Comparing Analysis on 3-D Kinematic Structure of Rainfall on Mei-yu Front by Dual- and Triple-Doppler Radar

 

1. Introduction

The retrieve technique using Dual(triple-)Doppler radar can improve the accuracy and the reliable. Since then, many methods have been developed using variation which improve the retrieve accuracy and reliable.

In order to study the 3-D wind structure of the meso-scale heavy rain on the Mei-yu front, China Heavy Rain Experiment and Study used triple-Doppler radar in AnHui province on 2002. 

2. MUSCAT Technology

This paper retrieved wind using MUSCAT method proposed by Bousquet et al(1998) on airborne radar. We developed MUSCAT to ground-based Dual-radar (Zhou et al, 2002a) and analysis the 3-D wind of heavy rainfall in Yangtze River (Zhou, 2002b, 2003). This method can remove the drawback of a dual-Doppler iterative method. It is more stable with high accuracy. 

3  Echo Evolution

   A band echo moved into the triple-radar network from west to east on 07:00(Beijing time) 24 June. It propagated quickly with some cells developed on it. The peak reflectivity is greater than 45dBZ. The rainfall lasted for more than 2 hours. The high reflectivity core began to decrease  after 9:35.

4  3-D Wind Analysis 

Four retrieve analyses were introduced: Hefei-Maanshan (hereafter as HM), Hefei- Wuwei (HW), Maanshan-wu(MW), Hefei- Maanshan- Wuwei (HMW). The first three analyses were dual-Doppler radar retrieve method, and the last one is triple-Doppler radar retrieve. The retrieve errors of four retrieve methods in the retrieve domain were different.

The horizontal velocity retrieved by the four analyses at 2.5km on 08:47 are given. The west-north dry-cold flow and west-south moisture flow combined and a convergence line was shown at the middle part of the retrieve area. The positions of the convergence line are quite similar retrieved by the four analyses . The HMW horizontal wind field was more smooth than the other retrieve methods. The upper level were all controlled by western wind shown by the four projects. The higher-reflectivity band was correspond to the convergence line. It indicated that the wind field is the kinematic foundation of the reflectivity field evolution. The divergence value retrieved by the dual-Doppler radar range varied in a large range which is corresponding with Ray's result (1980). 

 

 

Fig 1. shown the vertical cross section of velocities , reflectivity contours along x=40km at 08:47. Although there were some differences among them, the four analyses shown some same kinematic structure. They all displayed clearly that the inflow air are composed by two air flow  at the low level and outflow located at the upper layer. The two inflow air combined at the middle part the area which position was correspond to the convergence line. The kinematic configuration that convergence at the low level and divergence on the upper level can caused heavy updraft and was quite useful to the rainfall. A significant difference, however, is apparent in the velocity at the upper level along y=60km in project HM. The retrieve error which was the function of the distance between the retrieve grid and the radar location caused this difference.

The 3-D wind structure from other time was similar to the feature at 08:47. The significant feature of the storm shown by the four analyses was the convergence line at the low level which was caused by east-north cold-dry air and west-south moist air flow.

The characteristics of different analyses for the same data set at 08:47 are also given. The horizontal velocity were quite similar for the four analyses but the vertical velocity were quite different which is similar to the Ray's result(1980). The vertical velocity varied in a wind range for the Dual-radar analyses. It is shown that the advantage in increasing the number of radars is increase the retrieve accuracy. 

One reason of the retrieve error was caused by the radar volume scan time that the radar data were not scanned in the same time.

5 Conclusion

The four analyses were similar ,but there were some significant difference in error propagation, and in quantities, such as vertical velocity and divergence. The 3-D wind retrieved from the triple-radar was more accuracy than the Dual- Doppler radar. The significant feature of the storm was the convergence line at the low level. It play a important role on the rainfall.

Acknowledgments

This research was supported by key project funded by the Ministry of Science and Technology  of China(2002DIA20013), and National Natural Science Foundation of China((40375006).

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