A TRMM Precipitation Rate Analysis of the Extratropical Transitioning (ET) of Tropical Cyclones in the North Atlantic

            Tropical cyclones (TCs) pose a significant threat every year during hurricane season in the North Atlantic basin. About half of the TCs undergo what is known as an Extratropical Transition (ET), where the cyclone enters the mid-latitudes and eventually loses its tropical characteristics as it enters a baroclinic environment.  However, even as an extratropical cyclone, storms can cause damaging winds, rough seas, and extreme amounts of precipitation. This study focuses on the changing nature of precipitation as a TC undergoes ET. Previous research has shown that the rain fields of TCs expand ahead of the storm during the ET process. The current study hypothesizes that as more moisture is typically available to the TC surrounded by a tropical environment, heavier rain rates will decrease in areal extent as the TC becomes extratropical.  TC position data as well as the time when the TC became extratropical were obtained from the Hurricane Database (HURDAT2). All TCs meeting the following criteria were considered in the analysis: a) they existed for at least five days, b) they were classified as extratropical, c) they occurred during the period over which data from the Tropical Rainfall Mapping Mission (TRMM) were available. Data from TRMM 3B42 are available since 1998 in 3-hour intervals and were utilized to examine the rain rates of the TCs before during and after completion of the ET. A total of 74 TCs were analyzed. Precipitation rates were analyzed from a period of 72 hours before the designation of a TC as extratropical through 48 hours after this time creating a 5 day ‘window'. Within a Geographic Information System (GIS) a 500 km radius is used as a buffer to select rain rates associated with the TC. The numbers of points containing light to moderate rain rates (0.5-5 mm hr^-1) as well as rates greater than 5 mm hr^-1 were counted at each observation time, and the rate of change of this ratio during the 5 day period will be examined for each TC. This study will provide a better understanding of the changing precipitation rates within TCs that undergo ET as well as the timing of these changes relative to the storm's classification as extratropical.