A study of mixed-layer momentum evolution

Abstract

Evolution of mixed-layer momentum is studied using a zeroth-order jump model of the mixed layer. The geostrophic Ekman number for the mixed layer is found to be an important parameter in determining the behavior of mixed-layer momentum. Two types of parameterizations of the surface momentum fluxes, viz. linear and quadratic friction relations, are compared. For the stationary case, the two models become identical when the linear friction factor r is chosen to give the same cross-isobaric angle predicted by the quadratic friction model. Theoretically the linear friction relation provides exponential decay of inertial oscillation while the quadratic friction model introduces damping inversely proportional to time. In actual numerical simulations, however, these model differences are not very significant, especially when r which is taken as a function of time, is appropriately estimated. From the case studies of Cabauw data (Driedonks, 1981) some important features of mixed-layer momentum evolution during daytime are revealed. Similar to the thermodynamic evolution of the mixed layer, the momentum field also experiences diurnal changes which are seen through the diurnal variations of the relative contributions of surface friction, entrainment of momentum and large-scale pressure gradient forcing in the mean kinetic energy budget and the diurnal variation of cross-isobaric angles of the mixed-layer winds. However, for the days when advection and other large-scale effects are present, the predictions of the model are not favorable.