Mesoscale Evolution of an Extreme Precipitation Event over Long Island on 13 August 2014
A realistic WRF member at 1-km grid spacing was obtained, in which over 285 mm of precipitation fell in a narrow band at about the same locations as observed. We will highlight some of the low-level forcing and vertical circulations associated with the rainfall in this successful run and compare them with observations. Both radar and surface observations as well as 1-km WRF simulations illustrate a weak meso-low near Long Island that enhanced the low-level convergence, upward motion, and rain rates over this region. The mean storm motion was parallel to the orientation of the front, resulting in cells “training” over the same location for a few hours. The importance of latent heating and cooling on this evolution explored using WRF runs in which these processes are turned off in the model. Many WRF members using different NCEP analyses as initial conditions and physics underpredict the precipitation by a factor of two. We will show that the size of the nested domain has a significant impact on the evolution of the precipitation and the subsequent evolution of the intense precipitation band. A larger explicit precipitation domain (3-km domain) results in more spurious model convection forming to the south along the warm front during the first 6 hours of the simulation. This in turn perturbs (weakens) the low-level jet helping of transport moisture and instability to the location where cells initiate and grow over Long Island.