Occurrence Frequency of Snow Clouds with High Seedabilities over the Echigo Mountains - Statistical Evaluation of their Appearance Frequency with GMS IR Channel Data and Ground-Based Microwave Radiometer Data

In order to investigate the possibilities for snow pack augmentation by cloud seeding, the cooperative, orographic snow cloud project has been carried out by Meteorological Research Institute, Japan Meteorological Agency and Tone River Dams Integrated Control Office, Ministry of Construction in three winters since 1994. Appearance frequency of snow clouds with high seedability, that is, high possibilities that the cloud seeding would cause an enhancement of total snowfall amount (snow clouds of type (1)) or a shift of snowfall area downwind (snow clouds of type (2)), is examined over the Echigo Mountains during the winter season from Nov. 1994 through Mar. 1995. Monthly means of surface temperatures were about l degree C higher than usual so that we could call the winter a mild one. Snow cloud of type (1) ((2)) are defined as having top temperatures between - 5 degree C and - 15 degree C (- 15 not equal to and - 25 degree C), top height more than 2.5 km, cloud amount greater than 9/10, and one-hour averaged liquid water path more than 0.2 mm. Cloud top temperatures, heights and cloud amounts are calculated by a new algorithm, which is developed to detect relatively shallow clouds over the mountain areas, with GMS-4 infrared window channel data, surface temperature data from Automated Meteorological Data Acquisition System (AMeDAS) and aerological data. Liquid water path (vertically integrated cloud water contents) was measured with ground-based microwave radiometers. The results indicated that snow clouds of type (1) appeared in early and late winter (Nov., Dec. and Mar.) while those of type (2) appeared in mid winter (Dec., Jan. and Feb.). The appearance frequency of the both types of snow clouds were high over the windward slope of the mountains and the sum of appearance frequency of the both types reached 15-20% of the time during the winter season.