THE ESTIMATION OF SURFACE MOMENTUM FLUX UNDER UNSTABLE CONDITIONS FROM THE ATMOSPHERIC-PRESSURE FIELD

Data from FIFE (the First International Satellite Land Surface Climato logy Project (ISLSCP) Field Experiment) were used in an atmospheric bo undary layer bulk similarity formulation to derive values of the frict ion velocity, u (i.e., the shear stress), from surface geostrophic wi nd measurements under unstable conditions. Several stability-dependent forms for the similarity functions B (corresponding to the x directio n geostrophic wind, u(g)) and B(w) (corresponding to the scalar geostr ophic wind speed, G) were tested. Formulations to calculate u from G were more successful than those which used only u(g). However, the dep endency of B, B(w), or the geostrophic drag coefficient, C(G) = mu/G, on either stability or baroclinicity was observed to be relatively we ak. For FIFE, a constant value of B(w) = 1.99 agreed with the referenc e u values almost as well as the best stability-dependent formulation s. A constant value Of C(G) = 0.0883, used with measured values of G, gave an unbiased estimate of u, on average, with a correlation r = 0. 80.