Characteristics and sources of gravity waves in the summer stratosphere based on long-term and high-resolution radiosonde observations

Using long-term high-resolution operational radiosonde observation data from nine stations in the subtropics and mid-latitudes of Japan, this study performed statistical analysis of the dynamical characteristics of gravity waves (GWs). Wave generation by shear instability in summer was a particular focus because orographic GWs cannot propagate deep into the middle atmosphere through their critical layer in the lower stratosphere. The kinetic energy of summer stratospheric GWs is markedly large south of 37°N. Hodograph analysis revealed that GWs propagating eastward relative to the ground are dominant in summer. The percentage of GWs propagating energy upward (downward) is large above (below) the height at which the mean occurrence frequency of shear instability is high. The time series of the kinetic energy of stratospheric GWs exhibited statistically significant positive correlation with the occurrence frequency of shear instability slightly below the tropopause. These findings strongly suggest the possibility of excitation of summer stratospheric GWs by shear instability above the jet. The shear instability condition is satisfied more frequently in the region 30°–35°N. This is probably related to two characteristics of the background fields slightly below the tropopause: larger vertical shear of zonal winds at higher latitudes and lower static stability at lower latitudes.