MOIST BAROTROPIC INSTABILITY IN THE TROPICS

The characteristics of moist barotropic instability was studied using a primitive, inviscid model. Two types of mean zonal flow were used: shear mean flow, represented by a hyperbolic tangent function, and an easterly jet, represented by a hyperbolic secant-square function.;The growth rate of the most unstable mode in an easterly jet shows only small reduction compared to the nondivergent modes. The diabatic heating distribution and its magnitude have little effect on the growth rate due to the dominance of the barotropic instability of the jet. The most unstable modes are excited mainly by the conversion of zonal kinetic energy. The rate of generation of eddy available potential energy is an order of magnitude smaller and may be negative, in agreement with the analysis of Thompson et al. (1979).;The wave length of the most unstable divergent modes in both types of mean flow is longer than that of the nondivergent modes and agrees more with the observed wave length in the western Pacific and eastern Atlantic. The energetic study shows that the scale selection is determined mainly by the effectiveness of the wave to convert zonal kinetic energy. The rate of generation of eddy available potential energy, though it may be very large, does not differ too much from one wave number to another.;It is found that in the shear mean flow, with easterly wind to the north and westerly wind to the south, the heating will increase the growth rate of the most unstable mode only when the shear is not too strong. At large shear the heating will reduce the growth rate. The reduction in the growth rate is mainly due to the reduction of the barotropic conversion rate relative to the increase of the total wave energy. At large shear and moderate amount of heating rate, the growth rate is smaller when more heat is released in the lower troposphere even though this mode has a larger rate of generation of eddy available potential energy by heating. At small and large shear the generation of eddy available potential energy is the main source of energy while the barotropic conversion is small and may even be negative. The most unstable mode in a zonal flow similar to the observed flow in the western Pacific has wavelength 3800 km and westward phase speed about 8 m/s.