Linear Response of a Stratified Tropical Atmosphere to Convective Forcing

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  • 1 National Center for Atmospheric Research, t Boulder, CO 80307
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Abstract

The three-dimensional response of the tropical atmosphere to an isolated heat source is investigated using a primitive equation model linearized about a resting basic state on an equatorial β-plane. The model equations are solved by applying vertical and horizontal normal mode transforms. The thermal forcing was chosen to simulate the convection which occurs over tropical South America during the Southern Hemisphere summer. The vertical dependence of the forcing is such that the heating is zero at the top and bottom boundaries with a variable level of maximum beating. The model results with steady forcing are compared with the average circulation over tropical South America for a 19-day period in the Southern Hemisphere summer of 1979, and it is shown that the model reproduces many aspects of the observed circulation. The partition of the energy of the steady state response between the vertical modes is calculated, and it is shown that the internal mode with an equivalent depth of 208 m is dominant for a wide range of levels of maximum beating. The model results with transient forcing are compared with the results from a single equivalent depth version of the model, and it is shown that the horizontal structure is quite similar, but there are significant differences in the vertical structure of the response. The transient response is also compared with the steady state response.

Abstract

The three-dimensional response of the tropical atmosphere to an isolated heat source is investigated using a primitive equation model linearized about a resting basic state on an equatorial β-plane. The model equations are solved by applying vertical and horizontal normal mode transforms. The thermal forcing was chosen to simulate the convection which occurs over tropical South America during the Southern Hemisphere summer. The vertical dependence of the forcing is such that the heating is zero at the top and bottom boundaries with a variable level of maximum beating. The model results with steady forcing are compared with the average circulation over tropical South America for a 19-day period in the Southern Hemisphere summer of 1979, and it is shown that the model reproduces many aspects of the observed circulation. The partition of the energy of the steady state response between the vertical modes is calculated, and it is shown that the internal mode with an equivalent depth of 208 m is dominant for a wide range of levels of maximum beating. The model results with transient forcing are compared with the results from a single equivalent depth version of the model, and it is shown that the horizontal structure is quite similar, but there are significant differences in the vertical structure of the response. The transient response is also compared with the steady state response.

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