A Numerical Experiment on a Disturbance in the Tropical Easterlies*

A numerical experiment has been performed on a tropical disturbance embedded in easterlies to test the dependency of the vertical coupling on the value of Richardson number in tropics. By a 2-layer primitive equation model in sigma system of coordinates integrations have been performed for a period of 24 hours with two different sets of initial conditions in which the three dimensional structure of the initial easterly pattern is specified through pre-specified wind shear and different values of the Richardson number. The order of magnitude of vertical velocities, the time rate of change of kinetic energy and the phase speed of the disturbances is discussed for both the cases. The preliminary results indicate that, even the tropical motions have appreciable degree of vertical coupling and baroclinic nature in the cases of low Richardson number. The results seem to be of special interest in context of the fact that even the mean monthly values of static stability during summer are found to be very small and even negative in the middle and lower troposphere in the tropics. 1