Anomalous propagation effects on weather radar beam blockage corrections

Weather radars operating in complex orographic areas usually suffer from partial or total beam blockage caused by surrounding mountains. This shielding effect may restrict seriuosly 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, specially if quantitative rainfall estimations are required. In this paper we examine the effect of changing the radar beam propagation conditions upon the blockage correction. Particular results for the Vallirana weather radar, located near Barcelona (NE Spain), are presented.

Propagation conditions have been calculated from 12 Z radiosonde observations collected in Barcelona. After quality controlling the data, 862 radiosoundings were used to calculate duct occurrence and vertical refractivity gradients (VRG) for the layers 0 to 1 km and 0 to 4 km above sea level. The mode value of the VRG for the first 1000 m was –40 km-1 (standard propagation), the maxima and minima were –119 km-1 and –15 km-1 , respectively, and 2% of the cases presented VRG below –90 km-1. Ducting layers within the first 1000 m appeared in 37% of the cases and 60% of them had VRG below 300 km-1.

To evaluate the effects of anomalous propagation, the partial beam blocking correction scheme used in the NEXRAD Precipitation Processing System has been considered. This scheme is applied to beams partially shielded between 10% and 60% and consists of modifying equivalent reflectivity factor measurements by adding 1 to 4 dBZ depending on the degree of occultation. It is used with other corrections such as a test on the vertical echo continuity.

A number of clutter targets surrounding the Vallirana radar were selected to see the effects of varying the propagation conditions. The heights of the bottom and top of the main lobe radar beam were calculated at each site to evaluate the existence of total, partial or no beam blockage. Five clutter targets which presented partial beam blockage under normal conditions were chosen to examine the effects of changing the VRG. Assuming an homogeneous VRG for the whole radar beam and substituting the observed extreme VRG values, the differences found in occultation compared to standard propagation ranged from 5% to 20% (1 to 3 dBZ). It should be noted that if a more realistic heterogenous VRG were used to calculate the occultation, ducting layer effects would cause even greater differences.

Therefore, under anaprop partial beam blockage corrections based in assuming standard propagation may lead to wrong results. For instance, extreme cases where the partial beam blockage is incremented significantly but does not reach 60% would not be detected in a vertical echo continuity test but may produce a wrong correction of 4 dBZ. To detect such cases, information about the observed VRG in the radar coverage area, if available, might be incorporated in the correction schemes as a quality control.