Small-scale Disturbances in the Lower Stratosphere Revealed by Daily Rawin Sonde Observations

Abstract

Small-scale disturbances in the lower stratosphere are investigated with the aid of operational rawinsonde observations over Japan. The disturbances appear not only in wind but also in temperature fields. The disturbances show a clear seasonal variation: the intensity is large in winter and spring, in a similar manner to that of the mean zonal wind. They also have latitudinal and altitudinal dependency with the peak of intensity at 15km to 20km height around 40°N. The dominant vertical scale of the disturbances is 2 to 5km and they extend north and south with a scale of a few hundred kilometers. By applying a vertical band-pass filter, it is found that they show a wave-like form with a characteristic phase relation such that the lag of T' to u' and lag of u' to v' are both in between -180° and -90°. Supposing that they are inertia-gravity waves, the phase relation suggests that the wavenumber vector points to the direction of the north-west and downward and hence the waves transport the energy north-westward and upward relative to the background wind. This inclination of the propagation direction can be interpreted as a consequence of the selectivity due to the critical layers as well as the wave generation. From all of these results, we deduce that the generation and the characteristic structure of the disturbances are closely related to the subtropical jet at the tropopause level.