Horizontal mesoscale vortices in the atmosphere

Attention is drawn to classical hydrodynamic theory of a steady vortex moving near a plane in an ideal fluid, and accompanied by a mass M of fluid moving with it. What happens when the ideal fluid is replaced by a real one, subject to drag forces and energy dissipation, is then considered and a simple theory constructed for the effects of these on the dimensions of M and its rate of propagation. An experimental two dimensional vortex due to Schmidt (1911) is reanalysed, and observed wind data in the neighbourhood of shear lines examined, to show a similar structure to which the theory might apply. It is shown that application of the theory to observed atmospheric M‘s yields plausibly good results. Relative flow patterns in a sheared environment about an ideal vortex show that the latter may constitute a mechanism for turning the environmental fluid upside down. When this concept is applied to the atmosphere, very good qualitative agreement in the observed flow is found; a trajectory boundary in the environment of M, separating recently ascended low level air from recently descended high level air, is believed to have an important application in explaining cloud and humidity bands in frontal situations. Analogy with ideal vortices also suggests that environmental stability has a flattening effect on M.