Abstract
The purposes of this study are to clarify the regional and seasonal characteristics of cold frontal precipitation in Japan, and to understand the orographic controls on the precipitation and other meteorological elements during the passage of a cold front. Cold fronts occur repeatedly and compose weather patterns. Therefore, the analyses of cold frontal precipitation are also included in the synoptic climatology dealing with the accumulation of daily weather phenomena. Besides, in the region facing the Japan Sea the annual total amount of cold frontal precipitation is regarded as the second greatest, next to the precipitation due to winter monsoon. Consequently, it is necessary to research climatologically cold fronts, when the precipitation characteristics of Japan are discussed from the synoptic climatological point of view.
In this study the passage of a cold front has been defined as “the change of wind direction from southwest to northwest” and the precipitation from the cloud bands along the shear line has been discriminated as cold frontal precipitation. By means of the surface weather maps every 3 hours and the daily original registers of meteorological observation, the cause of each rainfall has been probed at the 140 stations all over Japan with the determination of the starting and finishing times of the cold frontal precipitation and the calculation of the amount of the precipitation.
Because cold frontal activity is an aspect of the heat exchange between high and low latitudes, its basic characteristics should be determined by the masses on both sides of the cold front, namely the cold air mass on the north and the warm air mass on the south, which affect the features of cold frontal precipitation to a considerable degree. Then, just before entering the Japanese Islands, the cold fronts have been classified on the basis of the equivalent potential temperature (θe) on the 850 mb level of the cold or warm air masses (Fig. 3). For the index stations Vladivostok in the cold air mass and Hachijojima in the warm air mass have been chosen, but in the case when a front passes through Northern Japan, Ust'Tyrma in the former and Hachinohe in the latter have been adopted (Fig. 2). The air masses have been divided into polar air mass (P), middle-latitude air mass (M), and tropical air mass (T) (Fig. 3). Then, the front belongs to the category of winter when the cold air mass is P and the warm air mass is M; it belongs to that of spring or autumn when both air masses are M ; and it belongs to that of summer when the former is M and the latter js T. From the cold fronts that passed over Japan for 6 years (1973 1978) 10 cases in each season, 40 cases in all, have been chosen. The dates and Be of the cold or warm air masses in the respective cases have been arranged in Table 1.
The criteria adopted when the objects of this study were chosen are as follows: 1. The cold front has been analyzed on the surface weather map drawn by the Forecast Department of the Meteorological Agency. 2. When the cold front passes, the wind change can be clearly observed. 3. At any station the cold frontal precipitation is 5mm or more. 4. The cold front passes without stopping. 5. There is no wave cyclone on the cold front before its passage over Japan. 6. The cold front is not a part of coupled cyclone.
The occurrence frequency of the cold frontal precipitation, which is 0mm or more and 5mm or more, has been illustrated in two figures (Figs. 4-a, b).
