Wind speed measurements in upper and lower boundary layer to determine regional momentum fluxes

Abstract

Radiosonde profiles in the atmospheric boundary layer (ABL) with other supporting measurements obtained during NOPEX Second Concentrated Field Effort over a complex area in Sweden were analyzed to determine regional momentum fluxes. Two methods were explored, which consist of (i) the combined similarity approach (CSA) for the entire ABL derived by assuming an overlap region between the mixed layer and surface sublayer and by joining two governing equations, and (ii) the surface sublayer profile equation derived from Monin-Obukhov similarity theory. The CSA has an advantage in that it does not require the knowledge of the regional roughness length which itself is not easy to determine. The surface sublayer profile equation needs the roughness length as input. This was estimated by a model of Grant and Mason (1990) for the upwind area with the size of approximately 26km2, from surface information such as the mean roughness height and density, with assumed drag coefficient of major obstacles, Dh/2=0.89, taken from previous study in a similar experimental area. A good agreement was found between the momentum fluxes derived from the two methods, and it shows the validity of the model used in the estimation of z0 and the assumed value of Dh/2=0.89 for the NOPEX region. The momentum fluxes from the eddy correlation method with an instrument installed at 44m on a tall tower were found to be much smaller than those derived from the profile equation with the wind speed measurements at the same height. The difference appears to be due to the smaller size of the footprint area of momentum flux than that of the wind speed measurements.