Effects of Wind Field Inhomogeneities on Doppler Beam Swinging Revealed by an Imaging Radar

Abstract

In this work, the accuracy of the Doppler beam-swinging (DBS) technique for wind measurements is studied using an imaging radar-the turbulent eddy profiler (TEP) developed by the University of Massachusetts, with data collected in summer 2003. With up to 64 independent receivers, and using coherent radar imaging (CRI), several hundred partially independent beams can be formed simultaneously within the volume defined by the transmit beam. By selecting a subset of these beams, an unprecedented number of DBS configurations with varying zenith angle, azimuth angle, and number of beams can be investigated. The angular distributions of echo power and radial velocity obtained by CRI provide a unique opportunity to validate the inherent assumption in the DBS method of homogeneity across the region defined by the beam directions. Through comparison with a reference wind field, calculated as the optimal uniform wind field derived from all CRI beams with sufficient signal-to-noise ratio (SNR), the accuracy of the wind estimates for various DBS configurations is statistically analyzed. It is shown that for a three-beam DBS configuration, although the validity of the homogeneity assumption is enhanced at smaller zenith angles, the rootmean-square (RMS) error increases because of the ill-conditioned matrix in the DBS algorithm. As expected, inhomogeneities in the wind field produce large bias for the three-beam DBS configuration for large zenith angles. An optimal zenith angle, in terms of RMS error, of approximately 9°-10°was estimated. It is further shown that RMS error can be significantly reduced by increasing the number of off-vertical beams used for the DBS processing.

Mean wind vector estimation using the velocity–azimuth display (VAD) method: an explicit algebraic solution

“…supporting and explaining results obtained by Cheong et al (2008), who have experimentally shown that the MSE or likewise the RMSE of the wind retrieval is significantly reduced by increasing the number of off-vertical beams in the Doppler beam-swinging technique in the presence of wind field inhomogeneities. Note, however, that for the vertical wind component only the random error can be reduced by an increase of N.…”

An assessment of the performance of a 1.5 μm Doppler lidar for operational vertical wind profiling based on a 1-year trial

“…It is well known that the wind field is not always horizontally homogeneous Cheong et al, 2008), this is mainly due to convection, gravity waves or shear induced turbulence. Characteristic temporal and spatial scales for turbulence are T = 10 s and L = 1 m. For thermally induced convective processes we typically have T = 5 min and L = 500 m. Thus, with reference to a full DL scan lasting about 3 min and with a scanning circle having height dependent diameters d C of about d C ∼ 300 m at an altitude of ∼ 550 m and d C ∼ 5360 m at ∼ 10 km it is often the case that due to the occurrence of turbulent motions there are rapid wind fluctuations along the scanning circle and accordingly the assumption of a horizontally homogeneous wind field is not fulfilled.…”

Investigation of the Planetary Boundary Layer in the Swiss Alps Using Remote Sensing and In Situ Measurements

“…c Profile of the corrected signal-to-noise ratio SNR cor retrieved from the wind profiler measurements and derived PBL height (horizontal line) recorded on the 7 July 2010 at 1200 UTC and the frequency-domain steps (Ruffieux and Stübi 2001;Vaisala 2007). An important assumption is a homogeneous wind field over the beams' separation (Koscielny et al 1984;Cheong et al 2008), which cannot be fully guaranteed in the complex topography of the Alps.…”