Variation of weather radar sensitivity at ground level and from space: case studies and possible causes

Abstract

Firstly, this paper summarizes the procedure for correcting the variation of sensitivity of radars at ground level, using a radar in space for reference, as already described in the literature. Around 10dB have to be added to the measured radar reflectivity Z (in mm6/m3), when increasing the range from 10 km to 100 km. In other words, instead of an r−2-dependence (where r is the range, i.e. the distance between the radar and the meteorological target), we find an apparent reduction proportional to r−3. A similar procedure is used to analyze the dependence of the TRMM radar, using the adjusted data of the radar at ground level for reference. We have to add around 3dB to the measured reflectivity, when increasing the distance from close to Nadir (10 km) to the edge of the swath (100 km). In other words, instead of a constant sensitivity over the swath of the satellite, we find a reduction of sensitivity with the range, proportional to r−0.3. Secondly, the paper illustrates possible causes of the systematic range dependence of both radars. The old, dominant cause - overshooting of precipitation - is already vastly discussed in literature. We describe the variability of the precipitation with three case studies. The variability of the precipitation is difficult to quantify. It is concluded to be responsible for the variable range dependence and for many disillusions with radar.