Uncertainties in radar echo top heights used for hail detection

Most operational hail detection algorithms for single-polarization radars are based on the analysis of the vertical profiles of radar reflectivity. At KNMI (Royal Netherlands Meteorological Institute) and RMI (Royal Meteorological Institute of Belgium) the probability of hail is derived from the height of the freezing level and the 45-dBZ radar echo top height (maximum height of the 45-dBZ echo). Echo tops are affected by errors on the measured reflectivity itself and by errors on the height assigned to these reflectivities. In this study we investigate the quality of radar echo top heights as a function of range and we discuss the implications for hail detection.

The method is based on the comparison between reflectivity measurements from two radars on the vertical cross section extending between these radars. In a first step, sampling errors related to the radar Volume Coverage Patterns are analyzed using idealized storm profiles. Subsequently, real reflectivity data for twenty-five thunderstorm episodes are compared. It is found that the quality of the maximum reflectivity measurements strongly deteriorates with range and that about half of this degradation can be attributed to overshooting effects. Height assignment differences between the two radars are limited to about 0.5 km. Errors on the reflectivity measurements strongly affect the frequency of 45-dBZ threshold exceedances. However, once the threshold is exceeded, errors in measuring the 45-dBZ echo top heights generally affect the derived probability of hail by less than 20 %.