Summertime Convective Rainfall in the New York City-New Jersey Metropolitan Region

Abstract

In this thesis, we examine the rainfall climatology of the New York City-New Jersey metropolitan region and investigate how urbanization, land-water boundaries, and complex terrain impact storm evolution. We develop long-term, high-resolution (1 km, 15 minutes) radar rainfall fields for the study region using the Hydro-NEXRAD algorithms and use these as a central tool for characterizing the regional rainfall climatology. We perform regional climate model simulations and sensitivity analyses using the Weather Research and Forecasting (WRF) model to examine the role of land surface processes in controlling spatial heterogeneities in the regional rainfall climatology. WRF simulations were performed for 5 warm seasons and also in case-study mode for severe thunderstorm systems that produce heavy rainfall over the New York City metropolitan region. We focus on severe thunderstorms that produce heavy rainfall through rainfall and lightning analyses for the 50 days with largest cloud-to-ground (CG) flash densities over the study region. A storm-tracking system is used with 3-D radar reflectivity fields for Lagrangian analyses of storm structure, motion, and evolution for the 50 severe thunderstorm days. Model and observational studies show that the NYC region is a hotspot for intense convection and heavy rainfall. These features are diminished for mean rainfall. Severe storms tend to propagate from southwest to northeast, along the axis of the Appalachian Mountains. For major thunderstorm systems, there is no evidence that storm elements split as they approach NYC, as suggested in previous studies for weaker storms. There is a systematic increase in storm area and, oftentimes, in measures of convective intensity as storms pass over NYC. These properties decrease as storm cells pass over the more

stable, marine environment. It is difficult to separate the effects of urbanization and land-water boundaries on storm evolution. We also examine the regional climatology of latent and sensible heat fluxes over the Princeton campus through analyses using long-term eddy covariance measurements and the Noah land-surface model (LSM). These analyses point to the critical role of heterogeneous surface fluxes on the regional water cycle, including the regional rainfall climatology.