Reflectivity, rain rate, and kinetic energy flux relationships based on raindrop spectra

The relationships between radar reflectivity factor Z rainfall rate R, and rainfall kinetic energy flux E were analyzed based on a multiyear raindrop spectra dataset recorded by a Joss-Waldvogel disdrometer in the Goodwin Cre ek research watershed in northern Mississippi. Particular attention was giv en to the climatological Variability of the relationships and the uncertain ty by which one rainfall parameter may be estimated from another. Substanti al variability for the coefficients of a power-law relationship Y = A(b)X(b ) between two rainfall parameters Y and X (where Y and X may stand for any paired combination of Z, R, and E) was found. The variability of the expone nt b, however, was small enough to support approaches of climatologically f ixed exponents to simplify radar rainfall estimation procedures. The multip licative factor A(b) should typically be adjusted on a storm basis. The unc ertainty of the estimation of one rainfall parameter from another, being a function of the difference in weighting of the drop size by the two paramet ers and the variability of raindrop spectra, was found to be approximately 50% for the Z-R relation, 40% for the GR relation, and 25% for the ZE relat ion. For extreme precipitation intensities (R greater than or equal to 100 mm h(-1)), this drop spectra-based uncertainty reduced to approximately 20% for all three relationships. The results exhibited significant sensitivity to the choice of method applied to determine the relationship between two rainfall parameters. Appreciable sensitivity of the relationship between ra infall parameters (i.e., power-law coefficients and drop spectra-based unce rtainty) to the number of raindrops registered per 1-min drop spectrum was also found.