Understanding precipitation enhancement over Long Island, NY using WSR-88D and high resolution simulations

Precipitation enhancement over small hills has been explained through a seeder-feeder mechanism, in which ice particles falling from aloft melt and grow via collision and coalescence within a feeder cloud of enhanced droplet concentrations over a hill. There are numerous hills over the Northeast United States, where this process has been documented to occur, but precipitation enhancement has also been observed just inland of the coast in areas of more limited topography, possibly because of frictional flow convergence between water and land. This study explores the precipitation enhancement that occurred over Long Island, NY on 1 December 2004. About 25 cm of rain fell over Long Island during a 6-h period of strong (15-20 m/s) southerly flow to the east of a developing extratropical cyclone, as compared to about 12 cm of rainfall over the adjacent waters. This region is unique since both small (25-50 m) hills and complex land-water boundaries may impact the precipitation distribution across the island.

This study utilizes the WSR-88D radar data (level-2) from Upton, NY (KOKX) over eastern Long Island. The radar volumes were interpolated to constant height levels every 500 m, and storm-total reflectivity averages and frequency plots were computed. During this event there was precipitation enhancement over Long Island from the surface to the freezing level (3-km), with the maximum average reflectivity over the small hills. The PSU-NCAR MM5 was run at 1.33-km grid spacing to determine the mechanisms for the precipitation enhancement. The enhanced precipitation over the 25-50 m hills in the control run suggests a seeder-feeder mechanism, but the depth of the enhancement (to 3 km) was much higher than the terrain height, since strong (20-30 m/s) southerly flow generated a weak vertically-propagating gravity wave and deep lift. Comparing a simulation using flat land (no hills) over Long Island with an all-water run suggests that 50% of the precipitation enhancement over Long Island was also the result of low-level frictional convergence. A simulation with hills but no gradients in surface drag between land and water also resulted in a weaker gravity wave, and 40% less precipitation than the control run, which suggests a synergistic interaction between the frictional convergence and enhanced gravity wave circulation over the island.