A seasonal and diurnal climatology of precipitation organization in the southeastern United States

Abstract

This article describes results from a new four-year (2009-2012) radar-based precipitation climatology for the southeastern United States (SE USA). The climatology shows that a size-based classification between mesoscale precipitation features (MPF) and isolated precipitation reveals distinct seasonal and diurnal variability of precipitation. On average, from 70 to 90% of precipitation is associated with MPF, generally less in the summertime and in southern coastal regions. MPF precipitation has a relatively small seasonal cycle except in Florida and the warm offshore waters of the Gulf Stream. In contrast, isolated precipitation has a dramatic seasonal cycle that outlines the SE USA coastline whereas the MPF precipitation does not, consistent with a thermodynamic mechanism for onshore isolated storms in coastal regions. In summer, the isolated precipitation preferentially forms offshore at night, and dramatically 'flips' inland by early afternoon. In contrast, MPF precipitation has no clear diurnal variations except in the southern coastal region in the summer, likely associated with sea breeze convection organized on the mesoscale. These results suggest that the MPF versus isolated precipitation system framework provides a useful basis for future studies of large-scale and local controls on precipitation and resulting implications for long-range predictability of precipitation.

Daily precipitation extremes in isolated and mesoscale precipitation for the southeastern United States

“…This domain is shown in Figure 1. An algorithm to identify IPF and MPF precipitation features, where MPF are identified as features 100 km or greater in maximum horizontal dimension and all smaller features identified as IPF, was applied for each 15 minute grid, averaged daily, and compiled for the entire year following the methodology described in our recent study [8]. The complete dataset will be available at an online archive prior to publication.…”

“…The MPF precipitation intensity has a less dramatic seasonal cycle, with precipitation across the domain year round but slightly higher peak intensities in the summer. The most extensive MPF precipitation occurs in spring in the northeastern domain, where baroclinic systems are most actively producing widespread precipitation [2,8]. The seasonal cycle of domain-averaged precipitation intensity for IPF and MPF (Figure 4) shows that in the winter and spring, MPF has 50% -60% higher values than IPF, while in summer IPF slightly exceeds MPF by about 5%.…”

“…Within those weather systems, precipitation may be organized as small, short-lived and spatially heterogeneous features, hereafter referred to as isolated precipitation features or IPF, or by larger, well-organized and generally longer-lived features, hereafter referred to as mesoscale precipitation features, or MPF [8]. In this context, an isolated thunderstorm is an example of IPF organization, while mesoscale convective systems or widespread frontal rain are examples of MPF organization.…”

“…In this context, an isolated thunderstorm is an example of IPF organization, while mesoscale convective systems or widespread frontal rain are examples of MPF organization. A similar framework has been used for climatological studies of precipitation systems regionally and globally using surface and space-based radar observations [8][9][10][11][12][13]. Differences in the organization of precipitating systems reflect the influence of the large-scale environment, for example, squall line mesoscale convective systems tend to form in environments of high shear and moderate convective available potential energy [14].…”

“…A recent climatology of precipitation organization in the SE US demonstrated that this framework of precipitation organization applied to a radar precipitation dataset was able to capture important differences in the seasonal evolution in precipitation organization [8]. The daily accumulated precipitation was averaged for each three-month season to determine whether the seasonal cycle differed with precipitation organization.…”