[1] High-frequency gravity waves generated by tropical deep convection play a major role in shaping the general circulation of the middle atmosphere. Special experiments were conducted to capture two convective events on 16 May and 5 June 2006 using VHF radar located at Gadanki (13.5°N, 79.2°E), a tropical Indian station. Control day observations were also made for necessary comparisons. Background wind and temperature information was obtained by GPS radiosonde flights launched from the same site. This work has utilized these valuable data sets to delineate characteristics of convectively generated gravity waves. A superposition of gravity waves is observed with different scales after the deep convective events. Vertical wave number spectra of radial velocities show steeper slopes and higher power spectral densities during convection which slowly reduce to their normal values. The present case studies suggest the mechanical oscillator mechanism to be a major source of convective gravity wave generation in the tropics. Estimates of vertical wind variances and momentum fluxes of short-period (<2 h) wind fluctuations show large enhancements on convective days in comparison to control days. The momentum flux frequency spectra revealed a higher contribution of 30-65 min wave periods to the mean profile in the lower stratosphere. The wavelet transform momentum flux spectra displayed the temporal variability and discretization of the gravity wave momentum fluxes in frequency and time.

Abstract. Wind observations made at Gadanki (13.5°N) by using Indian MST Radar for few days in September, October, December 1995 and January, 1996 have been analyzed to study gravity wave activity in the troposphere and lower stratosphere. Horizontal wind variances have been computed for gravity waves of period (2±6) h from the power spectral density (PSD) spectrum. Exponential curves of the form e ar have been ®tted by least squares technique to these variance values to obtain height variations of the irregular winds upto the height of about 15 km, where Z is the height in kilometers. The value of H, the scale height, as determined from curve ®tting is found to be less than the theoretical value of scale height of neutral atmosphere in this region, implying that the waves are gaining energy during their passage in the troposphere. In other words, it indicates that the sources of gravity waves are present in the troposphere. The energy densities of gravity wavē uctuations have been computed. Polynomial ®ts to the observed values show that wave energy density increases in the troposphere, its source region, and then decreases in the lower stratosphere.

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Abstract. High resolution (150 m) wind measurements from 13-17 July 2004 by Mesosphere-Stratosphere-Troposphere (MST) radar and 15-16 July 2004 by Lower Atmospheric Wind Profiler (LAWP) have been used to study the time variation of turbulence intensity. Layers of higher turbulence are observed in the lower stratosphere on 15-16 July which give rise to mixing in the region. Enhancement in short-period gravity wave activity and turbulent layers are observed after 22:00 LT which could be due to a dry convection event that occurred at that time. The breakdown of the convectively generated high frequency waves seems to have given rise to the turbulence layers. Wind shear is found to be high above the easterly jet, but very poor correlation is observed between square of wind shear and turbulence parameters in the region. The heights of the turbulent layers in the lower stratosphere do not correlate with levels of minimum Richardson number. A monochromatic inertia gravity wave could be identified during 13-17 July 2004. A non-linear interaction between the waves of different scales as proposed by Hines (1992) might also be responsible for the breakdown and generation of turbulence layers.

[1] VHF radar at Gadanki (13.5°N, 79.2°E) was operated continuously for 24 hours to obtain a unique set of high resolution (150 m in height and 2.5 min. in time) wind data in the troposphere and lower stratosphere. This high quality wind data have been used to estimate momentum flux of short period (<2 hr) wind fluctuations and the associated uncertainties. Error analysis has been carried out by varying the length of integration time. We show that an optimal time of averaging of about 15-16 hrs minimizes the error at a value nearly equal to the irreducible error observed by an ideal anemometer measuring u, v and w at a point. Citation: Dutta, G.,