Evaluation of 3-beam and 4-beam profiler wind measurement techniques

Most contemporary wind profilers estimate wind profiles from three- or five-beam measurements using the Doppler Beam Swinging (DBS) method. In the traditional 3-beam technique, the horizontal components of wind are derived from two orthogonal oblique beams and the vertical beam. However, vertical beam measurements are easily contaminated by ground clutter. On the other hand, the 5-beam profiler does not need to rely on the vertical antenna beam, and the horizontal components derived from the four oblique beams can be expected to have a better accuracy than the horizontal components derived from only three beams. One of the disadvantages of the 4-beam method is that it needs greater stationarity and homogeneity in the horizontal wind field. The four-beam method has been used since the early stages of the UHF wind profiler along with the three-beam method, but the difference of the two methods in accuracy in measuring horizontal components of wind has not been systematically documented. In this study, we report a detailed evaluation of the two methods by comparing data collected by a 5-beam profiler and processed in two different ways with winds collected by the collocated meteorological tower at the Meteorological Research Institute (MRI) field site in Tsukuba, Japan. Results show that, while the winds measured using both methods are in overall agreement with the tower measurements, some of the horizontal components of the 3-beam derived winds are clearly spurious when compared with the tower-measured winds or the winds derived from the four oblique beams. We found that many of these outliers in the 3-beam measurements were associated with periods of transition between clear-air and rain. Even after the removal of the rain data, the precision of the 3-beam method is still much less than that of the 4-beam method. These results suggest that spatial variability of the vertical airflow in non-rainy periods or hydrometeor fall velocities in rainy periods makes the vertical-beam velocities significantly less representative over the area across the three beams, and decreases the precision of the 3-beam method.