Observation of winter lightning in the Shonai area railroad weather project: preliminary results
The Shonai area railroad weather project has investigated fine-scale structure of wind gust using two X-band Doppler radars and the network of 26 surface weather stations since 2007. In 2009, the project was expanded and started lightning observation to investigate mechanism of lightning activity, particularly winter lightning in the Shonai area in Japan. Lightning activity is known to be related to microphysical and dynamical processes within storms. Hence, integration of continuous 3-D lightning monitoring (intracloud and cloud-to-ground lightning) and comprehensive high-density meteorological observation can provide useful index for predicting strong gust.
We developed a lightning monitoring system that consists of three VHF sensors and one LF sensor. Observed signals are digitized with two digital oscilloscopes and stored in a personal computer. Precise GPS time data are also recorded simultaneously. After operation test at Meteorological Research Institute, we installed this system in the north of Shonai area in October 2009. Now, we are monitoring lightning activities over the Shonai area. We will build three monitoring stations in the Shonai area and map lightning activities in three dimensions in 2010. Furthermore, in order to estimate the electrification of thunderclouds, we installed an electric field mill near Shonai Airport in October 2009. An array of the sensors will be constructed in 2010.
The lightning monitoring system has detected some lightning activities since October 2009. Using obtained data set, we have computed the azimuth and elevation of VHF radiation sources originated from lightning flashes. These data are compared with the radar echo, the electric field data, and the other observation data. In order to understand the characteristics of winter thunderclouds, we investigate the relationship between the temperature of echo top (20dBZ) and the altitude of -10 ºC level based on Michimoto (1989, 1993). The atmospheric vertical profile around the Shonai area was derived from Meso-scale Analysis (MANAL) data released by Japan Meteorological Agency (JMA). The analyzed period is about 4 months from 30 October 2009 to 9 March 2010. The result shows that the temperature of echo top is lower than -20 ºC and the altitude of -10 ºC level is higher than 1.7 km during relatively frequent lightning activities. Meanwhile during weak lightning activities the different environmental conditions are shown, which are the temperature of echo top is higher than -20 ºC and/or the altitude of -10 ºC level is lower than 1.7 km. This result is consistent with Michimoto (1989, 1993). Therefore, the process of charge separation and accumulation around -10 ºC temperature level is important to lightning activity. -->