Characteristics of sub-diurnal extreme precipitation-producing systems

Sub-diurnal extreme precipitation events are identified by examining hourly precipitation data from select rain gauges in the Midwestern United States during the period of 1956-2005. Time series of precipitation accumulations for 3- and 6-hour periods are fitted to the Pareto distribution to determine the 10-year return levels for the stations. An extreme precipitation event is defined as one that exceeds the 10-year return level over both a 3-hour and a 6-hour period. Return levels in the Midwest vary between 43 mm to 76 mm for 3-hour events and 54 mm to 93 mm for 6-hour events.

Characteristics of the precipitating systems responsible for the extremes are derived from the NCEP stage II/stage IV multisensor precipitation data. The precipitating systems are treated as objects that are identified using the automated procedure developed by Baldwin et al. (2005). Areas of continuous precipitation above a user-defined threshold (here, 6 mm) are defined as a single object. Characteristics considered include: object size, object duration, and object propagation speed, as well as the precipitation mean, variance, and maximum within each object.

Based on 45 extreme events, the mean lifetime of the precipitation system is 7 hours, and over the lifetime of these systems the mean precipitation is between 10 and 15 mm, and the mean size is 5,000 km2. The gauge data additionally show that most of the precipitation during the extreme events falls within one to two hours.