Springtime Onset of Isolated Convection across the Southeastern United States: Insights Using a Monsoon Framework

The southeastern United States (SE US) experiences about the same amount of precipitation in each season of the year. Embedded within the relatively flat annual cycle of precipitation is a clear summer maximum in rain from isolated convection (Rickenbach et al. 2015, QJRMS). Superficially, the seasonal cycle of isolated convection in the SE US resembles the annual precipitation evolution of a monsoon. While monsoon precipitation is driven by the seasonal reversal of a regional scale thermal circulation tied to the annual migration of the ITCZ, the summer convective season in the SE US emerges from changing characteristics of precipitation systems that occur year-round. The annual progression of seasonal precipitation regimes in the SE US, which include widespread rain and snow in winter, mesoscale convective systems in spring, isolated thunderstorms in summer, and tropical cyclones in fall. Though these regimes overlap considerably, they reflect a seasonal evolution and interplay of forcing mechanisms including the migration of the mid-latitude jet stream and associated baroclinic zones, the annual cycle of SSTs, and the intensification and zonal migration of the North Atlantic Subtropical High (NASH). This suggests that the monsoon framework might provide useful guidance to study the mechanisms controlling the annual cycle of the convective season in the SE US.

In this work, techniques and concepts relevant to studies of monsoon onset are used to examine the springtime onset of the isolated convection regime in the SE US for four separate years. Specifically, does isolated convection onset progress poleward like a monsoon, with the gradual seasonal advance of unstable air? Do the seasonal changes in the track of extratropical cyclones play a role in triggering this onset? What are the regional differences in the timing and amplitude of the springtime increase in isolated convection precipitation? What role does the North Atlantic subtropical high (NASH) play in the convective season onset?

Results suggest that the convective season onset occurs nearly simultaneously across the southeastern coastal plain and Gulf coast. The onset timing appears to result from the gradual seasonal advance of thermodynamic instability in concert with the establishment of southerly flow associated with the western migration of the NASH, often triggered by extratropical cyclone passage. The amplitude of the pre-versus-post isolated convection precipitation onset is much larger and more geographically uniform in the coastal plain and Gulf coast, consistent with the southerly advance of unstable air from the Gulf of Mexico in the NASH circulation. In contrast, in the northern domain, convective onset timing is more spatially variable, suggesting that the changing track and frequency of baroclinic frontal systems control onset there. This talk emphasizes the methodology of isolated convection onset, and on the geographic pattern of onset for four different years, with an overview of the mechanisms at work.

This work will establish a useful framework to study long-term changes in regional precipitation of the SE US in the context of ongoing longer-term observational and high-resolution modeling studies.