Time Series of Microphysical Structure and Lightning Polarity of a Group of Thunderclouds Generated Successively in a Line-shaped Area

The authors have constructed a hydrometeor classification (hereafter, HC) method using X-band polarimetric radars (X-pols) and have tried HC with our HC method (Kouketsu and Uyeda, 2011, presented the last Conference on Radar Meteorology by AMS in 2011). To validate our HC method, we used simultaneous data of an X-pol and balloon-borne instrument (in situ) observational data (Kouketsu et al., 2012, presented in EDAR2012) and we obtain the result that our HC method is reasonable for at least stratiform precipitation under moist environment, which accompanied obvious bright band.

To test our HC method, thunderclouds are a beneficial target because they contain various kinds of hydrometeor: rain, snow aggregate, non-aggregated ice crystal, graupel and perhaps hail. Under moist environment, a thundercloud contains less hail and it is suitable for HC using X-pols. Therefore, we tried HC with an X-pol for a summer thundercloud in 2010, generated in central area of Japan (Kouketsu et al., 2011, presented in the last AMS Conference on Radar Meteorology held in 2011). The result of HC was consistent with the riming electrification process proposed in Takahashi (1978, published in J. Atmos. Sci.); the volume of graupel (or ice crystal) region around or above -10 °C height indicated by HC agreed well with frequency of negative (or positive) cloud-to-ground lightning (CG) obtained with lightning location system (LLS) by Chubu Electric Power Co., Inc.

The previous study (Kouketsu et al., 2011) was conducted a simple thundercloud and we have to conduct more complicated system of thunderclouds to understand its microphysical structure. In this study, we examined the relationship between polarity of CGs and distribution of hydrometeors for a group of thunderclouds generated successively in a line-shaped area. The group of was generated at the south of Gifu Prefecture, the central Japan Area in the afternoon on August 25, 2010. We observed the group of thunderclouds with an X-pol of Nagoya University. The volume scan interval of the X-pol was 6 minutes with 15 elevations from 0.5° to 33.5°. After observation, we conducted HC with four polarimetric valuables obtained with the X-pol: radar reflectivity with horizontal polarization (Zh), differential reflectivity (Zdr), specific differential phase (Kdp) and correlation coefficient of horizontally and vertically polarized signals (Rhv). And for supplementary information, we used temperature data of ground and balloon sounding observational data. The polarity of lightning is important information because it suggests the presence of graupel and/or ice crystal and, therefore, it is useful barometer of validity of HC. To obtain the information of polarity and frequency of CG from the thundercloud, we used the data of Lightning Location System (LLS) performed by Tubu Electric Power Company.

The first thundercloud of the group was generated around 1530 JST (Japan Standard Time = UTC + 9 hours) below 0 C height. It moved to north-northeast slowly blown by southerly wind of upper air. It accompanied an anvil area blown by southerly wind and the anvil directed from south-southwest to north-northeast. Negative CGs and positive CGs were observed from 1600 and 1606 JST, respectively. Negative CGs were observed mainly below the reflectivity cores, which were identified as wet or dry graupel by our HC method, and positive CGs were observed mainly below the anvil areas, which are identified as dry snow aggregate or non-aggregated ice crystal by our HC method because weak reflectivity was observed. The group was composed of a few thunderclouds generated successively. The group survived for more than one and half hours. After 1700 JST, the group approached other groups of thunderclouds and we terminated analysis at 1700 JST.

There were three peaks of number of negative CGs and positive CGs during the analysis time (from 1530 to 1700 JST). Several minutes before each negative CG's peak, large volume of dry graupel regions were observed above 10 km height. After that, many negative CGs were observed. This can be considered that dry graupel obtained negative charge by riming electrification process (Takahashi, 1978) and fall toward ground to result in large electric intensity. On the other hand, there were three peaks of number of positive CGs during the analysis period that almost simultaneously occurred with negative CG's peak. Several minutes before those positive CG's peak, there were wide-spread area identified dry snow aggregate or non-aggregated ice crystal (anvil area). This is also can be considered that ice crystal had positive charge by riming electrification process and fall toward the ground, likewise dry or wet graupel.

In the conference, we will conduct further analysis and discuss the more detail of time series of relationship between microphysical structure and polarity of CGs for each thundercloud.