Estimating the Accuracy of Polarimetric Radar-Based Retrievals of Drop-Size Distribution Parameters and Rain Rate: An Application of Error Variance Separation Using Radar-Derived Spatial Correlations

The accuracy of retrieving the two drop size distribution (DSD) parameters, median volume diameter (D-0), and normalized intercept parameter (N-W), as well as rain rate (R), from polarimetric C-band radar data obtained during a cool-season, long-duration precipitation event in Huntsville, Alabama, is examined. The radar was operated in a special "near-dwelling" mode over two video disdrometers (2DVD) located 15 km away. The polarimetric radar based retrieval algorithms for the DSD parameters and rain rate were obtained from simulations using the 2DVD measurements of the DSD. A unique feature of this paper is the radar-based estimation of the spatial correlation functions of the two DSD parameters and rain rate that are used to estimate the "point-to-area" variance. A detailed error variance separation is performed, including the aforementioned point-to-area variance, along with variance components due to the retrieval algorithm error, radar measurement error, and disdrometer sampling error. The spatial decorrelation distance was found to be smallest for the R (4.5 km) and largest for D-0 (8.24 km). For log(10)(N-W), it was 7.22 km. The proportion of the variance of the difference between radar-based estimates and 2DVD measurements that could be explained by the aforementioned errors was 100%, 57%, and 73% for D-0, log(10)(N-W), and R, respectively. The overall accuracy of the radar-based retrievals for the particular precipitation event quantified in terms of the fractional standard deviation were estimated to be 6.8%, 6%, and 21% for D-0, log(10)(N-W), and R, respectively. The normalized bias was <1%. These correspond to time resolution of similar to 3 min and spatial resolution of similar to 1.5 km.