Abstract
The stability of medium scale disturbances is discussed in connection with the Rossby andRichardson numbers. In case where the Richardson number is of the order of magnitude unity, we may find the unstable disturbances whose wave lengths are about 1, 000km-2, 000km in middle latitudes.
Numerical experiments are also performed in order to simulate the rapid cyclogenesis of medium scale disturbances, which is frequently observed in the lower atmosphere near the southern part of the jet stream over Japan throughout all seasons except summer.
