THE SEVERE DEPLETION OF TURBULENT ECHO POWER IN PRECIPITATION OBSERVED USING THE CHUNG-LI VHF DOPPLER RADAR

Earlier investigations of atmospheric precipitation made with a VHF radar demonstrate that below the level of melting (i.e., 0 degrees C isotherm) the VHF echo intensities from 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 season of 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 observations a plausible mechanism is proposed to explain the abnormal depletion of refractivity echo power.