A statistical study of the structure, saturation and sources of inertio-gravity waves in the lower stratosphere observed with the MU radar

Abstract

Using wind data monitored by the MU radar at Shigaraki, Japan (35°N, 136°E) in the three years of 1986–1988, a statistical analysis is made of inertio-gravity waves (IGWs) with short vertical wavelengths (⩽ 4 km) and long ground-based periods (⩾ 10 h) which are dominant in the lower stratosphere in all seasons. The intensity of the IGWs is at its peak in winter when a strong subtropical westerly wind jet is situated over Japan. The zonal component of vertical momentum flux associated with the IGWs in winter is mostly negative and the magnitude decreases with altitude, suggesting a strong interaction between IGWs and the background wind field. The parameters of the wave structure are estimated by a hodograph analysis, taking into consideration the effect of vertical shear of the background wind that is not negligible in winter. The obtained parameters, such as the vertical and horizontal energy propagation directions, vertical and horizontal wavelengths, and intrinsic period, are compared between two periods with a strong subtropical jet including winter and with a weak jet including summer. The degree of saturation of IGWs, defined as the ratio of amplitude of the horizontal perturbation velocity parallel to the horizontal wavenumber vector to the intrinsic horizontal phase velocity, is also statistically and directly examined using the results of wave parameter estimation. It is shown that the degree of saturation is larger in winter than in summer and that there are few oversaturated IGWs even in winter. Moreover, in order to examine the generation mechanisms of the IGWs, an analysis is made from two points of view: geostrophic adjustment of the westerly wind jet and the topographic effect. The meridional propagation direction of IGWs is examined in a section of latitude and altitude relative to the jet axis using ECMWF operational data. Most of the IGWs observed in the 12–18 km height region (above the ground) in winter propagate meridionally toward the jet axis, indicating that geostrophic adjustment at least just as the jet axis is not the main generation mechanism of the IGWs. On the other hand, the characteristics of intensive IGWs propagating westward relative to the background wind in the 18–22 km height region in winter are in good accord with mountain waves excited in strong westerly winds near the surface.