Negative correlation between terminal velocity and VHF radar reflectivity: observation and plausible explanation

摘要

The correlation between precipitation backscatter P (or radar reflectivityZ) and reflectivity-weighted terminal velocity V_t has long been thoughtto be positive. Namely, the larger the magnitude of the terminal velocityis, the stronger the radar reflectivity will be, and vice versa. However, wewill show in this article the observational evidences of negative V_t–Pcorrelation made with the Chung-Li VHF radar. It is found that the negativeV_t–P correlation can occur in the regions from close to ground to wellabove the melting layer. In addition, there is a strong tendency for thenegative t–P correlation to occur around the bright band region. Inlight of the fact that the conventional model of single drop sizedistribution cannot explain this negative correlation, it is proposed thatthe drop size distribution responsible for the negative V_t–Zcorrelation is composed of two Gamma drop size distributions with respectivemean terminal velocities and radar reflectivities. The precipitationparticles of these two distributions are assumed to dynamically interact inthe way that the total numbers of the precipitation particles of the twoGamma distributions are varied and their reflectivities are also changedaccordingly. Theoretical analysis shows that the key factor determining thesign of the V_t–Z correlation is the ratio of the difference betweenrelative changes in the reflectivities of the two Gamma drop sizedistributions to the change in the total reflectivity. The V_t–Zcorrelation is negative (or positive) if the ratio is positive (ornegative). From these results, it follows that the V_t–Z correlationcould be considered to be the result of the redistribution of the radarreflectivies of the two Gamma drop size distributions caused by theinteraction of the precipitation particles between them. Differentinteraction processes of the precipitation particles, such as break-up andcoalescence, could give rise to the same V_t–Z correlation, depending onthe net change in the total reflectivity. In addition, the results also showthat the same interaction process might give opposite V_t–Zcorrelations. Therefore, great caution is advised when the V_t–Zcorrelation is employed to interpret the precipitation process.