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 modulated by diurnal thermodynamic forcing. With this shift from the winter to summertime precipitation regimes, the springtime transition to the convective season occurs rather abruptly within a few weeks. This spring transition has been shown to be characterized by an 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, data from the North American Regional Reanalysis (NARR) and the National Mosaic and Multi-Sensor Quantitative Precipitation Estimation (QPE) (NMQ) will be used to study the springtime onset of the convective season. Pentad averages of IPF will be created using the NMQ dataset to determine the progression of rapid IPF increase across the SE US during May and June of each of four years (2009-2012). We will determine the pentad of IPF onset, in a manner analogous to the onset of a seasonal monsoon. Next, various meteorological parameters from NARR will be analyzed using the Gridded Analysis and Display System (GrADS) to determine what the atmospheric conditions were shortly before, during, and immediately after onset. Dynamic and thermodynamic mechanisms such as the shifting jet stream and cyclone tracks, the strength and position of the North Atlantic Subtropical High (NASH) and southerly moisture advection in the NASH western ridge, passage of mid-latitude cyclones and their cold fronts, and increasing surface temperatures and convectively available potential energy will be analyzed to determine what role each plays in the transition to the onset of the convective season.