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Ensemble-based analysis of extreme precipitation events from 2007-2011




Lynch, Samantha, author
Schumacher, Russ, advisor
Johnson, Richard, committee member
Niemann, Jeffrey, committee member

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From 2007 to 2011, 22 widespread, multiday rain events occurred across the United States. This study makes use of the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers of Environmental Prediction (NCEP), and the United Kingdom Office of Meteorology (UKMET) ensemble prediction systems (EPS) in order to assess their forecast skill of these 22 widespread, precipitation events. Overall, the ECMWF had a skillful forecast for almost every event, with an exception of the 25-30 June 2007 event, the mesoscale convective vortex (MCV) over the southern plains of the United States. Additionally, the ECMWF EPS generally outperformed both the NCEP and UKMET EPS. 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. The 29 April-4 May 2010 case study used ECMWF ensemble forecasts to explore the processes responsible for the development and maintenance of a multiday precipitation event that occurred in early May 2010, due to two successive quasi-stationary mesoscale convective systems. Locations in central Tennessee accumulated more than 483 millimeters (19 inches) of rain and the city of Nashville experienced a historic flash flood. Differences between ensemble members that correctly predicted heavy precipitation and those that did not were determined in order to determine 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. For this particular case, the distribution of precipitation was found to be closely related to the strength of an upper-level trough in the central United States and an associated surface cyclone, with a weaker trough and cyclone being associated with more precipitation in the area of interest. The 23-28 April 2011 case study also used ECMWF ensemble forecasts to explore the processes responsible for the development and maintenance of a multiday precipitation event. This event was associated with persistent heavy rainfall, flooding more than six states lining the Mississippi River. In this case, the largest difference in the ensemble members' forecasts was the strength of the upper-level trough and associated occluded low, as well as the speed at which this system moved off to the east. These relatively small differences in the height field ultimately resulted in different forecasts of precipitation over the Mississippi Valley. This sensitivity to small-scale differences in the initial conditions highlights the importance of using ensembles for predicting the development of precipitation systems over both land and ocean. Comparison between the 29 April-4 May 2010 and 23-28 April 2011 widespread precipitation events provide information regarding which of the two case studies was better predicted in relation to both location and amount of precipitation. Heavy rainfall totals, exceeding the 100 and 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.


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