Gravity waves (GWs) play important roles in determining the general circulations and thermal structures of the atmosphere [e.g., Lindzen, 1981; Holton, 1982]. They are particularly important in the stratosphere and mesosphere, where they dissipate and deposit their momentum fluxes to the mean flow. Gravity waves are mostly generated in the lower troposphere by a variety of mechanisms [Fritts and Alexander, 2003]. Of these, the mountain waves generated by airflow over topography are believed to be one of the dominant sources particularly in the extratropics during winter [Nastrom and Fritts, 1992]. Moreover, mountain waves can transport significant momentum and energy vertically up through the atmosphere and the effect of which is essential to understand the middle atmosphere circulation and chemistry [Haynes, 2005; Bacmeister, 1993; Carslaw et al., 1998; Dornbrack et al., 2001; Fueglistaler et al., 2003; Eckermann et al., 2006]. There is a wealth of literature (including some important reviews) existing on this subject [Queney, 1947; Smith, 1979, 1989; Blumen, 1990; Baines, 1995; Wurtele et al., 1996; Smith et al., 2002; Fritts and Alexander, 2003; Kim et al., 2003]. The influences of orographic gravity wave drag on the climate and meteorology of the extratropical winter stratosphere and mesosphere, the Polar Stratospheric Cloud (PSC) formation and the ozone loss must be parametrized in global middle atmosphere models [e.g., McLandress, 1998; Pierce et al., 2003; Mann et al., 2005; Siskind et al., 2007]. However, constraints are lacking for the present parameterizations in global climate models to assess the resulting effects, especially the dynamics of short-scale waves [Eckermann et al., 2007].
 Modeled gravity wave properties require observational verification that is difficult to obtain. However, recent advances in satellite-based remote-sensing technology coupled with some innovative analysis techniques provide valuable information on small-scale gravity waves and their global properties throughout the atmosphere [Fetzer and Gille, 1994; Wu and Waters, 1996; Eckermann and Preusse, 1999; Tsuda et al., 2000; Wu, 2004]. Gravity wave momentum fluxes derived from these are normally used to constrain gravity wave parameterization in global models. However, estimations of wave momentum flux derived from satellite observations could not provide the needed constraints [Alexander and Barnet, 2007]. One reason is that estimation of wave momentum flux requires simultaneous observation of vertical and horizontal wavelengths and wave propagation direction [Ern et al., 2004]. Satellite-based instruments now offer appreciable resolution and precision for observing mesoscale gravity wave characteristics that can lead to reduced uncertainties in estimating momentum fluxes. Space-based observations also, in some cases, can provide detailed three-dimensional view of gravity waves (e.g., mountain waves) [Wu and Zhang, 2004; Eckermann et al., 2006; Alexander and Teitelbaum, 2007, 2011]. Using nadir-looking and limb-scanning satellite measurements, mountain waves have been observed over several parts of the world such as Antarctic Peninsula, Scandinavia, South Georgia Island, Andes, etc. [Eckermann et al., 2007; Alexander and Teitelbaum, 2007; Alexander et al., 2009; Eckermann and Preusse, 1999; Preusse et al., 2002; Alexander et al., 2008; Pitts et al., 2011]. However, the Himalayan region, and the adjacent Tibetan plateau, is less explored even though it comprises complex mountain ranges. This region also has profound dynamical and thermodynamic influences, affecting both the local and global climates [Boos and Kuang, 2010; Molnar et al., 2010]. They act as a physical barrier to the flow of air, leading to forcing of small-scale gravity waves to planetary-scale waves that can modify the atmospheric circulation [Trenberth and Chen, 1988; Barros et al., 2004]. In the present study, we analyzed a mountain wave event that occurred under the influence of strong westerlies flowing over the Himalayan region during a winter season. Gravity waves grow into significant strengths at places where the westerly wind was consistently strong from the lower troposphere to the upper stratosphere. The mountain wave event considered in the present study occurred during the winter season on 9 December 2008. The typical zonal wind structure that occurred during the winter over this region adjacent to the southwest of Tibetan plateau does not favor for the propagation of mountain gravity waves [Gong et al., 2011]. However, during this particular event, we observed weak wind changing its direction all over the troposphere that could favor mountain wave propagation into the stratosphere. The present study would be much informative to the gravity wave parameterization schemes as the Tibetan plateau is not very much dealt with in current GCMs [Kim et al., 2003]. The three-dimensional (3-D) properties of mountain wave event occurred over the western Himalayan mountain region are analyzed using the measurements made from the AIRS and MLS sounder instruments. The following sections provide information about the data sources, results, and summary of the present study.