Observations And Simulations Of Gravity Waves Over The Tibetan Plateau

In this paper, some statistics of gravity waves over the Tibetan Plateau are investigated based on observations obtained by GPS radiosondes which were launched over Gerze and Litang station during the PRC-Japan cooperative JICA project. The statistical results show that the sources of gravity waves can be either in the lower troposphere or in the upper troposphere, and the sources in these two cases are equally important; most gravity waves over Gerze and Litang station have the same frequency 1.5–3.5 times that of the Coriolis frequency, and the dominant vertical wavelengths of these waves are also mainly concentrated in 1.5–2.5 km, indicating that the two parameters had no significant spatial variation; For Gerze station, most gravity waves propagate West, East, and Southeast ward. And the gravity waves over Litang station mainly propagate North, Northwest, Southwest, Southeast, and East ward.Meanwhile, gravity waves in the troposphere occurring over complex terrain of the Tibetan Plateau in mid-May 2008 were also investigated, based on both high vertical resolution L-band radiosonde data from Ganzi station and mesoscale model MM5. The observational results show that the observed gravity wave has a vertical wavelength of about 2.7 km, a horizontal wavelength of about 104 km, an intrinsic frequency of about 4 f , which corresponds to an intrinsic period of 6.5 hours or so, an intrinsic phase speed of about 4.5 m/s, and a horizontal propagation direction of about 182.2 degree, which points to West direction, with wave energy propagating upward.The results from the simulations indicate that:(1) The mesoscale model MM5 successfully simulated the gravity wave in the troposphere occurring over complex terrain of the Tibetan Plateau, the gravity wave characteristics obtained from simulations were well consistent with the observations.(2) The gravity wave began to develop at 1600 UTC on 12 May 2008, and basically disappeared at 0300 UTC on 13 May 2008, lasting about 11 hours. The gravity wave was quasi-static along the direction of wave propagation, while propagating upstream and from northeast to southwest in the background flow.(3) The wave energy mostly concentrated in the lower troposphere, and mainly propagated upward throughout the entire evolutionary process of the gravity wave. During the development stage (before 2100 UTC 12 May 2008), the directions of wave front from vertical cross-sections were characterized by co-existence of upstream, downstream tilt, and approximately perpendicular to the ground with increasing altitude, while eventually became uniformly upstream tilt with increasing altitude as the wave developed, indicating that the directions of gravity wave energy propagation were not always upward uniformly during the whole evolution, but both of the upward and downward propagation directions were coexisted in the early stage of development.(4) The configuration structures for the zonal wind perturbation, the meridional wind perturbation, and the potential temperature perturbation of the gravity wave in the horizontal direction did not fully agree with the linear theory of inertia gravity waves, although they were well consistent with the linear theory in the vertical direction.(5) The regional topography primarily of Ganzi and its surrounding area was the most important source of the gravity wave in this study.