Diurnal variation of isolated convection over land and ocean in the southeastern United States: A case study

We present a one-week case study (observations and modeling) of the diurnal evolution of summer season rainfall and radar echo top height in the southeastern United States (SE US) and coastal ocean. The goal is to better understand the mechanisms for the transition of locally generated afternoon convection over land to nocturnal convection along the warm Gulf Stream and Gulf Coast ocean. This will aid in the interpretation of long-term precipitation climatology from spaceborne sensors, and will provide validation for regional climate simulations of precipitation variability in that region.

The SE US has a summertime precipitation regime, with intraseasonal variability from daily to weekly associated with diurnal and baroclinic forcing as well as the occasional tropical system. A four-year NEXRAD radar climatology for the summer months of June-August (Rickenbach et al., 2015 QJRMS) showed that the diurnal mode was characterized by afternoon rain maximum (1300 – 1600 LT) over land, greatest within a few hundred kilometers from the coast. Offshore the diurnal pattern reverses, with an early morning maximum (0400 – 0700 LT) over the ocean within the radar's maximum range of 200 km. Rickenbach et al. (2015) found that this diurnal pattern is associated mainly with locally generated isolated convection.

We selected the period 16-22 June 2010 for detailed analysis, because this case had the characteristics of the long-term climatology. For each 15 minute image of the 1 km x 1 km x 0.5 km NMQ NEXRAD radar product, rainrate and echo top height of objectively determined isolated features were compiled. A land-ocean mask was applied to the dataset. Time series of land and ocean rainfall and echo top height were produced, along with histograms of the evolving echo top height distributions. Results demonstrate that convection over the land was deeper and rainier than over ocean. Convection evolved independently over land and ocean, with timing consistent with the climatology, suggesting local thermodynamic instability drove the diurnal variation. We will present of NCEP NARR reanalysis data to test that hypothesis, as well as WRF simulations of the event.