A Comparison of Microwave Techniques for Measuring Rainfall

Abstract

Many hydrological and other scientific problems require nearly instantaneous measurements of rainfall rate. The primary purpose of this paper is to evaluate within a common framework a large number of techniques for nearly instantaneous microwave measurements of rainfall and to determine the range of rainfall rates best suited to the various techniques and estimators. The physical basis of a technique as transformed by measurement imperfections of real instruments determines the ultimate performance capability of any microwave rain estimator. While many of the defects of the measurement process apply to all techniques, the physics behind each estimator differs. A method is presented for objectively evaluating the physical bases of the techniques and for quantifying estimator performance for perfect instruments. These results, which represent the best possible performance expectations, are then tempered by standard measurement errors to yield more realistic results. Analysis demonstrates that in general the minimization of rainfall estimate errors over a wide range of rainfall rates requires the simultaneous application of more than one microwave rainfall measurement technique. While this theoretical exercise provides useful guidance, measurement realities highlight the need for definitive experimental studies.