1993 Volume 28 Issue 4 Pages 231-243
Cold air gravity currents flowing down over plateau-slope-plain topography have been analyzed numerically using non-hydrostatic flow model with boundary-fitted curvilinear coordinate system. Effects of the slope angle and the plateau height on dynamical nature of the gravity current have been discussed in terms of an advancing speed of the gravity current head (Vh) and energy balance of the head. Results are as follows:
First, temporal change of Vh, and final Vh values over the plain showed only minor var iations for change of the slope angle (φ=10°, 20°, 30°, 45°, 60°, 90° with Hp=500m). However, total kinetic energy of the gravity current head (Kt), which should have close relations with Vh, and can be estimated more precisely than Vh, responded clearly to the change of the slope angle, showing that when Φ=30°-60°, the initial potential energy of the cold air layer over the plateau can be most effectively transformed into the kinetic energy of the gravity current head over the plain. This can be explained by the examination of rate terms in the energy balance equationsuch as those of pressure-gradient force and buoyancy force.
Secondly, effects of the change of plateau height (Hp=500, 1000 m with Φ=45°) on both final Vh and Kt values over the plain can almost be evaluated by the difference of initial potential energy of the cold air layer over the plateaus; the final Vh value can be predicted by a semi-empirical formula Vh=k√g'H with k=0.84, where H stands for hypothetical depth of the cold air layer, consisting of plateau height and depth of the real cold air layer, and g' stands for the modified gravitational acceleration, which equals to (Δθ/θ) g with θ, the potential temperature of ambient air, and 40, the difference between those of the ambient air and the cold air.
