Radio Science, Volume 29, Number 5, Pages 1311-1320, September-October 1994 The severe depletion of turbulent echo power in precipitation observed using the Chung-Li VHF Doppler radar Yen-Hsyang Chu and Chun-Hsien Lin Institute of Space Science, National Central University, Chung-Li, Taiwan Abstract. Earlier investigationsof atmosphericprecipitation made with a VHF radar demonstrate that below the level of melting (i.e., 0øC isotherm) the VHF echo intensitiesfrom precipitation particles are generally much weaker than those from turbulent refractivity fluctuations by about 20-30 dB. However, in this article, using Chung-Li VHF radar data, the precipitation echoes in the height range from about 2.5 to 4 km, which is below the melting level during the seasonof spring over the Taiwan area, may far exceed the turbulent returns in intensity by about 15 dB, contradicting earlier observations. A comparison of the echo power profiles of the precipitation with those of refractivity fluctuations reveals that the weak backscatter from refractivity can be attributed to abnormally severe depletion of turbulent echo power in a convective cloud with fairly intense upward air motion and moderate precipitation. Furthermore, the correlation between hydrometeor terminal velocity and precipitation echo power is positive, while the air motion and turbulent echo power are anticorrelated. On the basis of these observationsa plausible mechanismis proposed to explain the abnormal depletion of refractivity echo power. 1. Introduction In the last two decades, VHF radars have been shown to be very important ground-based instru- ments in remote sensing of the troposphere, strato- sphere, and mesosphere. A number of important atmospheric parameters, such as three-dimensional wind velocity [Balsley and Gage, 1980; Rottger, 1981], tropopause height [Gage and Green, 1982; Gage et al., 1986], vertical wind shear [Sato and Woodman, 1982], turbulent strength [Hocking, 1985], aspect sensitivity [Tsuda et al., 1986; Wood- man and Chu, 1989], tropospheric and lower strato- spheric temperature profile observed in combina- tion with RASS (radio acoustic sounding system) [Masuda, 1988], etc., have been measured success- fully with VHF radars by many investigators. Re- cently, a number of experiments have been con- ducted with various VHF radars to observe atmospheric precipitation. Their results indicate that VHF radars have a significant capability for atmospheric precipitation measurements. For ex- ample, Fukao et al. [ 1985] investigatedthe behavior of precipitation in a cold front with a middle and upper atmosphere (MU) VHF radar and presented Copyright 1994 by the American Geophysical Union. Paper number 94RS01163. 0048-6604/94/94RS-01163508.00 average terminal velocity of raindrops below the melting layer. Wakasugi et al. [1986, 1987] deduced the size distribution of raindrops from the Doppler spectrafrom precipitation by using MU VHF radar. Larsen and Rottger [1987] reported the results of observing precipitation within a thunderstorm em- ploying the soundingsystem (SOUSY) VHF radar. Chu et al. [1991] reported a pronounced bright band in the echo power profile of typhoon precipitation observed by Chung-Li VHF radar and studied its general behavior in detail. All of these investiga- tions show that VHF radars not only can be em- ployed to observe atmospheric precipitation, but also can obtain more information about the precip- itating atmosphere than the conventional micro- wave meteorological radar can. VHF radar returns from precipitation are appar- ently composed of echoes from both precipitation particles and atmospheric refractivity fluctuations. According to the earlier observations [Fukao et al., 1985; Wakasugi et al., 1985; Larsen and Rottger, 1987; Chu et al., 1991], the VHF echoes below the melting level from precipitation particles are gener- ally much weaker than those from atmospheric refractivity fluctuations by about 15-30 dB, while in the height range of the bright band the precipitation echoes may be comparable to or slightly stronger than the latter by 3-5 dB. In view of the fact that the 1311