Effects of the North Atlantic Subtropical High on summertime precipitation organization in the southeast United States

In the southeastern United States (SE US), the western side of the North Atlantic Subtropical High (NASH), also known as the NASH western ridge, plays an important role in the variability of summertime precipitation. This study analyzes the effect of the location of the NASH western ridge on the daily variability of precipitation organization in the SE US. Daily North American Regional Reanalysis (Mesinger et al. 2006) 850 mb geopotential was used to determine the daily and mean summertime positions of the NASH western ridge. Based on the mean summertime position of the NASH western ridge, the study domain was divided into four quadrants and each day was classified according to the quadrant where the NASH western ridge was located. The National Mosaic and Multi-sensor Quantitative Precipitation Estimation (Zhang et al. 2011) was used to classify precipitation into mesoscale (MPF) or isolated precipitation features (IPF; Rickenbach et al. 2015). Finally, composites of summertime precipitation organization into IPF and MPF for each NASH western ridge quadrant were calculated.

As in previous studies, our results show that MPF contribute most of the total summertime precipitation in the SE US. In terms of precipitation organization, IPF rain was distributed more uniformly across the SE US in all quadrants, while MPF accounted for most of the regional rain differences between the four NASH quadrants. In each composite, the low-level jet along the NASH western ridge was collocated with the strongest MPF rain pointing to a dynamical role of the NASH in controlling precipitation organization. The implications of these results to future climate change will be discussed.