2017 2nd International Conference on Civil Engineering and Rock Engineering (ICCERE 2017) ISBN: 978-1-60595-513-1 Rainfall Characteristics Analysis in Landslides Area Based on Micro Rain Radar Zhiwang Wang, Suoying Mao and Xiao Han ABSTRACT It s very important to study the characteristics of precipitation structure and internal micro-physical processes for improving the monitoring and forecast of precipitation. This paper studies the time-height profile of the reflectivity factor, radar echo analysis and comparative analysis of precipitation of the two precipitation processes from 14:30 to 15:30 and 18:00 to 20:30 on 29 May 2013 in the Wangmo county, Guizhou province of China. The results indicate that there is a good correspondence between the radar echo detected by MRR and the precipitation process, and the radar echo detected by MRR can detect and reflect the precipitation process very well. The comparative analysis of precipitation shows that MRR has higher detection accuracy for weak rainfall. 1 INTRODUCTION It s of great significance to understand the characteristics of precipitation structure and internal micro-physical processes for improving the monitoring and forecast of precipitation. Currently used precipitation detection equipment, including Doppler weather radar, surface rain gauge and raindrop spectrometer. Doppler radar can observe high spatial and temporal resolution of a large-scale three-dimensional structure of precipitation, but the raindrop spectrum distribution inside the precipitation system cannot be obtained and the accuracy of precipitation estimation is low. At the same time, the radar beam cannot measure the near-surface 1 Zhi-wang Wang, Suo-ying Mao, Xiao Han, Changjiang River Scientific Research Institute, Wuhan, China; Research Center on Water Engineering Safety and Disaster Prevention of the Ministry of Water Resources, Wuhan, China; Research Center on National Dam Safety Engineering Technology, Wuhan, China. 685
precipitation echoes from far away. The surface rain gauge can accurately measure the surface precipitation, and the ground raindrop spectrometer can further detect the raindrop spectrum distribution near the ground, which helps to understand the microphysical features of the ground rainfall. However, the ground raindrop spectrometer cannot observe the vertical structure of precipitation and therefore cannot deeply study the micro-physical process of precipitation. Vertical-pointing radar can measure reflectance factor and raindrop spectral distribution from near ground to high altitude, which plays an important role in studying and analyzing precipitation micro-physical structure and improving the accuracy of radar precipitation estimation. The MicroRain Radar (MRR) produced by METEK in Germany is a commonly used vertical pointing radar for observing the vertical structure of precipitation. It uses continuous FM technology, with the operating frequency of 24 GHz and a wavelength of 12.5mm (K band). The MRR uses the Doppler effect to measure the raindrop size distribution at different heights (vertical 30 levels) through the relationship between the raindrop size, the scattering cross section and the falling speed, and derives the precipitation rate, the liquid water content, the particle drop speed and the radar reflectivity factor and other data. STUDY AREA AND DATA SETS The study area is located in Wangmo county, Guizhou province, China, where is in the subtropical monsoon climate zone with clear early spring and long summer, late autumn and short winter. It is not cold in winter, hot and humid in summer and rainy and hot in the same season. The precipitation in most areas is about 1220 mm. Due to the geological structure and landforms, most of the rivers in the territory are mountainous rivers, deep riverbed and more rapids in floodplains. Therefore, the characteristics of steep and steep rising of floods are obvious. A micro rain radar (MRR) produced by METEK in Germany and a rain gauge were installed in the study area. RESULTS AND DISCUSSION The MicroRain Radar (MRR) produced by METEK in Germany is a commonly used vertical pointing radar for observing the vertical structure of precipitation. It uses the continuous frequency modulation technology (FM-CW), operating frequency of 24GHz, wavelength of 12.5mm (K band). The MRR uses the Doppler effect to measure the raindrop size distribution at different heights (vertical 30 levels) through the relationship between the raindrop size, the scattering cross section and the falling speed, and derives the raindrop size distribution, the liquid water content, the particle drop speed and the radar reflectivity Factor and other data. 686
The case study selected for this study is the two precipitation processes in the Wangmo area from 14:30 to 15:30 and 18:00 to 20:30 on 29 May 2013. The timeheight profile of the reflectance factor is shown in Figure 1. There were total 31 floors in vertical direction of MRR, the height resolution was 100m, the maximum measurement height was 3100m, the sampling time interval was 10s. The minimum resolution of ground rain gauge was 0.5mm, the sampling time interval was 1 min. Figure 1. Time-height profile of reflectance factors during two precipitation process on 29 May 2013. The time-height profile changes of the radar reflectivity factors for the two precipitation processes are shown in Figure 1. Observations from 14:30 to 15:30 on May 29, 2013 showed that the MRR detected weak echoes ranging from 15 to 48.70 dbz. Rainfall was small between 18:00 and 20:30 and the radar echo detected by the MRR was also small, varying from 0 to 30.39 dbz. In the rest of the period, there was no precipitation, and the radar echo is basically less than zero. The vertical profile of the MRR average reflectance factor during the two precipitation events is shown in Figure 2. The rainfall between 14:30 and 15:30 was very heavy, the average echo intensity in the lower floors was also larger, reaching about 30 dbz. And average echo intensity in the lower floors decreased vastly when the altitude increased because the MRR echo intensity in the long distance decreased in heavy precipitation. The rainfall was small and the average echo intensity was small between 18:00 and 20:30. The MRR also decayed in the upper layer, but the attenuation in the weak precipitation was small. Radar echo analysis shows that there is a good correspondence between the radar echo detected by MRR and the precipitation process, and the radar echo detected by MRR can detect and reflect the precipitation process very well. 687
Figure 2. Vertical profile of average reflectivity factor of MRR during the two rainfall episodes on May 29, 2013 The MRR reflectivity over time during the two precipitation events is shown in Figure 3. The reflectivity measured by MRR at 100m, 200m, 300m and 400m altitudes was compared with the precipitation measured by the surface rain gauge. The hourly cumulative precipitation from the two precipitation processes (Fig 4) shows that the peak precipitation is consistent. The precipitation measured by MRR at the height of 100m agrees well with the precipitation observed by the rain gauge on the ground, and the deviation at the peak is less than 0.5mm The precipitation rates measured at 200m, 300m and 400m successively decreased, but the trend was basically the same. The comparative analysis of precipitation shows that MRR has higher detection accuracy for weak rainfall. CONCLUDING REMARKS Radar echo analysis shows that there is a good correspondence between the radar echo detected by MRR and the precipitation process, and the radar echo detected by MRR can detect and reflect the precipitation process very well. The comparative analysis of precipitation shows that MRR has higher detection accuracy for weak rainfall. 688
Reflectivity/dBz 60 50 40 30 20 10 0 10 20 30 40 100m 200m 400m 0 2 4 6 8 10 12 14 16 18 20 22 24 Time/UTC Figure 3. Curve of MRR reflectance over time for two precipitation events on May 29, 2013. 20 Hourly Cumulative Precipitation/mm 15 10 5 MRR 100m MRR 200m MRR 300m MRR 400m rain gauge 0 0:00 6:00 12:00 18:00 0:00 Time/UTC Figure 4. Hourly accumulated precipitation measured by MRR radar at 100m,200m,300m, 400m and rain gauge for the 29 May 2013 rain events. ACKNOWLEDGEMENT This research was financially supported by the National Natural Science Foundation of China (No.51379023), and the Public Welfare Research Project sponsored by Ministry of Water Resources of China (201501033-3). REFERENCES 1. Peters G., Fischer B., Mtinster H., et a1. Profiles of raindrop size distributions as retrieved by micro rain radars. Journal of Applied Meteorology, 2005, 44(12): 1930-1949. 2. Tridon F., Van Baelen J., Pointin Y. Aliasing in micro rain radar data due to strong vertical winds. Geophysical Research Letters, 2011, 38(2): 12-13. 689