Wind characteristics of CzeCOS’s ecosystem station Bílý Kříž

Eddy-covariance is a direct and accurate method to measure exchanges of greenhouse gases and energy between ecosystems and the atmosphere under good turbulent mixing conditions. Calm conditions can affect the accuracy of the eddy-covariance measurements by potential gas buildup below the measurement height. This study characterizes wind patterns and calm periods at Bílý Kříž Ecosystem Research Station, using four-year (2010-2013) wind data measured on an eddy-covariance tower at the site, located in Beskids Mountains, Czech Republic. The site is covered by young, dense Norway spruce forest on a 12.5° slope facing South. The results show that the prevailing wind direction was from S to SW direction (55 % frequency of occurrence), and to a much lesser extent from N, NE and NW but including the strongest winds above 12 m s-1, possibly during gale or storm events. Calm condition (u < 1 m s-1) accounted for 34 % of year time. Summer and Spring were the calmest seasons. Occurrence frequencies of calm conditions for the six-month periods from May to October (plant growing season) were higher than those for the remaining parts of the year (on average 39 % vs 28 % respectively). One should be aware of the calm periods in May-October when processing eddy-covariance data, since it is the time period of most contribution of forest ecosystem as a sink of CO2.