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

G D Nastrom,P B Rao,V Sivakumar

doi:10.5194/angeo-22-3291-2004

Abstract

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.

Highlights

The traditional method of estimating atmospheric turbulence kinetic energy (TKE) from spectral widths requires the application of correction factors to the measured spectral widths due to the interaction of the radar beam with the background wind (e.g. Atlas, 1964; Hocking, 1985; Nastrom, 1997)
While these correction factors are well known in principle, estimates of them are sometimes larger than the observed spectral widths (Hocking, 1986; Fukao et al, 1994; Kurusaki et al, 1996; Nastrom and Eaton, 1997; Narayana Rao et al, 2001; Nastrom and Tsuda, 2001; Satheesan and Krishna Murthy, 2002), especially during relatively strong winds, which implies relatively large uncertainty in them or in the observed spectral widths, since the true corrections cannot be larger than the observed spectral widths
The dual-beamwidth method employs the spectral widths measured simultaneously with two different beamwidths and the ratio of the magnitudes of the beamwidths is used under the assumption that the TKE is the same in the sample volumes viewed by both beamwidths

Summary

Introduction

The traditional method of estimating atmospheric turbulence kinetic energy (TKE) from spectral widths requires the application of correction factors to the measured spectral widths due to the interaction of the radar beam with the background wind (e.g. Atlas, 1964; Hocking, 1985; Nastrom, 1997). The dual-beamwidth method for measuring TKE using spectral widths from Doppler radar was recently introduced by VanZandt et al (2002). To exercise the dual-beamwidth method was conducted using the MST radar at Gadanki, India, during 24–29 April 2002 and 9–10 May 2002, and the results obtained are described in this paper. Because the beamwidth of the Gadanki radar could be modified in only one polarization at a time, the dualbeamwidth estimates will be best when the beam is parallel to the prevailing wind, as explained below. Scattered cumulus and towering cumulus formed in the afternoon, no significant precipitation occurred at the radar site

Method of analysis

Vertical profiles

Diurnal changes