ON THE FORMATION OF WIND-SHAPED TREES AND THE CLIMATIC MEANING OF THEIR DISTRIBUTION ON THE SOUTHERN RIDGE OF OZEGAHARA

Abstract

The wind-shaped tree is believed to be an indirect but effective means for detecting the wind field on any climatic scale in mountainous regions where meteorological stations are extremely sparce. In most of the preceding studies which dealt with the wind-shaped trees in the subalpine zone as well as in the lower part of the alpine zone, however, the genesis has not been studied as sufficiently as in the lower regions. There-fore, it is not always clear to what extent the wind condition can be deduced from the distribution maps of wind-shaped trees. In case of using the wind-shaped tree as an indicator of wind in these zones, an important problem is to know how a tree is deformed by wind and what wind a wind shaped tree actually represents. As an initial step to approach this problem, the author selected the southern ridge of Ozegahara as a field of study (Fig. 1-a), intending to elucidate the formation of the wind-shaped trees, especially the factors and mechanisms of their formation, and the climatic meaning of their distribution. This area is considered as one of the areas furnishing a key to the above mentioned problem for the following reasons ; 1) a detail analysis of the distribution of the wind-shaped trees (Abies mariesii) in relation to those of snow cover and the vegetation in this area (Fig. 2) would fail to support the generally held belief that the wind-shaped tree here shows the local distribution of the prevailing wind in winter, 2) the direction of the prevailing winter wind is obviously different from that of summer, and 3) there exists the main ridge running almost east to west. For the purpose of this study, the author took a synoptic climatoloical approach. First he analized the surface wind distributions on the ridge from the synoptic climato-logical point of view, and then, based upon the result of this analysis, he discussed the formation of the wind-shaped trees and the climatic meaning of their distribution. The analysis consists of three stages: The first stage is to detect the surfacewind distributions on the ridge under various upper wind conditions, so in detail as to be compared with the distribution of the wind-shaped trees. He carried out moving observations of the surface wind by means of portable anemometers and anemoscopes both in winter and in summer, and also by means of icings on trees and surface-snow-patterns in winter (Fig. 1-b). Thus, 23 ex-amples of the distributions were obtained, and each of them was examined if it meant the distribution under a certain synoptic wind condition (Table 1). on the main ridge, only one out of the 23 examples was the case in which the pattern of the surface wind-direction distribution coincides with that estimated by the wind-shaped trees (this was defined as W-type) (Fig. 5). All of the rest observations were fundamentally different from the W-type, and were classified into either N- or S-type (Figs. 3-a, 3-b, 4). on the other hand, all of the distributions obtained on the north-south ridge running from the main ridge to Mt. Nikura showed only one type, i.e. W-type, implying that the west-component wind blew at each station on the ridge. The second stage is to decide which synoptic wind conditions and flow patterns are associated with each type of the patterns of the surface wind-direction distributions. Comparing with the gradient wind at 700 mb surface over this area, the gradient wind at 850 mb proved to be more closely related to the patterns of the surface wind-direction distributions (Fig. 6, 7). Hence the gradient wind direction at 850 mb surface was chosen as a synoptic wind condition. This was classified into the following three flow patterns._??_ The third stage is to calculate the percentage frequency of appearance of each flow pattern for several years in consideration not only of seasons, wind speed, but also of the occurrence or non-occurrence of precipitation and the temperature of each flow pattern.