Vertical Wind Disturbances in the Troposphere and Lower Stratosphere Observed by the MU Radar

Abstract

An analysis is made of vertical wind disturbances (VWDs) observed by the MU radar (Shigaraki, Shiga, Japan) in terms of the wave structures, sources, and vertical momentum flux. First, it is shown through spectral and lag-correlation analyses that there are several common features in three observation periods when the VWDs appeared: large power in the vertical wind fluctuations exists below a height of ∼20 km and is distributed largely at lower frequencies. The decrease in power near 20 km is sharp and the correlation between vertical winds above and below the level is very small, suggesting that the height of 20 km is a special level for the VWDs such as a critical level. The disturbances below 20 km consist of several modes in a height-frequency space, some of which oscillate almost in phase in a wide height range spanning ∼8 km at the maximum. Second, using eight sets of the MU radar data, the relationship between vertical wind activity and horizontal wind near the surface is examined in order to identify the source of the VWDS. The activity of the vertical wind is closely related to both direction and strength of the horizontal wind near the surface at Yonago (about 250 km west of Shigaraki). This indicates that the VWDs are due mainly to gravity waves generated by the effect of topography with heights of ∼1000 m located between Yonago and Shigaraki. The features obtained through the wave structure analysis can be interpreted well as characteristics of topographically forced waves. Finally, vertical momentum flux associated with the VWDs is examined. Although it is generally difficult to examine the vertical momentum flux associated with quasi-stationary mountain waves from observations at one location, the estimation is possible to some extent when the spatial phase fluctuates largely according to temporal changes of the background wind. As a result, some of the VWDs' characteristics on the momentum flux consistent with the view of mountain waves are obtained.