Transition to the Summer Convective Season in the Southeastern United States

The southeastern United States (SE US) receives ample precipitation year-round in both wintertime-baroclinic and summertime-subtropical regimes. Associated precipitation phenomena include frontal precipitation, sea-breeze convection, mesoscale convective systems, and tropical cyclones, as well as diurnally-driven isolated convection. The timing and mechanisms for the seasonal transition of these regimes in the SE US have received little attention. Interestingly, our previous work (Rickenbach et al., QJRMS, 2015) suggests that the summer convective regime evolves in a manner analogous to a summer monsoon, though embedded within a constant year-round precipitation regime. In this context, understanding the transition to the summer convective regime in the SE US may have value as a metric to examine regional precipitation variability as global climate changes.

We explore the mechanisms responsible for the transition from the baroclinically-driven, synoptic-scale precipitation of the winter season to the thermodynamically-driven summer convective season in the SE US for the current and future climate. Our previous work revealed that rain from isolated precipitation features (IPF, generally isolated convection) increases rapidly in late spring, while mesoscale precipitation features (MPF) show minimal seasonal change. This study couples a surface radar-based dataset that distinguishes between mesoscale and isolated precipitation organization with analysis of North American Regional Reanalysis (NARR) thermodynamic and kinematic fields. Specifically, we examine the roles of atmospheric thermodynamics and dynamics in the abrupt transition to the summer convective (IPF) season in the SE US during the period 2002-2012.

Our hypothesis is that the gradual seasonal change in thermodynamics (temperature, humidity, CAPE, CIN) in springtime primes the atmosphere for a dynamic mechanism to cause the abrupt increase in IPF precipitation during early summer, in a manner similar to the onset of the summer monsoon in South America. We use NARR data in conjunction with the high-resolution National Mosaic and Multi-sensor Quantitative Precipitation Estimation (NMQ – ‘Q2’) NEXRAD-based and gauge-adjusted precipitation dataset to address this question. We apply an algorithm to the NMQ data to distinguish between isolated convection (IPF) and mesoscale systems (MPF), following Rickenbach et al. (QJRMS, 2015). We analyze hourly, daily and pentad NMQ-based IPF precipitation maps as well as pentad distributions of precipitation intensity for the four months (March-June) of the spring transition in the period 2002-2012 to determine the dates when the abrupt jump in IPF precipitation occurs. Using composite analysis, we explore how the slow thermodynamic priming of the atmosphere acts together with synoptic and large-scale dynamical features - such as upper-tropospheric jets, ET cyclones and associated low-level jets, and the position and intensity of the North Atlantic subtropical high (NASH) - to accomplish the timing and abruptness of the summertime convective season onset in the SE US.