertime Eastern US trough appears to represent classic Northeast US synoptic storms, with strong southerly flow of moisture up to 48 hours before the extreme event, strong Q-G forcing, and strong upward motion often related to Warm Conveyor Belts. The summertime version of the trough pattern has more limited moisture availability, and strongest forcing mechanisms to the north of the domain. The East-Central trough patterns represent storms travelling inland along the Atlantic coast through New England. Abundant moisture transported from the south ahead of the storms, and vigorous upward motion associated with the accompanying Warm Conveyor Belts, account for many of the precipitation extremes.
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