A Numerical Study on the Appearance of Target Clouds for Artificial Precipitation Experiments over Shikoku Island, Western Japan during the Baiu Season

Abstract

The appearance frequency of clouds with much cloud water and less precipitating water, which significantly influences cloud seeding to increase rainfall, is examined from the simulation results of a cloud-resolving model with a horizontal resolution of 1 km. The area of Shikoku Island, located in western Japan, was analyzed in the 2007 Baiu season.In this study, target clouds are judged using the threshold values of the vertically accumulated total amount of rainwater, snow and graupel (TRSG) less than 1.0 mm and that of cloud water exceeding 0.5 mm. Moreover, the available total rainfall amount is roughly estimated by vertically accumulating cloud water for TRSG < 1.0 mm.Between May and July 2007, target clouds appear at a rate of 6.4% in average over the central area of Shikoku Island. Even in June 2007 when observed rainfall amount was less than 50% of the 10-year mean value published by the Japan Meteorological Agency, the cloud appearance rate of 7.6% is comparable with that in July when rainfall reached the 10-year mean value. The diurnal variation of the appearance rate of target clouds is also examined. In June, target clouds appear at a higher rate (∼12%) in the afternoon. Moreover, available total rainfall amount of about 200 mm is estimated over the central area of Shikoku Island for three months.