Abstract :
Current gravity wave drag schemes alter the flow field of a numerical model by changing the velocity. It is argued that this is essentially the situation considered in the theory of geostrophic adjustment. The reduction of velocity at one moment does not necessarily lead to a reduction of the global mean velocity after adjustment in that case. It is best to discuss these processes in terms of angular momentum conservation, so that the initial loss of velocity can be seen as a reduction of the wind term which may be converted into a reduction of the mass term. Observations are used to see if this process can be detected in the angular momentum budget of the atmosphere. The data indeed reveal a rapid positive response of the mass term to the gravity wave drag.
