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Northeast Severe Weather as A Function of Flow Regime
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Wind data used in this study were obtained from the "Radiosonde Data of North America" CD-ROM, provided by the National Climatic Data Center. Dates and times of severe weather events were obtained from the Storm Data. Population density data was obtained from the 1990 census CD-ROMs. The previously defined latitude/longitude box (see MAP) was divided into 0.5º by 0.5º grid boxes, overlapping every 0.25º. Based on the block-group level data, area average population density was computed for each box. For each available severe weather report, the wind direction at 700 hPa, 500 hPa, and 850 hPa was obtained using the closest sounding time to the event. Events which occurred prior to 1400 local time were defined to be closest to the 12Z sounding, and events which occurred after 1400 local time were defined to be closest to the 00Z sounding. There is a potential for errors to occur if a frontal passage occurs between the 12Z morning sounding and the 00Z evening sounding. The events which are closer to the 00Z sounding could still potentially have been in the pre-frontal passage environment. Tests are being done on subsets of the dataset which are close to the sounding time to see if the subset is representative of the whole set. Wind rose plots were created for 850 hPa, 700 hPa, and 500 hPa flow. Wind directions were divided up into thirty six 10º bins, from 0º to 360º. The percent of high wind, hail and tornado events which occurred in each bin was totaled and plotted on wind roses. Each day on which severe weather occurred in the geographic area in this study was defined as a severe weather day. For each 0.5º by 0.5º box, the number of events per severe weather day was totaled for northwest flow days (greater than 280º), west flow days (260º to 280º),and southwest flow days (less than 260º) at 700 hPa. The 700 hPa level was chosen with the idea that it is high enough to reflect some of the large scale flow pattern, yet low enough to feel the influence of the topography. In addition, the ratio of events per southwest flow day to events per northwest flow day was calculated in order to identify which areas have a preference for severe weather to occur for different 700 hPa flow directions. A population density correction was performed on the Storm Data reports following the method used in Paruk and Blackwell (1994). The number of events was plotted against population density, and a linear regression analysis was performed. Based on the regression equation, a correction factor was developed which, when multiplied by the actual number of events, produced an 'adjusted' number of events. This corrected value assumes that all the events in the box with the highest population density are reported, and adjusts each of the other boxes to the number of events which would be reported if the population density was equal to that in the highest box.
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