The impact of coastal boundaries and small hills on the precipitation distribution across southern Connecticut and Long Island, New York

The modification of precipitation by the coastal land areas of Long Island (LI), New York, and southern Connecticut (CT) is examined for an extratropical cyclone over the northeast United States on 1 December 2004, which produced strong southerly flow (15-30 m s(-1)) below 900 mb and heavy precipitation over LI. The differential surface roughness at the coast and the hills of LI (30-80 m) and southern CT (100-250 m) enhanced the surface precipitation by 30%-50% over these regions compared with the nearby water region of LI Sound. The three-dimensional precipitation structures are shown using composite Weather Surveillance Radar-1988 Doppler radar data interpolated to a Cartesian grid, which is compared with a 4-km simulation using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). As the low-level stratification and flow increased at low levels, the MM5 produced a terrain-forced gravity wave over LI and CT upward through 6 km MSL. Precipitation enhancement (2-3 dBZ) occurred from the surface upward to around the freezing level (3 km MSL) across central LI and southern CT, while there was a localized precipitation minimum over LI Sound. A factor separation on a few sensitivity MM5 runs was performed to isolate the impact of small hills and differential friction across the LI coastline. Both the hills and frictional effects have similar contributions to the total precipitation enhancement and the vertical circulations below 3 km. The hills of LI enhanced the gravity wave circulations slightly more than the differential friction above 3 km, while there was little flow and precipitation interaction between the hills and differential friction. A sensitivity simulation without an ice/snow cloud above 3 km MSL revealed that the seeder-feeder process enhanced surface precipitation by about a factor of 4.