Orographic effects on local southerly winds in the Matsumoto Basin, central Japan

Abstract

Orographic effects on local southerly winds developing in the area from the southern part of the Matsumoto Basin to the northern part of the Ina Basin, central Japan, were analyzed statistically based on data from routine observations at meteorological stations. The development of local southerly winds is accompanied by intersification of local pressure gradients, which indicate the formation of local orographic lows in the Matsumoto Basin. The local southerly winds are classified into the NE and NW types based on the direction of the sea-level synoptic pressure gradient and occur under various pressure patterns. The typical pressure patterns for the NE type are the winter monsoon and coupled lows across Japan, and those for the NW type are stationary fronts covering Honshu (mainland Japan), lows over the Sea of Japan, and the transitional migrating high-to-trough pattern. For the NE type, winds at 850 hPa are at 250° or greater in the WSW, W, and WNW directions; for the NW type, winds at 850 hPa are at 250° or less in the WSW, SW, SSW and S directions. The local southerly winds tend to increase in strength and reach the maximum value to the wind direction of 250° at 850 hPa. Based on the Richardson number and Scorer's parameter (l), local southerly winds occur under stable conditions that induce the formation of leeside mountain waves. In addition, l values showed that winds of the NE type tend to have a vertical component and flow over mountains, while those of the NW type tend to split and flow around mountains. Differences in the l values between the NE and NW wind flow forms at 850 hPa were investigated statistically in relation to such local meteorological phenomena as the surface wind speed, local pressure gradient, and potential temperature of local southerly winds in the Matsumoto Basin. The results showed that, for the NE type, wind direction and speed at 850 hPa correlated significantly with local pressure gradient and potential temperature, suggesting intensification of the local orographic low and adiabatic heating due to air descending down the leeward side of mountains. For the NW type, wind speed at 850 hPa correlated significantly with local pressure gradient but not with potential temperature, suggesting that this type splits to flow around mountains and is not significantly influenced by the descending component and adiabatic heating. The correlations between wind in the upper layer and orographic phenomena accompanying local southerly winds are in basic agreement with the wind flow forms estimated using Scorer's parameter.