Quality of Ground-based WeatherRadar Observationsof Rainfall over the
Water Board District Regge and Dinkel of the Netherlands

Terence Darlington Mushore
Page No. 30-42

The relationships between radar reflectivity (Z) and rain-rate (R) adjusted to the climatology of the
Regge and Dinkel district were established using the window probability matching method. A
kernel of 3 by 3 pixels was used to spatially average radar reflectivity values coinciding with each
of the 9 rain-gauges for a time step of one hour. The relationships were established for a time
independent (bulk) calibration, for the seasonal calibration as well as for each of the years from
2006 to 2010 separately. The Z-R relationships obtained in each of the calibrations were compared
with the Marshall and Palmer Z-R relationship which is currently being used over the entire
Netherlands by the Royal Netherlands Meteorological Institute (KNMI). The reflectivity-rain-rate
relationships were found to vary from time to time over the period considered since different
relationships were obtained from each calibration. The variations are due to differences in rain
drop size distribution and hence predominant rainfall types in space and time. However, contrary
to the expectations, the Z-R relationships obtained using the window probability matching did not
improve the accuracy of radar rainfall estimation. The Regge and Dinkel area is located more than
100 km away from the de Bilt radar and this causes the unreliability of the rainfall estimates from
the radar (range degradation). The root mean square errors and mean absolute errors were higher
using the proposed Z-R relationships than when using the Marshall and Palmer Z-R relationship.
There is need to identify weather radar which is close to the Waterboard Regge and Dinkel district
and use it to monitor rainfall for the area. This may also imply the need to install weather radar
within the area to eliminate the impact of range degradation on accuracy of radar rainfall
estimates.
Keywords: windowprobability-matching, rain-rate, Z-R, errors, reflectivity, calibration, Regge
and Dinkel