WSR-88D Derived Rainfall Distributions in Hurricane Danny (1997)

A tropical cyclone (TC) poses a significant quantitative precipitation forecast (QPF) problem as evidenced by the recent tragic loss of life and property from rainfall during Hurricanes Mitch (1998) and Floyd (1999). Improved QPF is one of the primary objectives of the U. S. Weather Research Program (USWRP), and a specific goal of the Hurricane at Landfall effort under USWRP. Estimates of rainfall based on radar and other remote sensors offer promising avenues for improvement. The current level of QPF skill 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. While this "rule of thumb" provides a reasonable estimate of the peak storm total rain, it provides no information about the distribution of rain in space or time. There is also no adjustment in the rule for storm intensity, topography, or other dynamical or microphysical parameters. A major stumbling block to improving over this simple "rule of thumb" is a lack of a comprehensive climatology of TC precipitation, i.e., a description of the distribution of rain in space and time.

This study focuses on the distribution of WSR-88D radar-derived rain in space and time during Hurricane Danny from 17-21 September 1997. The WSR-88D radars provide 1-h mean rain estimates (R) over a domain extending to 230 km range from the radar with a 4 x 4 km resolution. Over the four days Danny, a category 1 hurricane, was visible from four different WSR-88D radars as it tracked slowly east-northeastward along the Gulf coast. The R estimates from the four radars provide an excellent opportunity to estimate the TC precipitation distribution in space and time as the storm passed along the Louisiana, Mississippi and Alabama coasts. First, R values from each radar are mapped into storm relative coordinates. The radial distribution of the mean R is calculated in 10 km radial bands out to 300 km from the storm center for each hour and for the total four-day period. The probability distribution of R is also computed for each 10 km radial band in 1 dBR (10 log₁₀R) steps from 0.3-300 mm h^-1 (-5 to 25 dBR) for each hour and the total four-day period. These distributions in range and intensity are compared to results from earlier studies done using rain gage estimates.