TRMM microwave imager rain estimates are used to quantify the spatial distribution of rainfall in tropical cyclones (TCs) over the global oceans. A total of 260 TCs were observed worldwide from 1 January 1998-31 December 2000, providing 2121 instantaneous TC precipitation observations. To examine the relationship between the storm intensity, its geographical location, and the rainfall distribution, the dataset is stratified into three intensity groups and six oceanic basins. The three intensity classes used in this study are tropical storms (TSs) with winds Ͻ33 m s Ϫ1 , category 1-2 hurricane-strength systems (CAT12) with winds from 34-48 m s Ϫ1 , and category 3-5 systems (CAT35) with winds Ͼ49 m s Ϫ1. The axisymmetric component of the TC rainfall is represented by the radial distribution of the azimuthal mean rainfall rates (R). The mean rainfall distribution is computed using 10-km annuli from the storm center to a 500-km radius. The azimuthal mean rain rates vary with storm intensity and from basin to basin. The maximum R is about 12 mm h Ϫ1 for CAT35, but decreases to 7 mm h Ϫ1 for CAT12, and to 3 mm h Ϫ1 for TS. The radius from the storm center of the maximum rainfall decreases with increasing storm intensity, from 50 km for TS to 35 km for CAT35 systems. The asymmetric component is determined by the first-order Fourier decomposition in a coordinate system relative to the storm motion. The asymmetry in TC rainfall varies significantly with both storm intensity and geographic locations. For the global average of all TCs, the maximum rainfall is located in the front quadrants. The location of the maximum rainfall shifts from the front-left quadrant for TS to the front-right for CAT35. The amplitude of the asymmetry varies with intensity as well; TS shows a larger asymmetry than CAT12 and CAT35. These global TC rainfall distributions and variability observed in various ocean basins should help to improve TC rainfall forecasting worldwide.

This study documents a new parametric hurricane rainfall prediction scheme, based on the rainfall climatology and persistence model (R-CLIPER) used operationally in the Atlantic Ocean basin to forecast rainfall accumulations. Although R-CLIPER has shown skill at estimating the mean amplitude of rainfall across the storm track, one underlying limitation is that it assumes that hurricanes produce rain fields that are azimuthally symmetric. The new implementations described here take into account the effect of shear and topography on the rainfall distribution through the use of parametric representations of these processes. Shear affects the hurricane rainfall by introducing spatial asymmetries, which can be reasonably well modeled to first order using a Fourier decomposition. The effect of topography is modeled by evaluating changes in elevation of flow parcels within the storm circulation between time steps and correcting the rainfall field in proportion to those changes. Effects modeled in R-CLIPER and those from shear and topography are combined in a new model called the Parametric Hurricane Rainfall Model (PHRaM). Comparisons of rainfall accumulations predicted from the operational R-CLIPER model, PHRaM, and radar-derived observations show some improvement in the spatial distribution and amplitude of rainfall when shear is accounted for and significant improvements when both shear and topography are modeled.

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Atlantic hurricane activity has been particularly high since 1995, with nine seasons recording more hurricanes than the long‐term average. The recognition that current activity is not the same as the long‐term historical average means that, for the purpose of catastrophe risk assessment, we need to be explicit as to the time period over which expected activity is evaluated. We have chosen to explore activities over a 5‐yr forward looking time window, which bounds the range of business applications for which catastrophe loss models are employed. This time horizon is also shorter than the pattern of past multidecadal periods of high and low activity. The methodology used to assess activity rates for the next 5 yr contains a blend of statistical analyses and an expert elicitation. A panel of experts was convened to discuss expected levels of activity for the next 5 yr across the Atlantic, along the U.S. and Caribbean coasts. The results indicate hurricane activities along the U.S. coast are expected to be between 20 and 35% higher than the long‐term average, depending on storm intensity. The implementation of these findings has included work to determine how increases are distributed by track type and by region, and the impacts on expected losses.