Heavy Rainfalls and Disasters

Abstract

There are various kinds of heavy rainfall. Some of them concentrate in a small area and others spread over considerably large areas. Primarily we will discuss about the scale of the meteorological phenomena. Possibility of the heavy rainfall can be found by the analysis of the global circulation system of the atmosphere. There must be amplification of the planetary wave near Japan to explain the heavy rainfall there, both in winter and summer seasons. In such cases, a potential energy increase is immediately converted into kinetic energy. In winter, warm moist current can be introduced into Japan by such amplified westerly waves. However, the position and the intensity of heavy rainfall is decided by atmospheric phenomena of smaller scale and complicated orographic cnditions. To explain these mechanisms, we have shown many examples in various districts. Moving periodic rainfall of relatively strong intensity can be explained by the so-called meso-scale meteorological disturbances or meso-scale spiral rain bands in typhoons. But, extraordinary strong and concentrated heavy rainfall may occur by the special conditions relating to the orography. By these analyses, we can understand the relation between the intensity and the area of heavy rainfall for each meteorological and orographic conditions. However, for the prevention of hydrological disasters, we must consider the relation between the scale of heavy rainfall and the scale of rivers and the other objects which can cause disasters. We have discussed the direct and the background cause of such disasters relating them with the scale of heavy rainfall.