Structure and Evolution of Rainfall in Numerically Simulated Landfalling Hurricanes

Abstract

Using an idealized landfalling model hurricane, the impact of different land surface characteristics on hurricane rainfall distribution before, during, and after landfall is investigated. Before landfall, maximum rainfall occurs on the right side of the storm track as a result of dry air intrusion from both the environmental flow behind the vortex and the land surface ahead of the vortex. These sources of dry air combine to destabilize the right side and stabilize the left side of the storm. Upon landfall, the rainfall maximum shifts to the left of the storm track over land, near the coast. Increased friction over land drives a region of convergence in the entire front half of the storm. While mean rainfall rates decrease, localized areas of large rainfall accumulations may occur as a result of this frictional forcing. Over land, the rainfall area broadens and mean rainfall rates decrease. No differences are detected in inner-core rainfall rates between cases, but outer-core rainfall rates and rainfall coverage increase with moister land surfaces. Hence, significant differences in rainfall accumulations occur depending on moisture availability of the land surface. Since reconnaissance planes cannot fly over land, forecasters are often forced to make extrapolations from reconnaissance data over water. They should take extreme caution in doing so, since rainfall distributions may change suddenly upon landfall as different forcing mechanisms take over.