THE INVESTIGATIONS OF THE ATMOSPHERIC PRECIPITATIONS BY USING CHUNG-LI VHF RADAR

In this paper the results of precipitation observed by Chung-Li VHF radar during the passage of typhoon Susan through the Taiwan area on June 2, 1988, are presented. We find that VHF radar can not only observe the falling speed of the rain drops but can also detect that of the ice particles. The observational results show that the former is about 6-8 m/s, while the latter is about 1 m/s, consistent with those observed by the conventional Doppler meteorological radar. The echo power due to precipitation is generally far smaller than that due to atmospheric refractivity fluctuations. However, it is enhanced strikingly at the height of melting layer (about 5 km in Taiwan area), and its value can be greater than that due to refractivity fluctuations by about 5 dB. Moreover, it also shows that the feature of bright band only occurs in the echo power profile of precipitation, and it cannot be observed in that of atmospheric refractivity. The echo power profiles of the vertical and oblique beams (with zenith angle 17-degrees) are also examined. They indicate that there is no aspect sensitivity for the echoes scattered from the atmospheric refractivity fluctuations during the precipitating environment. However, for the echo power of precipitation the feature of aspect sensitivity does exist above the melting layer and disappears below this altitude. The correlation between the horizontal wind speed and the Doppler spectral width of precipitation is found to be fairly high, with a correlation coefficient of 0.86. This suggests that the beam-broadening effect due to the advection of horizontal wind may be one of the important factors contributed to the precipitation spectral width and has to be taken into consideration when the precipitation spectra are analyzed.