Areal rainfall estimates using differential phase

Radar polarimetric methods for rainfall measurements have received increasing attention in recent years. The one based on the estimate of specific differential phase K_DP uses the relation: R=aK_DP^b where R is rain rate. This method has several advantages compared to the conventional one which utilizes radar reflectivity factor Z. Differential phase is immune to radar miscalibration, microwave attenuation, partial beam blockage. It is less contaminated by hail and is less affected by drop size distribution variations. Because K_DP is a radial derivative of the total differential phase Φ_DP(K_DP=½dΦ_DP/dr), and the exponent b in the above equation is close to unity, the rainfall integrated over the radial interval (r₁,r₂) is approximately proportional to the difference between Φ_DP values at the ends of the interval: Φ_DP(r₂)-Φ_DP(r₁). Similarly, areal rainfall is determined by the values of differential phase on the areal contour and, therefore, is not affected by distribution of differential phase inside the area of interest. This idea was first suggested by Raghavan and Chandrasekar (1994) as a useful technique with potential to obtain Area-Time Integral rain accumulations. In this paper the authors examine this technique using the data obtained with the 10-cm wavelength polarimetric radar and rain gauge data from the Agricultural Research Service (ARS) micronetwork in Oklahoma