Abstract
Comparisons are made between horizontal wind measurements carried out using a VHF-radar system at Aberystwyth (52.4°N, 4.1°W) and radiosondes launched from Aberporth, some 50 km to the south-west. The radar wind results are derived from Doppler wind measurements at zenith angles of 6° in two orthogonal planes and in the vertical direction. Measurements on a total of 398 days over a 2-year period are considered, but the major part of the study involves a statistical analysis of data collected during 75 radiosonde flights selected to minimise the spatial separation of the two sets of measurements. Whereas good agreement is found between the two sets of wind direction, radar-derived wind speeds show underestimates of 4â6% compared with radiosonde values over the height range 4â14 km. Studies of the characteristics of this discrepancy in wind speeds have concentrated on its directional dependence, the effects of the spatial separation of the two sets of measurements, and the influence of any uncertainty in the radar measurements of vertical velocities. The aspect sensitivity of radar echoes has previously been suggested as a cause of underestimates of wind speeds by VHF radar. The present statistical treatment and case-studies show that an appropriate correction can be applied using estimates of the effective radar beam angle derived from a comparison of echo powers at zenith angles of 4.2° and 8.5°.
Highlights
IntroductionRoÈ ttger (1981) pointed out that with relatively broad radar beams, this aspect sensitivity of the echo power could cause a discrepancy between the bore-sight radar beam direction and the eective beam direction for relatively small zenith angles, and this could lead to an underestimate of a measured horizontal wind speed
During the past two decades considerable attention has been paid to the use of stratospheric-tropospheric radars to improve our understanding of mesoscale processes and to contribute to numerical weather forecasts
RoÈ ttger (1981) pointed out that with relatively broad radar beams, this aspect sensitivity of the echo power could cause a discrepancy between the bore-sight radar beam direction and the eective beam direction for relatively small zenith angles, and this could lead to an underestimate of a measured horizontal wind speed
Summary
RoÈ ttger (1981) pointed out that with relatively broad radar beams, this aspect sensitivity of the echo power could cause a discrepancy between the bore-sight radar beam direction and the eective beam direction for relatively small zenith angles, and this could lead to an underestimate of a measured horizontal wind speed Support for this suggestion was provided by estimates for two days of the horizontal wind component with beam zenith angles of 2°±12° by Tsuda et al (1986) and by comparisons of radar wind measurements at zenith angles of 4.2°, 6° and 12° with balloon-based measurements in sets of 20±30 balloons for each zenith angle by Astin and Thomas (1991). The correction for aspect sensitivity of horizontal wind estimates derived using nominal o-vertical beam angles of 6° has made use of power measurements at zenith angles of 4.2° and 8.5° to derive the eective beam angle. Hooper and Thomas (1995) have demonstrated that a combination of power measurements in the 6° and vertical directions over-corrects the radar wind estimates
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