Differential propagation phase shift and rainfall rate estimation

This paper presents two methods for computing differential propagation phase shift (ϕ DP ) using time series data form a coherent radar with alternate switching between two linear but orthogonal polarizations. An analysis of the statistical error in ϕ DP estimate shows that ϕ DP can be estimated with less than 0.5° standard error using time and range averaging. To evaluate the usefulness of ϕ DP for estimating rainfall rate (R) vis‐a‐vis the Z DR method, a discussion on the sensitivity of R to standard errors in ϕ DP as well as Z H and Z DR is presented. It is shown that the relation, differential propagation phase constant Δϕ versus R , is relatively insensitive to drop size distribution (DSD) variations and thus can yield more accurate R estimate, even when a direct relationship between Δϕ and R is assumed. However, standard error in Δϕ estimate causes large inaccuracies of R at low values ( R < 50 mmh −1 ), thus limiting its usefulness to higher rainfall rates. It is also shown that Δϕ can be used as a third remote measurable in conjunction with Z H and Z DR to determine a three parameter gamma DSD. Another use of Δϕ is likely to be in hydrometeor type identification, especially hail in severe storms.