Wind-stilling in the light of wind speed measurements: the Czech experience

ABSTRACT: Changes of instruments often give rise to significant break-points in wind-speed series. This is particularly applicable when measurements are automated, a process that started in the Czech Republic in the mid-1990s, when standard universal anemographs were progressively replaced by the Vaisala WAA251 sensor (cup anemometer) and the WS425 sensor (ultrasonic). Parallel wind speed measurements, by universal anemographs and Vaisala sensors, at the Doksany (2000-2016) and Kocelovice (2000-2016) stations enabled differences in the 2 types of measurement to be analysed. Vaisala sensors measure, on average, higher wind-speeds than universal anemographs, particularly in calm situations and at low wind speeds. The differences between the 2 types of instrument do not depend on wind direction. Linear trends of homogenised daily mean wind-speed series from only a universal anemograph or combined from universal anemograph and Vaisala sensors generally exhibit no important differences in their significance or values. This could indicate that observed decreasing trends in mean daily wind-speeds (‘stilling’) cannot be attributed to changes of wind-speed measurements to automated devices; with respect to only 2 stations being investigated, this is biased by high uncertainty. In contrast, important differences in the 2 types of measurement, both in linear trends and their significance, appear for series of 3 daily readings (07:00, 14:00 and 21:00 h LMT).