Variability of the stable and unstable atmospheric boundary-layer height and its scales over a boreal forest

Radiosondes releases during the NOPEX-WINTEX experiment carried out in late winter in Northern Finland were analysed for the determination of the height h of the atmospheric boundary layer. We investigate various possible scaling approaches, based on length scales using micrometeorological turbulence surface measurements and the background atmospheric stratification above h. Under stable conditions, the three previously observed turbulence regimes delineated by values of z/L (L is the Obukhov length) appears as a blueprint for understanding the departures found for the suitability of the Ekman scaling based on L-E = u(*)/f (u(*) is the friction velocity and f the Coriolis parameter). The length scale L-N = u(*)/N (where N is the Brunt-Vaisala frequency) appears to be a useful scale under most stable conditions, especially in association with L. Under unstable conditions, shear production of turbulence is still significant, so that the three scales L, L-N and L-E are again relevant and the dimensionless ratios mu (N) = L-N/L and L-N/L-E = N/f describe well the WINTEX data. Furthermore, in the classical scaling framework, the unstable domain may also be divided into three regimes as reflected by the dependence of u(*)/f on instability (z/L).