[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. 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.
Abstract. We have analyzed wind velocities measured with high-resolution Global Positioning System (GPS) radiosondes which have been flown continuously from Hyderabad for 120 h with an interval of 6 h. Hodograph method has been used to retrieve inertia gravity waves (IGWs) parameters. Background winds are removed from the time series by detrending, whereas polynomials of different orders are removed to obtain the fluctuations from individual profiles. A Butterworth filter is used to extract the monochromatic IGW component. Another filter finite impulse response (FIR1) is tried in a similar manner to test the effects of filters in estimating IGW characteristics. Results reveal that the fluctuation profiles differ with the change in polynomial orders, but the IGW parameters remain same when a Butterworth filter is chosen to extract the monochromatic wave component. The FIR1 filter produces results with a broader range. The direction of wave propagation can be confirmed with additional temperature information.
[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.,
Abstract. MST radars are capable of measuring vertical motion along a vertically directed beam. We present 8 years (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) averaged profile of vertical velocity in the troposphere and the lower stratosphere over Gadanki (13.5 • N, 79.2 • E), a tropical station. A downward mid-troposphericw is observed with a reversal of sign around 10 km and a further reversal can also be seen at ∼17 km. A significant diurnal and semidiurnal variation in vertical wind is observed for all heights with subsidence during the evening hours. Seasonal variability of vertical wind is also found to be quite appreciable. Vertical velocities have been derived using symmetric pairs of off-vertical beams and a comparison has been made with direct vertical beam measurements. Vertical components estimated from E-W and N-S radial velocities do not match and are also found to have discrepancy with direct measurements. Plausible causes of the discrepancy have been investigated with the help of some case studies. Vertical shear in horizontal wind, gradients in horizontal velocities and echo power imbalance may be some of the factors responsible for the observed discrepancy.
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