Identifying Thermodynamic Controls on Convective Onset in the Southeastern United States Summer

Within the Southeastern United States (SE US) summer, deep convection occurs frequently and is difficult to forecast. This study examines thermodynamic conditions supporting deep convection onset of isolated convective systems, as well as mesoscale convective systems (MCSs). Days of interest for convective onset are identified using the ground-based precipitation radar and are matched to radiosonde and ERA5 reanalysis data to study the large-scale thermodynamic environment supporting deep convective initiation in the SE US. Ground-based radar is also used to classify organized multi-cellular convection. Comparing thermodynamic environments of locally-developing isolated systems and propagating organized convective systems, organized convective systems are sustained by moister environments than those in which isolated convective systems develop. Examining variability in the moisture vertical structure, we find that moisture is higher in the lower free troposphere in environments supporting MCSs, compared with environments supporting isolated deep convection initiation. By using geostationary imagery to identify cloud top heights (CTH), the temporal evolution of the transition from shallow to deep convection is explicitly identified and analyzed for our isolated deep convective cases. Matching ERA5 thermodynamics to our CTH analysis of the shallow-to-deep transition reveals a buildup of column moisture in the many hours preceding the convective transition. Overall, low values of CAPE, elevated and sustained total column water (TCW), and the importance of a moist surrounding environment are indications that convection within the SE US is highly dependent on moisture-based variables and have traits comparable to tropical convection. Finally, preliminary results from local field experiments (Summer 2021, Charlottesville, VA) explore physical mechanisms – e.g. dry air entrainment – relating deep convection to its thermodynamic environment in a new way.