Frequency analysis on suitable convective clouds for artificially increasing rainfall in the northern part of Kyushu, Japan, using meteorological satellite (GMS-5)

Many droughts (shortage of water) have broken out by extreme small amount rainfall in recent Japan. So far, in order to prevent these droughts, artificial rainfall methods with eAgI' or 'dry ice' have been widely used in Japan. However, these methods have many problems, which a large amount of overcooling liquid in the cumulus cloud was not able to be converted into precipitation efficiently. So as to solve these problems, new artificial rainfall method using liquid carbon dioxide (LC) was proposed by Fukuta (1996).

1.The new method of artificial rainfall using LC The injection of LC materials into super-cooled convective clouds cause strong evaporative particles per gram of LC materials by homogeneous nucleation. The number of artificially formed ice particles keeps approximately constant after the injection of LC materials during the ascent of an artificially formed thermal. No competition process for limited super-cooled liquid water among ice particles in thermal provides an important advantage for effective growth of ice particles. From this point of view, LC seeding method is still more advantageous than AgI seeding method, which causes competition process due to drastic increase in the number of ice particles in low temperature. In addition, in order to enhance the efficiency of seeding, seeding operation by an aircraft is designed to be carried out at a low supercooled portion near 0°C in a young developing cumulus so that ice particles to grow effectively into enough size to fall out within a limited lifetime of a cumulus.

2. The conditions for the artificial rainfall experiment to succeed The most important condition so as to succeed artificial rainfall is to find the suitable convective cloud on the seeding area. 1) The cloud for the artificial rainfall must be the convective cloud (cumulus cloud). 2) The thickness of the cloud must be 2000m or more. 3) The temperature of the cloud must be 0°C or less . 4) The wind direction of the ground is from northwest to north.

3. Method of finding suitable convective clouds for artificial rainfall These suitable convective clouds occur in winter season, because the cold air moves toward the East China Sea and the Japan Sea. The resultant wind flows strongly in these regions. 1)The convection cloud is judged from a visible image of the meteorological satellite (GMS-5). 2)The height of the cloud-top is estimated from the air temperature of aerological observation data and the visible data obtained from GMS-5. 3)The height of the cloud-bottom is estimated from the air temperature and the dew point temperature of aerological observation data. 4)The thickness of the cloud is estimated from the difference between the height of cloud-top and the height of cloud-bottom.

4.Result of these analyses 1)The number of rainy days is 1484 in winter-time of the 15 years (1990 to 2004). 2)The number of winter-type pressure pattern precipitation is the biggest (28%). 3)The number of suitable convective cloud in January is the biggest (38%) in winter-time of the 6 years (1999 to 2004).