The evolution of quasi-linear convective systems encountering the northeastern U.S. coastal marine environment

Though the evolution of quasi-linear convective systems (QLCSs) over central U.S. land areas has been well documented, there is considerably less known about the interaction of QLCSs with land-ocean boundaries. It is unknown why some systems quickly decay when encountering the coast while others maintain their intensity. This becomes particularly important in the coastal regions of the northeastern U.S. (i.e. New York City and Long Island) where millions of people live, since many of these QLCSs can produce damaging winds as well as flash flooding.

To better understand the different QLCS evolutions, we manually examined NOWrad radar reflectivity imagery from the 2002-2007 warm seasons (May-Aug) and identified 73 QLCS events that encountered the northeastern U.S. coast. We classified these events into 4 different categories based on their evolution upon encountering the coastline. There are 32 events that decay at the coastline, 18 events that slowly decay upon reaching the coast (i.e. show no signs of decay at coastal boundary but show signs of decay once over the water and within 100 km of the coast), 9 events that maintain their intensity and decay more than 100 km from the coast, and 6 events that organize along the coast.

To investigate the different QLCS evolutions in the context of the surrounding ambient conditions, we created feature-based composites using North American Regional Reanalysis (NARR) data centered on the point at which the line crosses the coast at the closest 3-hr NARR time prior the crossing. Preliminary results show that for events that decay at the coast, the convective line is collocated with a surface pressure trough as well as a 1000 hPa thermal ridge, with MUCAPE values of ~1250 J kg^-1. The line forms within a 900-800 hPa frontogenesis maximum, between a region of warm air advection to the east and cold air advection to the west. A similar pattern exists for slowly decaying events, though the average MUCAPE is only ~750 J kg^-1, and the magnitudes of frontogenesis and temperature advection are greater than for decay events. For maintaining events, the synoptic pattern is different. Upon reaching the coast, the convection is collocated with a 1000 hPa baroclinic zone, while the surface trough is ~250-300 km to the west. Maintaining events form in a localized region of warm air advection, with little 900-800 hPa frontogenesis and MUCAPE of ~1000 J kg^-1.

To understand the differing processes between decaying and maintaining QLCS events, 2 events will be contrasted: the 23 July 2002 decaying event and the 31 May 2002 maintaining event. These cases form under similar synoptic conditions with one important difference; there is 900 hPa warm air advection ahead of the convection during the 31 May event, with cold air advection during the 23 July event, similar to the composites. Using Weather Research and Forecasting (WRF) simulations down to 2-km grid spacing with a 500 m nest, we will examine the role of the land-sea boundary in the evolution, including the source of air ingested into the storms as well as the role of the low-level temperature advection ahead of the system.