Characteristics of atmospheric waves in the upper troposphere observed with the Gadanki MST Radar—RASS T.V. Chandrasekhar Sarma a,n , Yasu-Masa Kodama b , Toshitaka Tsuda c a National Atmospheric Research Laboratory (NARL), Gadanki 517 112, AP, India b Department of Earth and Environmental Sciences, Hirosaki University, Hirosaki, Aomori 036 8561, Japan c Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Kyoto 611 0011, Japan article info Article history: Received 30 November 2009 Received in revised form 6 August 2010 Accepted 11 August 2010 Available online 18 August 2010 Keywords: RASS MST radar Gravity wave Atmospheric tides abstract Indian MST Radar at Gadanki (13.46% o N, 79.17% o E) working at 53 MHz was operated continuously in Radio Acoustic Sounding System (RASS) mode with six distributed acoustic exciters during 22–25 August 2007 for about 69 h in RASS and turbulence echo modes alternately, with one cycle spanning about 20 min. Temperature and wind velocity were observed in the altitude range 3.6–12 km most of the time and 3.6–20 km, respectively, with a range resolution of 150 m. Presence of an inertia–gravity wave in the lower stratosphere was identified. Perturbations in the wind and temperature fields in the troposphere, however, indicated a mixture of waves with a wide frequency range, within which a dominant periodicity of 8 h with a vertical wavelength of 4 km was revealed by two-dimensional spectral analysis. Further, signatures of diurnal tide were also observed and the temperature phase profile was found to exhibit a close match with the Global Scale Wave Model (GSWM02). From the RASS virtual temperature profiles, time–height section of Brunt-V ¨ ais ¨ al¨ a frequency squared was computed to deduce the background atmospheric stability, which showed stable layered structures with slow downward phase progression. Outgoing Longwave Radiation (OLR) over Gadanki and TRMM precipitation data over peninsular India was used as an indicator of convective activity. During the beginning of the observation period, even though lower OLR was seen, corresponding convective activity or precipitation was not seen in the TRMM data. On 24 August, enhanced temperature perturbations could be related to widespread precipitation shown by TRMM. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction VHF Radio acoustic sounding system (RASS) has been estab- lished as a reliable technique for simultaneous ground-based remote profiling of vector wind velocity and atmospheric virtual temperature in the lower atmosphere on a continuous basis (e.g., Marshall et al., 1972). RASS is a unique technique that can operate during day and night in nearly all weather conditions. Matuura et al. (1986) showed the capability of this technique to profile atmospheric virtual temperature up to the lower strato- sphere for the first time, and it was also demonstrated in the tropics by Sarma et al. (2008). Continuous temperature profiles are used to clarify the behaviour of temperature perturbations associated with meteorological phenomena, atmospheric waves and the stability structure (Neiman et al., 1992; Tsuda et al., 1992; Alexander et al., 2007; Alexander and Tsuda, 2008a). RASS consists of a wind profiling radar and collocated acoustic sources. Acoustic excitation is generated at a wavelength that is half of the radar transmission wavelength, so as to obtain echoes by means of Bragg scatter from the propagating acoustic wavefronts. As the sound speed generally decreases with an altitude in the troposphere, the frequency of an excitation required for obtaining radar backscatter at different altitudes is different in order to keep the wavelength relationship with the radar transmission (Masuda et al., 1992). Therefore, an FM-chirped acoustic pulse is normally employed to expand the height coverage of RASS observations (Sarma et al., 2008). The radar measures the wind velocity and the propagation speed of acoustic wavefronts, C a (ms 1 ), which is related to ambient temperature T (K) T ¼ C a k h   2 ð1Þ The constant k h (JK 1 kg 1 ) in Eq. (1) is given by k h ¼ gR M   1=2 ð2Þ Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.08.010 n Corresponding author. Tel.: +91 8585 272008  272024; fax: + 91 8585 272021  272018. E-mail address: tvcsarma@narl.gov.in (T.V. Chandrasekhar Sarma). Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1020–1030