Errors of Remapping of Radar Estimates onto Cartesian Coordinates
Friday, 19 December 2014
Recent upgrades to operational radar rainfall products in terms of quality and resolution call for re-examination of the factors that contribute to the uncertainty of radar rainfall estimation. Remapping or gridding of radar polar observations onto Cartesian coordinates is implemented using various methods, and is often applied when radar estimates are compared against rain gauge observations, in hydrologic applications, or for merging data from different radars. However, assuming perfect radar observations, many of the widely used remapping methodologies do not conserve mass for the rainfall rate field. Research has suggested that optimal remapping should select all polar bins falling within or intersecting a Cartesian grid and assign them weights based on the proportion of each individual bin’s area falling within the grid. However, to reduce computational demand practitioners use a variety of approximate remapping approaches. The most popular approximate approaches used are those based on extracting information from radar bins whose centers fall within a certain distance from the center of the Cartesian grid. This paper introduces a mass-conserving method for remapping, which we call “precise remapping”, and evaluates it by comparing against two other commonly used remapping methods based on areal weighting and distance. Results show that the choice of the remapping method can lead to large errors in grid-averaged rainfall accumulations.