Effect of mountain convexity on the locally strong “karakkaze” wind

Abstract

This study numerically examined how the locally strong “Karakkaze” wind in the Kanto Plain of Japan is affected by terrain shape, particularly by a convex feature in the mountain range. Our method involved running idealized numerical simulations using the Weather Research and Forecast model with a horizontal grid spacing of 3 km. The results revealed that a strong-wind region formed in the lee area of the convex feature, hereafter the semi-basin, and leeward of the semi-basin. In contrast, weak-wind areas formed adjacent to the strong-wind region. These results were consistent with the basic features of the observed surface wind pattern of the Karakkaze during the winter monsoon. However, such a flow pattern did not appear in the numerical simulation with a mountain range that lacked a convex feature. Sensitivity experiments were also conducted to evaluate the detailed effects of a mountain range with convex-ity. Sensitivity experiments with different convex shapes revealed that strong winds appeared within and leeward of the semi-basin when the aspect ratio of convexity (ratio of the wave amplitude to the wavelength of the con-vexity) exceeded about 0.5. Sensitivity experiments on terrain shape suggested that saddles in the mountain range were not essential to the formation of the Karakkaze, but they could affect its strength. Sensitivity experiments on the mountain Froude number, Frm, showed that locally strong winds within and leeward of the semi-basin appeared only when the Frm was in the range 0.42 – 1.04. Sensitivity experiments with surface heat fluxes (SHFs) showed that the basic structure of the strong-wind region in the leeward plain of the convex feature did not depend strongly on SHFs. However, the addition of SHFs reduced the surface wind speed, but increased the size of the strong-wind region.