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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 27, issue 4
Ann. Geophys., 27, 1631–1642, 2009
https://doi.org/10.5194/angeo-27-1631-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: 11th International Workshop on Technical and Scientific Aspects...

Ann. Geophys., 27, 1631–1642, 2009
https://doi.org/10.5194/angeo-27-1631-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  06 Apr 2009

06 Apr 2009

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

C.-L. Su, Y.-H. Chu, and I.-Y. Lo C.-L. Su et al.
  • Institute of Space Science, National Central University, Chung-Li, R.O.C. Taiwan

Abstract. The correlation between precipitation backscatter P (or radar reflectivity Z) and reflectivity-weighted terminal velocity Vt has long been thought to be positive. Namely, the larger the magnitude of the terminal velocity is, the stronger the radar reflectivity will be, and vice versa. However, we will show in this article the observational evidences of negative VtP correlation made with the Chung-Li VHF radar. It is found that the negative VtP correlation can occur in the regions from close to ground to well above the melting layer. In addition, there is a strong tendency for the negative t–P correlation to occur around the bright band region. In light of the fact that the conventional model of single drop size distribution cannot explain this negative correlation, it is proposed that the drop size distribution responsible for the negative VtZ correlation is composed of two Gamma drop size distributions with respective mean terminal velocities and radar reflectivities. The precipitation particles of these two distributions are assumed to dynamically interact in the way that the total numbers of the precipitation particles of the two Gamma distributions are varied and their reflectivities are also changed accordingly. Theoretical analysis shows that the key factor determining the sign of the VtZ correlation is the ratio of the difference between relative changes in the reflectivities of the two Gamma drop size distributions to the change in the total reflectivity. The VtZ correlation is negative (or positive) if the ratio is positive (or negative). From these results, it follows that the VtZ correlation could be considered to be the result of the redistribution of the radar reflectivies of the two Gamma drop size distributions caused by the interaction of the precipitation particles between them. Different interaction processes of the precipitation particles, such as break-up and coalescence, could give rise to the same VtZ correlation, depending on the net change in the total reflectivity. In addition, the results also show that the same interaction process might give opposite VtZ correlations. Therefore, great caution is advised when the VtZ correlation is employed to interpret the precipitation process.

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