Comparison of Radar against Gauges
The radars in De Bilt and Den Helder both record 288 pseudoCAPPI reflectivity images each day. The measured radar reflectivity factor Z of each pixel is converted to surface rainfall intensity R using the following equation:
The surface rainfall intensities are accumulated to obtain estimations for the 24-hour rainfall accumulation, i.e., from 08 UTC until 08 UTC the next day, for each pixel. The network of about 325 volunteers provides rainfall gauge accumulations over the same period. At 14 UTC, between 200 and 250 gauge observations of the previous day are available for further processing. Pairs of radar and gauge accumulations are collected by extracting for each gauge observation the value of the overlying radar pixel. The radar-gauge pairs with information on date, location in radar image, and range/azimuth from radar are collected in a "growing" Pair Table.
In further analyses of the radar-gauge accumulation pairs (R-G pairs), the following quantity is analyzed:
It has been found by Koistinen and Puhakka (1981) that the logarithm of the radar-gauge accumulation ratio is distributed according to a normal distribution. The known distribution of the RG quantity facilitates the characterization of the distribution radar-gauge pairs. The factor of 10 in RG puts the radar-gauge ratios on a decibel scale, which is convenient for interpretation of the results.
Once a day the 250 most recent radar-gauge pairs in the Pair Table with a gauge accumulation of at least 3 mm, a minimum range of 15 km and a maximum range of 200 km are selected. These radar-gauge pairs are randomly split into a verifcation set of 75 pairs and an adjustment set with the remaining pairs. The bias and the standard deviation of radar-gauge pairs is calculated using the verification set.
Only an adjustment of the radar-gauge pairs as a function of the range to the radar has currently been implemented. The adjustment radar-gauge pairs (in dB) are fitted to the following formula:
From this fit the Offset and the Curvature can be obtained at each day. The Offset contains information on the overall systematic deviations, while the Curvature contains information on the over/under estimation of precipitation by radar with increasing range.
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Hans Beekhuis