Three-Dimensional Distribution of VHF Lightning Radiation Sources in Winter Thunderstorms

We have conducted field observations, which we have called “The Shonai Area Railroad Weather Project.” The Shonai area is located on the coast of the Japan Sea. The project was designed in 2007 to investigate the fine-scale structure of wind gusts using two X-band Doppler radars and a network of 26 surface weather stations in order to develop an automatic strong gust detection system for railroads. We have focused on total lightning activity in winter to investigate the mechanism of the winter lightning discharge process and the application to the prediction of strong wind gusts. Thus motivated, we developed a three-dimensional (3D) VHF lightning mapping system for winter thunderstorms.

We investigate 3D distribution of VHF lightning radiation sources observed on November 30, 2010. The detailed structure of the thundercloud was observed with the X-band radar. The horizontal distribution of the VHF sources was consistent with the area of radar echo. The vertical distribution of the VHF sources was 1-5 km in altitude, corresponding to the echo-top height (about 5 km) observed by RHI scan. In particular, 22% of the source was located at 2.5-3 km in altitude. The -10 degree C level retrieved from JMA-MANAL was 2.8 km at the same time. Hence, we estimate that the electric charges accumulated around the -10 degree C level of 2.8 km. The vertical distribution of the VHF sources is related to the atmospheric temperature level. Similar tendency is shown in the other cases (e.g., Dec. 3, 2010; Dec. 14, 2011; Dec. 17, 2011). These results support the numerical study on rimming electrification mechanism (Takahashi 1984) and activity diagram for winter lightning (Michimoto 1993; Nishihashi et al. 2011).

In this presentation, we will also show the spatial-temporal relationship between lightning activities and vortex evolutions in winter thunderstorms.