Importance of no-rain measurements on the comparison of radar and rain gauge rain rate

This study evaluated four possible cases of comparing radar and rain gauge rain rate for the detection of mean-field bias. These four cases, or detection designs, consider in this study are: (1) design 1-uses all the data sets available, including zero radar rain rate and zero rain gauge rain rate, (2) design 2-uses the data sets of positive radar rain rate and zero or positive rain gauge rain rate, (3) design 3-uses the data sets of zero or positive radar rain rate and positive rain gauge rain rate and (4) design 4-uses the data sets of positive radar rain rate and positive rain gauge rain rate. A theoretical review of these four detection designs showed that only the design 1 causes no design bias, but designs 2, 3 and 4 can cause positive, negative and negative design biases, respectively. This theoretical result was also verified by applying these four designs to the rain rate field generated by a multi-dimensional rain rate model, as well as to that of the Mt Gwanak radar in Korea. The results from both applications showed that especially the design 4, which is generally used for the detection of mean-field bias of radar rain rate, causes a serious design bias; therefore, is inappropriate as a design for detecting the mean-field bias of radar rain rate.