Observations of precipitation associated with a cold front using a VHF wind profiler and a ground-based optical rain gauge

In this paper the VHF radar returns from hydrometeors and refractivity fluctuations associated with a cold front are analyzed. A composite analysis of the precipitation echo intensity and the vertical air velocity indicates that the vertical air velocity plays a vital role in the formation of the bright band. The observations show that the VHF radar reflectivity from precipitation at the height around the melting layer may be enhanced in the condition of weak vertical air velocity, while the bright band may be disrupted if the upward vertical air speed is a large as 1.2 m/s. The intense updraft may also diminish the echo intensity from refractivity fluctuations through the mechanism of turbulent mixing, as first suggested by Chu and Lin [1994]. The corresponding observational evidence will be presented and discussed in this paper. Comparing the time series of the VHF precipitation echo intensity aloft with that of the surface rainfall rate indicates there is a systematic time delay in the two, which increases approximately linearly with height. The drop-off rate of the time difference is approximately equal to the average fall velocity of the raindrop. A regression analysis of the precipitation data between range-corrected VHF radar reflectivity P_r and ground-based rainfall rate R using the equation P_r=αR^β is also made. It shows that β has a tendency to decrease with height, while α is nearly height-independent.