Dynamical Weather Radar Beam Blockage Correction

Weather radars operating in complex orographic areas usually suffer from partial or total beam blockage caused by surrounding mountains. This shielding effect may restrict seriously the use of the lowest antenna elevation angles which provide the most useful information for rainfall rate estimation at ground level. Therefore, beam blockage correction schemes have to be applied in order to minimize the effect of topography, especially if quantitative rainfall estimations are required.

Each CINRAD(ChIna New generation RADar) site has a unique, hybrid scan “look-up table” in the form of a polar grid at 1 km by one azimuthal degree defining which one of the four lowest elevation angles to use at each polar grid point. It is generated once for each CINRAD using off-line procedures. The inherent assumptions are that the radar always scans at the predefined elevation angles, and the beam propagates according to standard refraction.

However, because the beam does not always propagate according to standard refraction, the partial occultation corrections are not always appropriate. A dynamical weather radar beam blockage correction method was presented in this paper. As long as the obscuration percentages were greater than 10% (an adaptable parameter) the reflectivity value in the obscured range bin was replaced by a new value. In order to calculate the new value, a multiplicative factor were calculated determined based on 1.5° and 0.5° elevation reflectivity value which were closest to the obscured range bin and themselves were not been obscured. Then the new value were calculated with the multiplicative factor times the 1.5° reflectivity value corresponding to the 0.5° obscured bin.

By comparing the radar rainfall estimates based on the dynamical method with the estimates based on the beam blockage correction method which using a unique hybrid scan "lookup table" for each radar (called the static method in this paper) using rain gauge mean-areal rainfall measurements from 01:00 UTC 20 June 2003 to 00:00 UTC 12 July 2003 for Xianghongdian subcatchment (1410km²) in the upper reaches of Pihe river, Anhui Province, P.R.C., it was shown that the radar rainfall estimates derived from the dynamical beam blockage correction method were better than the estimates from the static method.

Keywords: Weather radar, Quantitative Precipitation Estimates, Radar Beam Blockage Correction