Range resolution dependence of VHF radar returns from clear-air turbulence and precipitation

Abstract

With employing 1.5 h of the data observed by the Chung-Li VHF radar, the range resolution dependences of the VHF backscatter from refractivity fluctuation and precipitation are investigated in this article. It indicates that the atmospheric layer structure of refractivity seems to play a role in governing the range resolution dependence of clear-air turbulent echoes. Observations shows that the VHF clear-air echo power ratios for 4 to 2 μs pulse lengths are close to 3 dB in the middle or bottom side of the layer, while the ratios are significantly greater than 3 dB in the top side of the layer. The analysis of the precipitation echo power ratio for 4 to 2 ms pulse lengths shows that basically the ratios above 3.0 km are close to 3 dB, but enormously smaller than 3 dB below 3.0 km. The feature of extraordinarily small echo power ratios below 3.0 km is also observed for the radar returns from refractivity turbulence. The radar recovery effect is thought to be a primary factor responsible for the severe diminution of the echo power ratios at the lower altitudes. In addition, statistical analysis reveals that the range resolution effect on the first and second moments of the Doppler spectra for the radar echoes from clear-air turbulence and precipitation is insignificant and negligible. The dependences of the coefficient A and power B in the power-law approximation V t= AP B r to the terminal velocity V t and range-corrected echo power P r are examined theoretically and experimentally. The results show that the coefficient A (powers B) is inversely (positively) proportional to the range resolution, in a good agreement with the observations. Because of the strong dependence of coefficient A and power B on the radar pulse width, it suggests that great caution should be taken in comparing the power-law expressions V t= AP B r established from the radar returns obtained with different range resolutions.