A Numerical Study of the Local Downslope Wind "Yamaji-kaze" in Japan

Part 3: Numerical Simulation of the 27 September 1991 Windstorm with a Non-hydrostatic Multi-nested Model

By Kazuo Saito

Meteorological Research Institute, 1-1, Nagamine, Tsukuba, Ibaraki, 305 Japan


A three-dimensional non-hydrostatic anelastic model using a radiative-nesting lateral boundary condition is presented, and is applied to the simulation of the Yamaji-kaze on 27 September 1991. A 2.5 km-resolution version of the model, which is double-nested with the operational hydrostatic model (Japan Spectral Model; JSM) of the Japan Meteorological Agency (JMA), successfully simulates the downslope wind and the associated internal hydraulic jump in the lee of the Shikoku Mountains.

The non-hydrostatic anelastic model, which was used in a previous study (Saito, 1993), is modified so that it can express the time-changing environmental information through the lateral boundary condition. Variational calculus is used in order to obtain a three-dimensional mass-consistent initial wind field which satisfies the anelastic continuity equation and the upper and lower boundary conditions. The Orlanski-type radiation condition is employed for the lateral boundary condition of the inner model, using interpolated grid point values of the outer model as the external reference value. The validity of the nesting procedure is shown by a comparison with the non-hydrostatic linear analytic solution over a three-dimensional mountain.

The Yamaji-kaze of the 27 September 1991 windstorm caused by Typhoon 9119 is presented, and realistic simulations are conducted using the 10 km and 2.5 km resolution non-hydrostatic models which are nested with JSM. The 10 km-resolution model reproduces well the time change of the synoptic field by the approach of T9119, but does not simulate a conspicuous downslope wind. On the other hand, the 2.5 km-resolution model reproduces well the downslope wind and the associated Yamaji-kaze front (internal hydraulic jump in the lee of the Shikoku Mountains). The characteristics of the model-reproduced wind field and the response of the jump to the magnitude of the environmental wind are generally consistent with th e conceptual models of the Yamaji-kaze which have been proposed in previous studies (Saito and Ikawa, 1991a; Saito, 1993).

Sensitivity experiments show that both the ground temperature and ground roughness affect the intensity of the downslope wind. In case of the Yamaji-kaze, not only the orographic characteristics of the Shikoku Mountains, which were emphasized in previous studies, but also the presence of the Hiuchi-nada (southern part of the Inland Sea in the lee of the Shikoku Mountains) contributes to intensifying the surface wind in the coastal plain through the difference in roughness between sea and land.