A comparison of the ambient conditions favoring different organizational convective structures over the Northeast US

The organizational structures of all warm season convection are determined by the ambient dynamic and thermodynamic conditions in which they develop. Over the Northeast U.S, the Appalachian terrain and cooler Atlantic coastal waters can also influence the development and organization of these structures. This presentation will highlight the environmental conditions that support the development of 3 main convective structures (cellular, linear, nonlinear) over the complex surface conditions of the Northeast U.S.

For this study, organizational structures were identified through the manual examination of 2-km NOWrad radar reflectivity imagery over the Northeast. From the 2002-2007 NOWrad data, 47 cellular events, 38 linear events, and 42 nonlinear events were selected. A day is classified as an event if at least 90% of the convection over the Northeast exhibits one particular organizational structure (i.e. cellular). To highlight the environments associated with the 3 structural types, feature-based composites were constructed using the North American Regional Reanalysis (NARR). Composites were centered on the convection (feature-based) location, which allowed for more robust synoptic and thermodynamic signals than geographically fixed composites. To compare the ambient conditions between cellular, linear, and nonlinear convective events, several variables related to the ambient conditions from the composite events were averaged over a 1.5x1.5^o latitude-longitude box centered on the compositing point. A bootstrap method was applied to the area-averaged variables to test for statistical significance between the 3 groups (i.e. cellular, linear, nonlinear).

In the mean, cellular convection develops within moderately unstable air (surface CAPE ~1250 J kg^-1) and weak large-scale forcing for ascent at all levels. About 60% of cells are orographically initiated from 900 hPa upslope flow over the Appalachian terrain, as well as along sea breeze convergence zones in the Atlantic coastal plain. On average, linear convection organizes within the axis of a surface trough and is supported by 900 hPa frontogenesis. Near the developing linear convection, instability is modest with CAPE values ~1250 J kg^-1 and θe is high (~344 K). During linear events, the mean precipitable water (37.9 mm) is significantly higher than during cellular and nonlinear events. The deep layer (0-6 km) shear is ~12.5 m s^-1, with most of that shear in the lowest few km. Nonlinear convection is forced by relatively deep baroclinic systems, with forcing for ascent from low-level warm advection and upper-level jet circulations. Nonlinear convection organizes in significantly lower CAPE (~200 J kg^-1) and higher 0-6 km shear (16.9 m s^-1) as compared to other convective structures.