Abstract
Abstract. This study quantifies the uncertainties in winds measured by the Aberystwyth Mesosphere–Stratosphere–Troposphere (MST) radar (52.4° N, 4.0° W), before and after its renovation in March 2011. A total of 127 radiosondes provide an independent measure of winds. Differences between radiosonde and radar-measured horizontal winds are correlated with long-term averages of vertical velocities, suggesting an influence from local mountain waves. These local influences are an important consideration when using radar winds as a measure of regional conditions, particularly for numerical weather prediction. For those applications, local effects represent a source of sampling error additional to the inherent uncertainties in the measurements themselves. The radar renovation improved the signal-to-noise ratio (SNR) of measurements, with a corresponding improvement in altitude coverage. It also corrected an underestimate of horizontal wind speeds attributed to beam formation problems, due to pre-renovation component failure. The root mean square error (RMSE) in radar-measured horizontal wind components, averaged over half an hour, increases with wind speed and altitude, and is 0.8–2.5 m s−1 (6–12% of wind speed) for post-renovation winds. Pre-renovation values are typically 0.1 m s−1 larger. The RMSE in radial velocities is <0.04 m s−1. Eight weeks of special radar operation are used to investigate the effects of echo power aspect sensitivity. Corrections for echo power aspect sensitivity remove an underestimate of horizontal wind speeds; however aspect sensitivity is azimuthally anisotropic at the scale of routine observations (≈1 h). This anisotropy introduces random error into wind profiles. For winds averaged over half an hour, the RMSE is around 3.5% above 8 km, but as large as 4.5% in the mid-troposphere.
Highlights
Radar wind profilers have become an established tool for measuring wind profiles in the boundary layer, the free troposphere and lower stratosphere, and the lower mesosphere
In this paper we examine in detail how the error characteristics of a Doppler beam swinging (DBS) VHF wind profiler – the Natural Environment Research Council’s (NERC) Mesosphere– Stratosphere–Troposphere (MST) radar at Capel Dewi, near Aberystwyth in mid-Wales (52.4◦ N, 4.0◦ W) – changed after extensive refurbishment of the system in 2011
Post-renovation speeds, corrected for aspect sensitivity, show no overall difference (Fig. 1d), but wind speed is overestimated below 10 km and underestimated above (Fig. 1b)
Summary
Radar wind profilers have become an established tool for measuring wind profiles in the boundary layer (at UHF wavelengths), the free troposphere and lower stratosphere (at lower-UHF and VHF), and the lower mesosphere (at VHF wavelengths). Worthington et al (1999b) created maps of aspect sensitivity, using both the MU (Middle and Upper Atmosphere) radar in Japan and the NERC MST radar at Aberystwyth (used in this study) They showed that the peak in echo power can be displaced from the zenith, and that displacement direction was correlated with the direction of wind shear. Thomas et al (1997) made a comparison with 75 radiosondes launched from Aberporth, 50 km southwest of the radar site They concluded that the radar underestimated horizontal wind velocities by 4–5 %, and that aspect sensitivity corrections were necessary to remove the bias.
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