Abstract
Stagnant flow which appears upstream of a ridge when the airflow is stably stratified is studied by observing wind data and also by wind tunnel experiment.
Stagnant flow in the atmosphere is shown to have a strong relation to the nocturnal inversion and disappears together with it. Since in the wind tunnel, thermal conditions are kept unchanged, the stability condition is controlled by changing flow velocity. Then, the flow features are dependent on flow velocity. But when the stagnant characteristics are described with respect to the Froude number given by Fγ=U/(gHΔθ/θ)1/2 both the results in the wind tunnel and atmosphere show qualitative agreement.
A stagnant flow layer where airflow is stagnant or slightly opposite to the ambient flow is found to appear when the Froude number is less than some critical value. The critical Froude number above which the stagnant flow disappears is estimated to be about 2.3.
