Characteristics of Inertia Gravity Waves Generated by Sub-tropical Westerly Jet Stream over the Sub-Himalayan Region

Gravity waves play a major role in the dynamical coupling between different atmospheric layers. Importance of vertical wind shears associated with jet streams in the generation of gravity waves is well-recognized. An experimental campaign for investigating the tropospheric dynamical characteristics at sub-Himalayan region was carried out during 6-17 March 2015 with balloon borne GPS radiosonde observations of the altitude profiles of wind and temperature at 3 hourly interval, simultaneously from two locations at northeast India which are separated by 50 km: Guwahati (26.1°N, 91.8°E) and Umiam (25.7°N, 91.9°E). Satellite observations of IR and water vapour band brightness temperature, MERRA reanalysis and the radiosonde observations show that these locations were manifested by dry troposphere with cloud-free sky, descending limb of Hadley cell and core of the subtropical westerly jet stream (SWJ). The 3-hourly GPS radiosonde observations carried out continuously for 12 days at the above stations were used to investigate the characteristics of gravity waves, potentially generated by the vertical wind shears associated with SWJ. The observations carried out at two stations enable investigations on gravity wave propagation characteristics. During the experiment period, the SWJ core at both stations was located between 11-13 km with peak westerly wind speeds of 55-60 ms-1. Vertical wind shear associated with the jet stream typically varied between 10 to 30 ms-1km-1 between the jet core and 15 km altitude. Anomalies in the temperature and zonal and meridional wind profiles at both these stations show that gravity waves with vertical wavelengths of 1.2-2 km propagate upward above the jet core up to 30 km altitude with amplitudes of 3-6 ms-1 in wind and 2-3 K in temperature. Wind hodograph analysis shows downward phase propagation with period of 20-30 hr, which is close to the inertial period ( 28 hr) at these locations. Phase differences between the wave perturbations at the two locations were <60°, indicating long horizontal wavelengths of >300 km. Similar results were also obtained from the dispersion and polarization relationships. This study shows the importance of inertia gravity waves produced by wind shear associated with westerly jet stream in the vertical transport of energy into the stratosphere.