Tropical wave instabilities: convective interaction with dynamics using the Emanuel convective parameterization

Abstract

Wave-CISK and evaporation-wind feedback modes, also known as WISHE (wind-induced surface heat exchange) modes, are investigated using a two-dimensional (x-p), hydrostatic, nonrotational model linearized about a basic state in radiative-convective equilibrium with no vertical shear. Cumulus convection is parameterized using version 1.22 of the Emanuel convective parameterization scheme, a mass flux scheme that includes the effects of evaporatively driven unsaturated downdrafts. It is found that the only unstable modes are long-wavelength WISHE modes. All wave-CISK modes are damped, though the longest-wavelength modes have nearly netural growth rates. It is demonstrated that the presence of evaporatively driven unsaturated downdrafts plays a major role in damping both short-wave WISHE and wave-CISK modes in the model. The model favors approximately the same horizontal scale as observed for the Madden-Julian oscillation (40-60 day wave) but the phase speed is too large by a factor of ~4-5. -from Authors