Evaporation-Wind Feedback and the Organizing of Tropical Convection on the Planetary Scale. Part II: Nonlinear Evolution

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  • 1 Department of Geophysics, Tohoku University, Sendai, Japan
  • | 2 Department of Earth System Science and Technology, Kyushu University, Kasuga, Japan
  • | 3 Department of Geophysics, Tohoku University, Sendai, Japan
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Abstract

The nonlinear evolution of the quasi-linear (QL) evaporation-wind feedback (EWFB) instability obtained in Part I of this study is investigated in a two-level model of the aqua-planet atmosphere. In this model, the QL-EWFB instability causes tropical convection to organize on the planetary scale and a wavenumber one Kelvin wave-like structure dominates the east-west circulation in the tropics. An increase of the static stability that is in phase with the surface evaporation stabilizes the EWFB mode. For large surface humidity, a hierarchy of convective structures appear as a result of the nonlinear adjustment of the QL mode at large amplitudes. Isolated grid-size individual convective zones move randomly, while a wavenumber one envelope of this convection propagates eastward at a constant speed.

In the conditionally unstable parameter regime, the model atmosphere is found to be stable on the planetary scale, but it can be conditionally unstable on the scale of individual convection events. The EWFB and conditional instabilities are not mutually exclusive as in the QL model but cooperate in organizing convection. The development of the fast-growing conditional instability acts to stabilize the large-scale atmosphere, allowing the EWFB mechanism to organize convection into a wavenumber one structure.

Abstract

The nonlinear evolution of the quasi-linear (QL) evaporation-wind feedback (EWFB) instability obtained in Part I of this study is investigated in a two-level model of the aqua-planet atmosphere. In this model, the QL-EWFB instability causes tropical convection to organize on the planetary scale and a wavenumber one Kelvin wave-like structure dominates the east-west circulation in the tropics. An increase of the static stability that is in phase with the surface evaporation stabilizes the EWFB mode. For large surface humidity, a hierarchy of convective structures appear as a result of the nonlinear adjustment of the QL mode at large amplitudes. Isolated grid-size individual convective zones move randomly, while a wavenumber one envelope of this convection propagates eastward at a constant speed.

In the conditionally unstable parameter regime, the model atmosphere is found to be stable on the planetary scale, but it can be conditionally unstable on the scale of individual convection events. The EWFB and conditional instabilities are not mutually exclusive as in the QL model but cooperate in organizing convection. The development of the fast-growing conditional instability acts to stabilize the large-scale atmosphere, allowing the EWFB mechanism to organize convection into a wavenumber one structure.

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