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

MEASUREMENTS OF ATMOSPHERIC TURBULENCE WITH THEDUAL-BEAMWIDTH METHOD USING THE MST RADAR ATGADANKI, INDIAG. Nastrom * , P.B. Rao + , V. Sivakumar #*St. Cloud State University, St. Cloud, Minnesota 56379 USA+ADCOS-2, National Remote Sensing Agency, Hyderabad 500037 AP India#Universite de la Reunion, Cassin-97715 St. Denis-C9 France1. IntroductionA brief experiment was conducted during April and May, 2002, using the MSTradar at Gadanki (13.47 o N, 79.18 o E) operating at 53 MHz with average power apertureproduct of 7x10 8 Wm 2 to test the dual-beamwidth method of estimating the turbulencekinetic energy (TKE) recently introduced by VanZandt et al. (2002). Because thebeamwidth can be modified on only one polarization at a time at Gadanki, an ellipticalbeam was used with a modified dual-beamwidth analysis. Estimates of the TKE from thedual-beamwidth method and the traditional method using corrections for beam-, shear-, andwave-braodening are very similar in regions of light winds (~15 ms -1 ) the traditional method often gives TKE0 on the beams parallel to the prevailingwind. It is suggested that the problems with the traditional method are due to uncertainty ofthe effective width of the radar beam. The data from May extend over a full diurnal period,and the diurnal range of TKE is found to be about 5 dB below about 12 km and near thetropopause, with maximum values during local afternoon.2. DataThe Gadanki radar antenna array consists of 1024 crossed 3-element Yagi antennascovering 130x130 m. Peak transmitted power is 2.5 MW obtained from 32 transmitterseach feeding a sub-array of 32 Yagis. The one-way half-power full beamwidth of the fullantenna is about 2.9 o .During this experiment two beamwidths were interleaved each hour. A narrowbeamwidth (2.9 o ) was obtained using the full antenna. By disconnecting 16 subarrays fromeach polarization of the antenna a second beamwidth was obtained, about 5.8 o (i.e., the ratioof the broad and narrow beams is about 2). Because the broad beam could be formed onlyin one polarization at a time, the resulting beam was elliptical (5.8º by 2.9º degrees) and ourapplication of the dual-beamwidth method accounts for this ellipticity. Our observationalstrategy included a total of 10 beam positions each hour for each beamwidth: vertical andtoward the 4 cardinal directions for 10 o and 15 o zenith angles. Range-resolution was 150 mfrom 3.6 to 24.9 km. Data from the vertical beam are not used in this study.Typically, 6-8 profiles were obtained with each beam direction and beamwidth eachhour. Observations were made usually from 1100-1700 local time on 24, 25, 26, 27, and 29April 2002. Another set of observations were taken over a diurnal cycle from 1500 9 May-1500 10 May 2002. Hourly medians are used for the analyses below.210Local weather conditions during late April and early May 2002 were very hot, withdaily maximum temperatures near 40 o C every day (the sun is directly overhead at noon at