Quantitative Precipitation Forecasting in Hurricanes: Issues and Opportunities

The tropical cyclone (TC) poses a significant quantitative precipitation forecast (QPF) problem as evidenced by the recent tragic loss of life and property from rainfall in Hurricanes Mitch (1998) and Floyd (1999). Estimates of rainfall based on radar data, satellite imagery, and numerical models offer promising avenues for improvement, however, the prediction of TC rainfall beyond 6 h has little skill for all but the most general indications. The simplest form of QPF for TCs in the United States is to predict the peak storm rainfall amount using the "rule of thumb" first proposed by Kraft in the late 1950s, which states that the maximum storm rainfall amount will be 100 inches divided by the storm forward motion in knots. This rule of thumb provides reasonable estimates of the peak storm total rain, competitive with that from numerical models.

Improved QPF in TCs is one of the primary goals of the U. S. Weather Research Program (USWRP) effort on TC landfall. Improved QPF is also one of the three main foci of the USWRP. The varied nature of precipitation makes the QPF topic very complex. Convection may be forced by many sources: dynamical, local thermal effects or density currents, topography, and radiative effects. Although much of the significant precipitation occurs in conjunction with these convective clouds, stratiform clouds also account for significant precipitation accumulations over extended intervals. All of these mechanisms are active in TCs, yet the vortex structure acts to dynamically constrain the smaller scale circulations that often confound better QPF. In short, the TC provides a perfect laboratory for testing many of the QPF techniques. If we can't improve QPF in TCs then QPF in more typical meteorological situations may be hopeless.

Opportunities to improve QPF in landfalling TCs seem particularly promising because of improved understanding of TC precipitation mechanisms, quantitative precipitation estimation (QPE) from remote sensors, and improvements in horizontal resolution of operational and research models so that moist non-hydrostatic processes, which occur on small scales, may be represented better. However, a major stumbling block to improved QPF in TCs is a lack of a comprehensive climatology of TC precipitation, a description of the distribution of rain in space and time. Few precipitation climatologies exist for TCs in the United States, and other TC basins have similarly limited climatologies. To date there has been no attempt to compare these limited climatologies over a basin or between basins. New QPE opportunities (e.g., WSR-88D, TRMM) offer a unique opportunity to develop TC rain climatologies for the US and globally. These climatologies can be used to validate numerical models and thereby improve QPF in TCs. Opportunities are identified and a strategy for comparison proposed.