Tropical Wave Instabilities: Convective Interaction with Dynamics Using the Emanuel Convective Parameterization

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

Wave-CISK and evaporation-wind feedback modes, also known as WISHE (wind-induced surface heat exchange) modes, are investigated using a two-dimensional (xp), 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 neutral 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. A general analytical two-dimensional model designed to work with any convective parameterization is used to show that the unusually high phase speeds are most likely a result of a time lag in the vertical transport of water vapor by 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 (xp), 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 neutral 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. A general analytical two-dimensional model designed to work with any convective parameterization is used to show that the unusually high phase speeds are most likely a result of a time lag in the vertical transport of water vapor by the Emanuel convective parameterization.

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