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The Cold-Core Temperature Structure in a Tropical Easterly Wave

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  • 1 Department of Earth and Atmospheric Science, York University, North York, Ontario, Canada
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Abstract

The purpose of this study is to identify what effects are responsible for the observed temperature field, in particular the lower-level cold core, in the trough region of a convectively active tropical easterly wave disturbance. GATE Phase III A/B- and B-scale data were used in the analysis, and the divergence equation, a first-order balance condition proposed by Cho and Jenkins applicable to slowly evolving or near-steady-state large-scale convectively active tropical circulation systems, and the standard nonlinear and linear balance equations were chosen as the framework in which to assimilate the observational data in order to understand the spatial anticorrelation between temperature changes and latent heating. It is shown that all three balance conditions reproduced the thermal structure of the easterly waves that passed over GATE during Phase III. Despite the differences in formulation and form, the simpler standard balance equations were as accurate as the first-order divergence equation in diagnosing the temperature field. The central result of the analysis is that the lower cold-core temperature anomaly observed in the near-trough region of easterly wave disturbances is not a direct consequence of the distribution of latent heat released by cumulus clouds, but reflects instead a balance of forces that dominate the momentum field and that the agreement between the observed and diagnosed temperature fields is dominated by the rotational component of the flow. This has possible implications for a first-order model of the interaction between cumulus-scale and large-scale equatorial wave motions, and for the distinction between developing and nondeveloping easterly waves in terms of the ambient vorticity field, which may aid future numerical investigations of tropical cyclogenesis.

Abstract

The purpose of this study is to identify what effects are responsible for the observed temperature field, in particular the lower-level cold core, in the trough region of a convectively active tropical easterly wave disturbance. GATE Phase III A/B- and B-scale data were used in the analysis, and the divergence equation, a first-order balance condition proposed by Cho and Jenkins applicable to slowly evolving or near-steady-state large-scale convectively active tropical circulation systems, and the standard nonlinear and linear balance equations were chosen as the framework in which to assimilate the observational data in order to understand the spatial anticorrelation between temperature changes and latent heating. It is shown that all three balance conditions reproduced the thermal structure of the easterly waves that passed over GATE during Phase III. Despite the differences in formulation and form, the simpler standard balance equations were as accurate as the first-order divergence equation in diagnosing the temperature field. The central result of the analysis is that the lower cold-core temperature anomaly observed in the near-trough region of easterly wave disturbances is not a direct consequence of the distribution of latent heat released by cumulus clouds, but reflects instead a balance of forces that dominate the momentum field and that the agreement between the observed and diagnosed temperature fields is dominated by the rotational component of the flow. This has possible implications for a first-order model of the interaction between cumulus-scale and large-scale equatorial wave motions, and for the distinction between developing and nondeveloping easterly waves in terms of the ambient vorticity field, which may aid future numerical investigations of tropical cyclogenesis.

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