On the Radar Detectability of Distant Precipitation

Abstract

Weather radar has played a significant role in detection of precipitation area, however, it has a weak point that its detectability lowers according as the distance to the precipitation increases. The lowering of detectability is due mainly to the following four factors.
1) Spread of microwave with its propagation
2) Attenuation of microwave by atmospheric gases, clouds and precipitation
3) Increasing of beam height with distance
4) Decreasing of “fraction of beam filled” with distance Among the above factors, 3) and 4) are difficult to examine, related to the three dimentional reflectivity distribution within target storm. Then the author tries to estimate statistically the effect of the respective factors on the lowering of detectability.
It seems likely that the effect of “increasing of beam height with distance” is negligible within ca. 130km (the minimum attainable beam height of 2000m) on an average. And it would seem that within ca. 130km the lowering of detectability is due mainly to the effects of “spread of microwave” and “attenuation by atmospheric gases ”.