Dynamic and Thermodynamic Mechanisms for the Onset of the Southeastern United States Convective Season

The southeastern United States (SE US) receives ample precipitation year-round. In the winter , precipitation primarily comes from synoptic-scale baroclinic systems and cold fronts. Meanwhile , precipitation in the summer over the SE US is primarily the result of convection. With this shift from the winter to summertime precipitation regimes , spring is the transition period to the convective season , and this transition occurs rather abruptly. This shift can be described as a sudden increase in precipitation from isolated precipitation features (IPF) while precipitation from mesoscale precipitation features (MPF) stays relatively unchanged over the SE US. IPF is defined as small , short-lived , and spatially heterogeneous features while MPF is defined as larger , well-organized , and generally longer-lived precipitating features. To study the springtime transition to the convective season , the SE US was split into twenty-seven 2°x2° boxes. Precipitation data for March-August from the National Mosaic and Multi-Sensor Quantitative Precipitation Estimation (QPE) (NMQ) for the years 2009-2012 is used to determine onset using an objective method based on IPF precipitation in each of the twenty-seven boxes for each year and for the four-year average. Meteorological data from the North American Regional Reanalysis (NARR) is analyzed to determine potential dynamic and thermodynamic mechanisms that cause onset of the convective season in the SE US. Thermodynamic variables analyzed include convective available potential energy (CAPE) , surface temperature , and specific humidity. Dynamic variables analyzed include 500 hPa geopotential height , mean sea level pressure (MSLP) , and 850 hPa wind speed and direction. Daily composites of NARR are generated for May and June , while pentad average composites are generated for April-July for each year. Pentad averages of IPF will be created using the NMQ dataset to determine the pentad of onset. Three different sensitivity tests are also conducted to determine how sensitive onset is to the threshold criteria used to determine onset. It was found that the timing of onset varies from year to year , and there is no regional progression of onset in the SE US. Along with that , IPF behavior varies quite greatly across the SE US. Despite this variation in onset timing within the four years and variation in IPF behavior across the SE US , there are similarities in meteorological conditions in the pentads immediately leading up to and during onset. The North Atlantic Subtropical High (NASH) becomes established over the SE US one to two pentads before onset , priming the atmosphere for onset by bringing warm air and moisture from the Gulf of Mexico into the SE US. As the NASH becomes established , CAPE and specific humidity increase over the SE US , providing instability and moisture for IPF precipitation to develop over the SE US. At 500 hPa , either a ridge or zonal flow is present over the SE US at the time of onset , which aids in the NASH staying established over the SE US. The results of this research have begun to provide a new framework to better understand precipitation variability in the SE US.