Influence of vertical stratification on motion in a differentially heated rotating annulus

Understanding the influence of vertical stratification on large scale motions is of prime geophysical interest for both the stable and unstable cases. Here we present a generalization of the Lorenz (1962) model which includes externally imposed stratifications of either sign. Various flow regimes are determined, according to the values of the thermal Rossby number Ro T the nondimensional rotation rate Ω and the ratio of temperature differences in the vertical, δ, and horizontal directions, δT. It is found that the stability curve in the Ro T ,R plane, while preserving its general shape which was found by Hide (1953,1969) for δT=0, moves towards the lower right with increasing δT>0, other conditions being equal, and to the upper left for the slightly unstable case. The curve separates the Hadley axisymmetrical regimes from the Rossby wave regimes. Introduction of a bulk Richardson number helps to separate regular and irregular wave regimes for the case of unstable stratification. Laboratory experiments with a differentially heated (both horizontally and vertically) rotating annulus confirm qualitatively all the main conclusions of the theoretical analysis for a broad range of the parameter space in which the experiments were performed. The results obtained here may also explain the damping of cyclones in the Martian atmosphere after the onset of global dust storms (Ryan and Henry, 1979) and the attenuation of synoptic activity in numerical experiments modelling the atmospheric consequences of nuclear war (Schneider, 1985). The results are also related to the nature of the stable stratification of the majority of planetary atmospheres even for conditions when the prime radiative equilibrium temperature gradient is unstable.