CISK AND EVAPORATION-WIND FEEDBACK WITH CONDITIONAL HEATING ON AN EQUATORIAL BETA-PLANE

The effect of conditional heating on unstable Kelvin waves generated by CISK and evaporation-wind feedback was investigated with multi-level models on the equatorial beta-plane. In agreement with previous analytic results, the scale-selection catastrophe of linear heating is modified but not eliminated by conditional heating. There is only one region of upward motion, but it contracts to the smallest zonal scale allowed by diffusion or model resolution. The pathology is most severe in the CISK-unstable case. With evaporation-wind feedback, Kelvin-wave instability is possible apart from CISK; growth rate in this case increases slowly with decreasing diffusion, but contraction is still observed. We note incidentally that eastward moving evaporation-wind modes can be generated on the beta-plane with no mean wind, contrary to previous assumptions requiring an easterly background flow. While evaporation-wind feedback is an improvement over CISK, as far as scale selection is concerned, zonal phase speeds remain too large compared to the observed propagation of tropical intraseasonal oscillations and superclusters, unless (1) the parameter q controlling the strength of moisture convergence feedback lies close to the CISK-neutral point, (2) the parameter A controlling the strength of evaporation-wind feedback is small, and (3) the heating profile is assumed proportional to vertical velocity. Small positive phase speeds are obtained under these conditions, for example, in a two-level model.