Lightning and In-cloud Ice Phases in the East Asian Monsoon

During the past fifteen years more than 200 videosondes have been launched into monsoon rains from 15 different locations in East Asia. The numbers of graupel and of ice crystals in rainwater contents of 1 g m were compared with lightning activity in the storms. There were more graupel and ice crystals in areas of increased lightning activity. Lower graupel and ice crystal number densities extended from the equatorial western Pacific to Shanghai. Much higher graupel and ice crystal number densities existed within narrow bands along the maritime continent. Ice particle concentrations differed by two orders of magnitude between the maritime continent and the open ocean as lightning activity also varied by the same magnitude. INTRODUCTION Data from the TRMM-LIS satellite show a considerable difference between lightning activity over a maritime continent and that observed over the open ocean of the western Pacific in eastern Asia (Christian et al., 2003). Both laboratory work and model studies suggest riming electrification as the major charge separation mechanism in thunderstorms (Takahashi, 1978, 1984). If this is true, number concentrations of both ice crystals and graupel should be lower over the open ocean than over the maritime continent and different precipitation processes should be expected as well. VIDEOSONDE Videosonde data have been used extensively in the investigations reported here. The primary features of videosonde are an induction ring that measures the electric charge on the particles and a video camera that records particle images (Fig.1, Takahashi, 1990). An infrared light is set parallel to the line of sight of the camera and interruption of the beam by a particle larger than 0.5 mm diameter triggers a flash lamp set above the lens. Twice a second a regular flash monitors particles down to 0.2 mm diameter. The induction ring, 70 mm in diameter and 10 mm high, is mounted at the top of the sonde. Particles enter through a cone above the sonde and the signals are logarithmically amplified. Charges are detected in the range of 0.1-200 pC. Some of the videosondes contain a new component that monitors ice crystals. The crystals fall on transparent film, and their images are converted to frequencies between 10 Hz and 1 MHz and sent to a ground station by a 1680 MHz carrier wave where they are projected on a TV screen at 10X magnification and recorded on videotape. The size and shape of each image are measured. For every 500 m height, the total mass and number density are calculated for each category of shape. As Braham (1990) commented in relation to aircraft measurement data that include wide size ranges, the concentrations of the small crystals reported herein are probably underestimated by as much as one order of magnitude with consideration of lower collection by videosonde. However, a proportional relationship has been maintained between ice particle concentrations obtained from videosonde data and actual, in-cloud total ice particle concentrations. Also, because the collision efficiency of small crystals to graupel is less, they may also contribute less to charge generation. RESULTS Data from videosondes launched from the shore of the Japan Sea during the winter monsoon and from southern Kyushu during the Baiu season revealed several important findings. 1) Intense space charges appear in a cloud as pairs, consisting of graupel in the lower levels and ice crystals in the upper ones. Graupel below the –4 °C level are predominantly positively charged while above –12 °C level, they are predominantly charged negatively(Fig.2) . 2) Space charges carried by precipitation particles increase with increasing number concentrations of graupel and ice crystal(Fig.3). 3) In areas where space charges carried by particles are sufficient to produce lightning, the average charge on the precipitation particles may be as high as 50 pC(Fig.4). 4) As cloud-top height increases, the space charge on particles also increases, and, when it reaches a certain critical value, there is a sudden increase in the frequency of lightning (Fig.5). ! Figure 5. Lightning flash rates in Kyushu area (upper-left) versus peak particle space charge and cloud