Variance method to determine turbulent fluxes of momentum and sensible heat in the stable atmospheric surface layer

Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined from the standard deviations of longitudinal wind velocity and temperature, sigma(u) and sigma(T) respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather stations, where they replace the standard cup anemometer-wind vane system. With methods such as the one described in this note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It is shown that a 'variance' dimensionless height (zeta(sigma)) defined from sigma(u) and sigma(T) is highly related to the 'conventional' dimensionless stability parameter zeta = z/L, where z is height and L is the Obukhov length. Empirical functions for zeta(sigma) are proposed that allow direct calculation of heat and momentum fluxes from sigma(u) and sigma(T). The method performs fairly well also during a night of intermittent turbulence.