Mechanisms for the Summertime Precipitation Minimum off the Georgia and Carolina Coasts

The Gulf Stream off the Georgia and Carolina coasts supports a band of heavy precipitation along the warm current that is a robust feature of the seasonal and annual precipitation climatology. During the summer season, rain from onshore afternoon convection is separated from the Gulf Stream precipitation by a relative minimum, most notably off the Georgia and South Carolina coasts. A National Mosaic and Multi-sensor Quantitative Precipitation Estimation (NMQ)-NEXRAD based four-year precipitation climatology (Rickenbach et al. 2015, QJRMS) show that this offshore summertime minimum is very clearly seen for isolated precipitation features, but not for mesoscale precipitation features. The hypothesis is that this offshore rain minimum is linked most closely to processes that control small-scale, locally generated convection through the diurnal cycle of solar radiation, rather than, for example, the propagation of mesoscale convective systems associated with baroclinic circulations.

The present study further investigates the summertime minimum in precipitation off the Georgia and Carolina coasts. The goals are to verify and to extend the climatology in time and space with an independent satellite precipitation dataset, and to examine mechanisms for the summertime offshore rain minimum. Since the NMQ product is limited in coverage offshore to the Gulf Stream, we analyzed Tropical Rainfall Measuring Mission (TRMM) 3B-42 three-hourly precipitation data (0.25^o spacing) for the period 1998-2014 to compare the seasonal to the four-year NMQ-NEXRAD climatology of Rickenbach et al. (2015). We will compare the composite diurnal and seasonal changes of TRMM precipitation to the NMQ-NEXRAD climatology. Then, we use National Centers of Environmental Prediction (NCEP) reanalysis data to analyze the diurnal and seasonal changes in kinematic and thermodynamic parameters, to evaluate the role of regional circulation features (land-ocean circulations modified by coastal shape, Gulf Stream thermally-induced circulations), as well as large-scale moisture advection determined by the strength and position of the North Atlantic subtropical high.