MST radar and radiosonde observations of inertia-gravity wave climatology over tropical stations: Source mechanisms

In this paper, possible source mechanisms for the generation of inertia-gravity wave activity over a tropical station, Gadanki ( 13.5 degrees N, 79.2 degrees E), are investigated using a long-term data set obtained from Indian mesosphere-stratosphere-troposphere ( MST) radar. The gravity wave analysis is carried out in two different height regions, namely, 4-14 and 17-21 km, representing the troposphere and lower stratosphere, respectively. Clear seasonal variation in the wave activity has been noticed in both regions with maximum ( minimum) in winter ( monsoon) in the troposphere. But it is maximum ( minimum) in monsoon ( winter) in the lower stratosphere. This kind of winter enhancement in the wave activity is not expected at this tropical site. Interestingly, the contribution of the meridional component to the total kinetic energy ( E-k) is found to be dominant rather than zonal in the winter except during 1997-1998. Topography seems to be the likely source for the generation of wave activity during winter in the troposphere. The influence of this topography is also reflected in the nearby radiosonde stations, Chennai ( 13.0 degrees N, 80.2 degrees E) and Bangalore ( 12.9 degrees N, 77.6 degrees E), which are located at radial distances of 128 and 190 km from Gadanki, respectively. Although two major sources, that is, strong convection and wind shears, coexist during monsoon season, strong wind shear seems to be the likely source of the wave activity. Large interannual variability in the wave activity is also noticed from 9 years ( September 1995 to December 2004) of data. Good consistency is observed between the wave activities observed from nearby ( Chennai) radiosonde and Gadanki MST radar data sets. Making use of a network of radiosonde observations operated by India Meteorological Department, we also present the latitudinal variation of wave activity. From the latitudinal variations it is observed that large-scale systems can also influence the generation of the gravity wave activity over larger areas.