A climatology of convective systems over the Northeast U.S. and their structural evolution from the lee of the Appalachians to the Atlantic Coast

Convection over the Northeast U.S. is modified by the underlying Appalachian terrain and the relatively cool Atlantic coastal waters during the warm season. The impacts of the terrain and marine air on the evolution of the various organizational structures of convection have not been quantified over the Northeast. This presentation will focus on modification of convection from the lee of the Appalachians to the coastal ocean, highlighting some of the convective structures and evolutions within this region.

The different modes of convection over the Northeast were identified for two warm seasons (2007 and a random warm season from 2002-2006). To allow for a direct comparison between Northeast and central Plains convection, the same convection classification approach was applied to the Northeast as in previous literature studying convection over central U.S. This involved examining 15-minute NOWRAD (2-km grid-spacing) radar imagery and classifying the convection into three types of cellular convection (individual cells, clusters of cells, and broken lines), five types of quasi-linear systems (bow echoes, squall lines with trailing stratiform rain, lines with leading stratiform rain, lines with parallel stratiform rain, and lines with no stratiform rain), and nonlinear systems. The Northeast was divided into 4 domains (upslope, high terrain, coastal plain, coastal ocean) and the spatial initiation of each convective element was documented with respect to these domains. The time of initiation was recorded and binned into 6-hr time periods (00-06 UTC, 06-12 UTC, 12-18 UTC, 18-00 UTC).

Convection over the Northeast displays the same structural distribution as the Central Plains, with a few exceptions. Cellular convection over the Northeast develops in clusters, while there are more isolated cells over the Plains. Over the Northeast, cellular initiation is favored over the high terrain during the early afternoon, with some preferred genesis regions occurring in the lee of the Appalachians. Once developed, there is a tendency for these cells to evolve into quasi-linear systems that propagate eastward towards the coastal ocean. Only a fraction of these systems reach the coast and those that survive decay over the ocean waters. The composite evolution of various convective structures will be highlighted.

The evolution of convection modified by the high terrain will be categorized and composited, highlighting features such as a pressure trough, an axis of convergence at the surface within the lee, the orientation and magnitude of the flow from 925-850 hPa across the barrier, the ambient CAPE and vertical shear. For convection encountering a marine layer, it is hypothesized that the source of the air ingested into the system plays a role in the evolution of that system. For example, a cool, deep marine layer will more likely cause convection to decay or become elevated as compared to a less cool and shallower layer. The various evolutions will be categorized and composited examining the depth of the marine layer, the magnitude of CAPE over the land verses the ocean, and the lower-tropospheric wind shear over land verses the ocean.