INTERACTIONS BETWEEN GRAVITY-WAVES AND COLD-AIR OUTFLOWS IN A STABLY STRATIFIED UNIFORM-FLOW

Interactions between gravity waves and cold air outflows in a stably s tratified uniform flow forced by various combinations of prescribed he at sinks and sources are studied using a hydrostatic two-dimensional n onlinear numerical model. It is found that the formation time for the development of a stagnation point or reversed flow at the surface is n ot always directly proportional to the Froude number when wave reflect ions exist from upper levels. It is shown that a density current is ab le to form by the wave-outflow interaction, even though the Froude num ber is greater than a critical value. This is the result of the wave-o utflow interaction shifting the flow response to a different location in the characteristic parameter space. A density current is able to fo rm or be destroyed due to the wave-outflow interaction between a trave ling gravity wave and a cold air outflow. This is proved by performing experiments with a steady-state heat sink and an additional transient heat source. In a quiescent fluid, a region of cold air, convergence, and upward motion is formed after the collision between two outflows produced by two prescribed heat sinks. After the collision, the indivi dual cold air outflows lose their own identity and merge into a single , stationary, cold air outflow region. Gravity waves tend to suppress this new stationary cold air outflow after the collision. The region o f upward motion associated with the collision is confined to a very sh allow layer. In a moving airstream, a density current produced by a he at sink may be suppressed or enhanced nonlinearly by an adjacent heat sink due to the wave-outflow interaction.