85th AMS Annual Meeting

Sunday, 9 January 2005
Warm Season Climatology of Convective Evolution Over the Coastal Northeast U.S
Michael Charles, Stony Brook University, Stony Brook, NY; and B. A. Colle and J. S. Tongue
The rapid evolution of convective systems approaching the coastal metropolitan region of New York City (NYC) is a significant forecast problem. The purpose of this study is to show the distribution and evolution of summertime convection over the coastal Northeast U.S. in order to improve the forecasting of severe weather. This study constructs a cloud to ground lightning climatology over the Northeast, a composite of the synoptic-scale flow, and a few case examples.

A five-year (2000-2004) climatology of cloud to ground lightning data was compiled for June and August over the Northeastern U.S. During some years (2000 and 2004) there was a minimum in lightning strikes along the coast in June as compared to interior locations, which suggests that the relatively cool and more stable marine boundary layer (MBL) weakened the convection. However, some years did not show much marine impact in June (2002 and 2003); therefore, there may be other factors that helped to maintain convection near the coast, such as baroclinic forcing and instability aloft.

To begin separating these factors, lightning cases were defined as those days that New Jersey, southeast New York or Connecticut received >10 lightning strikes. These were then divided into two types of convection: frontally forced ( 100 km ahead of a cold front or 200 km ahead of a warm front) and non-frontally-forced. For the non-frontally-forced cases, a distinct gradient in strikes exists near the coast in June and August, suggesting that non-frontally-forced convection frequently weakens as it interacts with the MBL. In contrast, the frontally-forced cases have little decrease in lightning toward the coast.

To relate the results with upper-level flow, NCEP/NCAR reanalyses of 700mb monthly mean heights from June and August were analyzed. The climatological average flow regime for June is nearly zonal aloft over the Northeast. This occurred in June 2000, 2001 and 2004, and was associated with a significant coastal gradient in lightning. However, when there was an anomalous trough over northern New England (June 2002) or an upstream trough centered over the Great Lakes (June 2003), which were associated with several frontal events, there was a significant amount of coastal lightning.

A few case examples will be presented to illustrate the different types of convection. Future work will involve examining convective mode (organized/long-lived convection vs. disorganized/short-lived convection) as a possible factor in lightning distribution, as well as mesoscale model simulations of events.

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