THE STRUCTURE OF KATABATIC FLOWS DOWN A SIMPLE SLOPE

An analysis of the katabatic flow structure is presented, based on dat a collected along a simple slope over a two-month period. Four meteoro logical stations were deployed along the nearly two-dimensional east s lope of Mt. Hymettos in Greece. The study focuses on the momentum and energy budgets of the flows during carefully selected stationary perio ds with negligible ambient winds. In order to achieve a proper selecti on, the mean features of the observed flows are presented along with t heir response to external winds. The observed profiles are successfull y compared with the theoretical Prandtl profiles. Bulk quantities of t he katabatic layer are compared with proposed experimental values in o rder to check the representativity of the observed flows. These compar isons offer a further test of theoretical considerations and assist in discussions of processes affecting mean-flow features. The flows were similar to 20 m deep, with velocities 1-2 m s(-1). During steady-stat e periods the katabatic acceleration was mainly balanced by the surfac e friction, the contribution of advective terms being small. The estim ated interfacial drag was less important than the surface drag, which proved large compared with usually suggested values. Within the experi mental uncertainties, values of the drag coefficient of the order of 1 0(-2) seem appropriate. The steady-state energy balance showed that th e downslope advection balanced the turbulent heat flux to the ground s urface.