A SYNOPTIC CLIMATOLOGICAL ANALYSIS ON LOCAL ANTICYCLONES, CYCLONES AND FRONTS IN CENTRAL JAPAN

Abstract

In Central Japan, orographically and thermally induced anticyclones, cyclones and fronts appear very frequently in winter. Takayama High is one of such local anticyclones, which is formed either thermally on clear nights by long wave radiation or mechanically by upper winds. In the latter cases, it usually accompanies Matsumoto Low simultaneously. In the thermal cases, cold air cooled by radiation in the mountaineous region flows down along large valleys and produces local discontinuity lines on the Hokuriku coasts of the Sea of Japan and on the southern coast of Boso Peninsular. On this account, the Takayama High has been studied mainly in relation to the Hokuriku Front, which brings heavy snowfalls in the Hokuriku region. A local cyclone, Suruga-wan Low, is frequently formed when the winter monsoon becomes weaker. The previous studies made it clear that a winter maximum and a summer minimum of its appearance are caused by the thermal effects of water and air temperatures of the Suruga Bay and its vicinity. It has already been stated that the origins of the local front, Boso Front, are closely related to (1) the local high in thermal cases and (2) the convergence of two or more currents branched off topographically from the same Pc air mass with a local cyclone on the Sagami Bay or the Suruga Bay. The purpose of this paper is to clarify their synoptic climatological features. The results are as follows; i) On the meso-scale charts, the occurrence of the Takayama High is 12.3 days per month on an average from December to February, the Suruga-wan Low 9.8 days and the Boso Front 21.3 days. The frequencies of occurrence of the Takayama. High are found most frequently in December, but those of the Suruga-wan Low are in Febuary and the Boso Front in January. This is accounted for by the fact that each of them corresponds with different synoptic pressure patterns. ii) Most frequently the Takayama High persists for 18—20 hours, forming at 17—19h and disappearing at 10—12h on the next day. The shape of the Takayama High is influenced by the direction of upper winds (Fig. 1 (a) (b)) and the differences of the pressure gradient from the center have a close relation to the differences of the direction. In general, it is very difficult to define the size of a local anticyclone, but if the Takayama High is defined by the outermost closed isobar, the areas of the Takayama High of the W—SW type are about 3 times as large as the one of the NW type (Fig. 2 and 4). The maximum of the area of W—SW type Takayama High is 39.1×103km2, and the one of NW type is 9.1×103km2. iii) In the case of the Takayama High of the W—SW type, a local cyclone is formed from time to time in the Matsumoto district by orographical effects. When a Matsumoto Low is formed, strong southerly wind blows and air temperature becomes higher in the district. An indicator of the relative intensity of the Matsumoto Low, ΔP (the difference of sea level pressure at Takayama minus Matsumoto) is obviously related to the W—SW upper wind (mainly 850—700mb); The stronger the wind of these levels is, the greater ΔP occurs simultaneously (Fig. 5). Furthermore, ΔP is related clearly to the temperture of these levels, when 700mb temperature is above -13°C. It seems that warm strong wind, whichh is observed in association with the Matsumoto Low, is caused by a fohn effect. This is verified by facts that ΔT (the difference of air temperature at Matsumoto minus its vicinity) has a relation to the temperature of the height of the mountain-tops (about 700mb) (Fig. 9) and to the wind direction of the levels, and also by the distributions of air temperature, wind (Fig. 8) and relative humidity at the surface level.