Abstract
The instability of medium-scale disturbances in a moist atmosphere is discussed comparing with the characteristics of medium-scale disturbances in a dry atmosphere. In case of a dry atmosphere, the prefered mode of medium-scale disturbances is expected when the Richardson number Ri is of the order of unity. In case of a moist atmosphere, we assume the condensation processes due to the large-scale upward motion of the conventional baroclinic wave and also due to the (sub-grid scale) convective motion. In this paper, the released latent heat of convective motion is treated in a way of the so-called CISK mechanism.
The parameter λ which represents the ratio of the vertical p-velocity due to the frictional convergence in the planetary boundary layer to that of the conventional baroclinic wave is introduced. The instability of medium-scale disturbances is discussed analytically in cases where λ_??_1 and λ_??_1. It is shown that the unstable disturbance where λ_??_1 corresponds to the modified conventional baroclinic unstable wave. However, the unstable disturbance where λ_??_1 shows the different characteristics in a sense that there is no steering-level and the trough axis tilts eastward in the westerlies.
The role of the Rossby number in the meridional direction, Ry on the growth rate is also discussed when Ri∼0(1) and λ_??_1.
