Proceedings with Extended Abstracts (single PDF file) - Radio ...

STUDIES ON MOMENTUM FLUXES USING MST RADAR WINDSOBSERVED AT GADANKI (13.5°N, 79.2° E), INDIAD. Narayana Rao 1 , I.V. Subba Reddy 2 , P. Kishore 3 , S.P. Namboothiri 3 , K. Igarashi 3 ,K. Krishna Reddy 4 and S.V. Bhaskara Rao 3 .1 National MST Radar Facility, P.B. No.: 123, Tirupati – 517 502, India2 Department of Physics, Sri Venkateswara University, Tirupati - 517 502, India3 Communications Research Laboratory, 4-2-1 Nukui-kita machi, Kognei-shi Tokyo 184-8795, Japan4 Yokohama Institute of Earth Sciences, Frontier Observational Research System for Global Change(FORSGC), 3173-25, Showa-Machi, Kanazawa – KU, Yokohama, Kanagawa – 236-0001, JapanABSTRACTVHF Doppler radars provide a unique database to estimate vertical flux of horizontal momentumin the troposphere and lower stratosphere. Estimation of the oblique momentum flux involves two methods:1) using three beams – one vertical and two oblique, and 2) using four beams – two pairs of oblique beamssystematically offset from the vertical. The rapid steerability of the Indian MST radar allows to make three –and four beam measurements simultaneously. The momentum fluxes measured by the two methods arealmost the same for wind fluctuations in a fairly long period range (longer than 5 h). We choose frequencybands corresponding to periods of 30 min-2h, 2-8 h, 2-16h and 2-24 h. Vertical profiles of the zonal andMeridional flux in each frequency band were found to be consistent, in general, with the total flux. Zonalfluxes were small at lower levels and increasingly negative ( westward) at higher heights. The dominantcontributions to the Meridional flux occur in the lower-frequency band.IntroductionThe importance of gravity wave effects on various dynamical processes has been welldocumented in the upper and middle atmosphere. However, less is known about these waves in thethe lower atmosphere especially in tropics. The upward propagating gravity waves transport energyand momentum in different regions of the atmosphere along with their propagation to produceeffects at upper heights [Hines, 1972] indicated that the influence of such waves at upper heightsmight be significant. Most of the theoretical and modeling studies were primarily concentrated onthe effect of gravity waves in the mesosphere and thermosphere, but later it has been found thatthese effects are also important in the troposphere and stratosphere. In early observations it isidentified that for lower heights, the momentum divergence caused by mountain waves [Newton,1971], and the importance of such divergence in the evolution of the general circulation wasemphasized by Lilly [1972]. Furthermore, the lack of height coverage at a given time in aircraftflights is a limitation. Vincent and Reid (1983) presented a technique in which ground-based radarscan be used to measure fluxes directly and this technique has been applied to both mesosphericheights [Vincent and Fritts, 1987; Tsuda et al., 1990] and tropospheric and lower stratosphericlevels [Fritts et al., 1990; Thomas et al ., 1992]. Because of the continuous operation of multipleinstruments, long term studies have been conducted to quantify the variability and latitudinaldifference of gravity wave activity [e.g. Vincent and Fritts, 1987; Manson and Meek, 1993;Vincent, 1994]. It has been found that from theoretical and observational studies that the highfrequency gravity waves with small horizontal scales are the majority carriers of momentum. ~70 %of the momentum and the associated zonal drag is due to gravity waves with observed periods lessthan one hour [Fritts and Vincent, 1987]. Chang et al., [1997] used ST radar data from ChristmasIsland to study tropospheric gravity waves and they found a broad range of frequency associatedwith gravity wave activity in the troposphere. A method of Vincent and Reid [1983] is used toestimate the vertical flux of zonal and meridional momentum fluxes from the radial velocities intwo orthogonal beams measured by the Indian MST Radar at Gadanki (13.48º N, 79.18º E) ispresented. These flux values are quite well with the values obtained from previous observations.342