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Ensemble-Based Analysis of Extreme Precipitation Events from 2007-2011

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October 19, 2012
Samantha Lynch
Hosted by Russ Schumacher (advisor), Dick Johnson, Jeffrey Niemann (Civil and Environmental Engineering)

Abstract

From 2007 to 2011, 22 widespread, multiday rain events occurred across the United States. This study will conduct numerical model evaluation of three ensemble prediction systems – the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers of Environmental Prediction (NCEP), and the United Kingdom Office of Meteorology (UKMET) using the area under the relative operating characteristic (ROC) curve. After analyzing the 22 widespread precipitation events, the ECMWF appears to have produced more skillful forecasts for each of the events, with an exception of the 25-30 June 2007 event, the MCV over the southern plains of the United States. To better evaluate the ECMWF, two widespread, multiday precipitation events were selected for closer examination: 29 April - 4 May 2010 and 23-28 April 2011.

From 1 to 3 May 2010, persistent heavy rainfall occurred in the Ohio and Mississippi River Valleys due to two successive quasi-stationary mesoscale convective systems, with locations in central Tennessee accumulating more than 483 millimeters (19 inches) of rain, and the city of Nashville experiencing a historic flash flood. This study used operational global ensemble forecasts from the ECMWF to diagnose atmospheric processes and assess forecast uncertainty in this event. Utilizing ensemble members, several methods are used to examine the processes that led to the development and maintenance of this high precipitation system. Differences between ensemble members that correctly predicted heavy precipitation and those that did not were determined, in order to pinpoint the processes that were favorable or detrimental to the system's development. Statistical analysis was used to determine how synoptic-scale flows were correlated to area-averaged precipitation. The heavy precipitation throughout Nashville, Tennessee and surrounding areas occurred ahead of a deep upper-level trough located over the central United States. The distribution of precipitation was found to be closely related to the strength of this trough and an associated surface cyclone, with a weaker upper-level trough and cyclone being associated with more precipitation in the area of interest. The same technique was used to assess forecast skill and uncertainty for the 23-28 April 2011 widespread rain event. This event was associated with persistent heavy rainfall, flooding more than six states lining the Mississippi River. The heavy precipitation throughout the eastern Unites States is found to be closely related the strength of this trough and an associated occluded cyclone, as as well as the speed at which the upper-level trough and associated surface cyclone push off to the east. These relatively small differences in the wind and pressure fields led to important differences in the precipitation forecasts and highlighted some of the uncertainties associated with predicting these extreme rainfall events.

Finally, a comparison is made between how well these two widespread, multiday precipitation events, from 29 April - 4 May 2010 and 23-28 April 2011, were predicted by the ECMWF. It was found that heavy rain fall totals, exceeding the 100 to 150 mm threshold, were better anticipated for the 29 April - 4 May 2010 event, while location of the precipitation was better predicted for the 23-28 April 2011 widespread rain event.