The Effect of Shear and Topography on Rainfall Forecasting with R-CLIPER

This study documents enhancements to the rainfall climatology and persistence model (R-CLIPER) used operationally in the Atlantic to forecast hurricane rainfall accumulations. Although R-CLIPER has shown skill at estimating the mean amplitude of rainfall across the storm track, it assumes the storms to be azimuthally symmetric. The new implementations take into account the effect of shear and topography, both known to significantly affect the spatial distribution of rainfall in a tropical cyclone (TC). The shear effect is modeled by introducing spatial asymmetries in the rainfall estimates, based on findings described in previous studies, while the effect of topography is modeled by evaluating the flow advection along the terrain slopes. Comparisons of rainfall accumulations predicted from the operational R-CLIPER model, the new enhanced R-CLIPER tool, and observations show significant improvements in the spatial distribution and amplitude of rainfall when both shear and topography are accounted for.

The new model has numerous potential applications. First, the model with shear and topographic enhancements will be tested at the National Hurricane Center (NHC) during the 2006 season. The model outputs also show value for the assessment of financial losses associated to TC inland flooding. The modeled rainfall can for example be used as input to hydrological models to determine flood depths, or can directly be linked to knowledge on historical flood losses (through the use of insurance claims). The rainfall model as well as some of the applications will be described in this paper.