Latitudinal and seasonal variations of inertial gravity wave activity in the lower atmosphere over central China

Gravity wave activities and background dynamical structure in the troposphere and lower stratosphere ( TLS) over five stations at latitudes from 10 degrees N to 40 degrees N were statistically studied by using the data from Radiosonde observation on a twice daily basis at 0800 and 2000 LT. The background dynamical structure exhibits evident latitudinal and seasonal variations and has a profound influence on inertial gravity waves in the TLS. In the analyses of inertial gravity waves, according to the background structures, the observation height coverage is divided into two segments, which are the tropospheric ( 0-10 km) and lower-stratospheric segments ( 18-25 km). The observational results indicate that the tropospheric jet is the most important excitation source for gravity waves both in the troposphere and lower stratosphere, and it plays different roles in determining the morphology of gravity waves in these two segments. The jet-excited gravity waves in the troposphere can propagate both upward and downward, and only part of the upward-propagating waves can penetrate into the stratosphere because of the Doppler shifting by the jet, while in the lower stratosphere, gravity waves excited by the tropospheric jet propagate upward. Most differences between the tropospheric and lower-stratospheric results can be explained from the linear dispersion relations of gravity waves and the Doppler shifting by the strong tropospheric jet. However, such an explanation is qualitative rather than quantitative. Generally, the observations reveal that the tropospheric gravity waves are mainly controlled by their excitation sources, implying the wave characteristics may be regarded as ( at least an indicator of) the wave excitation source characteristics. These results suggest that in order to attain a more realistic source parameterization for gravity waves propagating in the middle and upper atmosphere, more attention should be paid to the gravity waves in the troposphere. Moreover, the causes of the pronounced peak of lower-stratospheric gravity wave intensity at tropical latitudes revealed by previous observations are also discussed.