Measurements of atmospheric turbulence with the dual-beamwidth method using the MST radar at Gadanki, India

Abstract

A brief experiment was conducted during 24-29 April and 9-10 May 2002, using the MST radar at Gadanki, India, to test the dual-beamwidth method of estimating the turbulence kinetic energy (TKE). Because the beamwidth can be modified on only one polarization at a time at Gadanki, an elliptical beam was used with a modified dual-beamwidth analysis. Estimates of the TKE from the dual-beamwidth method and the traditional method are very similar in regions of light winds (<~10ms-1). In regions of stronger wind (>~15ms-1) the traditional method often gives TKE<0 because the beam-broadening correction is greater than the observed spectral width. It is suggested that some of the problems with the traditional method are due to the uncertainty in the effective width of the radar beam. In all regions the modified dual-beamwidth method gives TKE>0 on the beam parallel to the prevailing wind; on this beam the estimates depend only on the ratio of the beamwidths, which is presumably well-known, and the observed spectral widths. The values of TKE from the dual-beamwidth method are approximately constant with height at 0.2m2s-2 from about 5 to 7.5km during the afternoon during both April and May (all April observations were made between 9:00 and 17:00 local time), and then decrease rapidly to about 0.02m2s-2 by about 9km. The data from May extend over one full diurnal period and the diurnal range of TKE during this period is found to be about 5dB below about 12km and from about 15 to 19km, near the tropopause, with maximum values during local afternoon.