The Spatial and Temporal Distribution of Organized Convective Structures over the Northeast and Their Ambient Conditions

Abstract

Organized convective structures over the northeastern United States were classified for two warm seasons (May?August) using 2-km composite radar [i.e., the National Operational Weather Radar (NOWrad)] data. Nine structures were identified: three types of cellular convection (clusters of cells, isolated cells, and broken lines), five types of linear convection (lines with no stratiform precipitation, lines with trailing stratiform precipitation, lines with parallel stratiform precipitation, lines with leading stratiform precipitation, and bow echoes), and one nonlinear system. The occurrence of all structures decreases from the western Appalachian slopes eastward to the Atlantic coast. Isolated cellular convection forms primarily during the morning to late afternoon (1200?2100 UTC) mainly over the high terrain. Clusters of cells form primarily over the Appalachians and the Atlantic coastal plain during the daytime (1200?0000 UTC). Linear convection is favored from midafternoon to early evening (1800?0000 UTC) over land areas. Nonlinear systems develop mainly from midafternoon to late evening (1800?0600 UTC) over the inland areas and over the coastal zone during the early morning (?1200 UTC). Composites using the North American Regional Reanalysis (NARR) highlight the ambient conditions for three main convective structures: cellular, linear, and nonlinear. Cellular convection initiates with limited quasigeostrophic forcing and moderate instability [i.e., average most unstable CAPE (MUCAPE) ?973 J kg?1]. A majority of cells develop in orographically favored upslope areas. Linear convection organizes along surface troughs, supported by 900-hPa frontogenesis and an average ambient MUCAPE of ?1011 J kg?1. Nonlinear convection organizes along warm fronts associated with larger-scale baroclinic systems, and the MUCAPE is relatively small (?207 J kg?1).