Meteorologische Zeitschrift (Dec 1998)
VAD winds from C band Ericsson Doppler Weather Radars
Abstract
The VAD (Velocity Azimuth Display) technique for retrieving winds from a single Doppler weather radar is well known as a robust and simple one, though still not used on its merits. Precipitation generally gives VAD winds, but it is less known that even in high latitudes during the warmer seasons the Doppler weatherradars of today record enough clear air echos to give wind profiles in the planetary boundary layer. There are, however, few verifications of VAD winds in general, and hardly any concerning clear air. In this paper mainly VAD winds from one Ericsson Doppler Weather Radar in Jonsered, Gothenburg (57.723° N, 12.172° E, 164 m above MSL) are compared to Radiosonde Winds (RAWINDs) from Landwetter (57.668° N, 12.296° E, 155 m above MSL). The sites are about 10 km apart, and the radiosonde is within the range used for theVAD (30 km). The VAD soundings were made each hour, and the radiosoundings every 6th (00, 06, 12 and 18 UTC). About seven months of data were available (9 Dec. 1994 to 14 Feb. 1995 and 28 Jun. to 30 Nov. 1995). The LORAN C system was used to retrieve the RAWINDs during the first period (Dec. 1994 to Feb. 1995), and the OMEGA system was used during the second period. The comparisons are made using the five standard pressure levels, 925, 850, 700, 500 and 400 hPa, corresponding to heights above MSL of about 750, 1450, 3000, 5600 and 7200 m. As overall results, the average differences, in m/s, between the VAD and RAWIND were largest at the lowest (geometrically) levels, and higher up more or less constant with height. This is remarkable, since the wind speed increases with height, and the relative differences thus decrease with height. As an example, the average of the magnitude of the wind vector differences was 3.2 m/s at 925 hPa, but about 2.8 m/s at the higher levels. The differences also tend to be somewhat larger for winds retrieved from clear air echos. In the planetary boundary layer during summer, that is up to about 800 hPa, the availability of VAD winds was about 90 %. The availability decreases with height, and at 400 hPa it was 15 % for the whole period. Comparisons are also performed between VAD winds and winds from a limited area model, HIRLAM.
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