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A. M. Treguier and R. L. Panetta

Abstract

The presence of multiple zonal fronts in the Antarctic Circumpolar Current (ACC), a phenomenon described as zonation, has been confirmed by many observations suggesting that the fronts are circumpolar in extent. In the context of quasigeostrophic turbulence models, Panetta has shown that fronts associated with long-lived zonal jets develop spontaneously in very wide baroclinically unstable regions (of width large compared with the Rossby radius of deformation). The present paper examines the relevance of this mechanism to the zonation of the ACC, using a quasigeostrophic wind-forced channel model. Multiple jets appear when the forcing scale is wide enough. and they have horizontal scales and a strength that compares well with observations. However, in the presence of large-scale random topography, a different dynamical regime emerges in which the flow structure depends on the topographically controlled stationary eddies.

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Silvio N. Figueroa, José P. Bonatti, Paulo Y. Kubota, Georg A. Grell, Hugh Morrison, Saulo R. M. Barros, Julio P. R. Fernandez, Enver Ramirez, Leo Siqueira, Graziela Luzia, Josiane Silva, Juliana R. Silva, Jayant Pendharkar, Vinicius B. Capistrano, Débora S. Alvim, Diego P. Enoré, Fábio L. R. Diniz, Praki Satyamurti, Iracema F. A. Cavalcanti, Paulo Nobre, Henrique M. J. Barbosa, Celso L. Mendes, and Jairo Panetta

Abstract

This article describes the main features of the Brazilian Global Atmospheric Model (BAM), analyses of its performance for tropical rainfall forecasting, and its sensitivity to convective scheme and horizontal resolution. BAM is the new global atmospheric model of the Center for Weather Forecasting and Climate Research [Centro de Previsão de Tempo e Estudos Climáticos (CPTEC)], which includes a new dynamical core and state-of-the-art parameterization schemes. BAM’s dynamical core incorporates a monotonic two-time-level semi-Lagrangian scheme, which is carried out completely on the model grid for the tridimensional transport of moisture, microphysical prognostic variables, and tracers. The performance of the quantitative precipitation forecasts (QPFs) from two convective schemes, the Grell–Dévényi (GD) scheme and its modified version (GDM), and two different horizontal resolutions are evaluated against the daily TRMM Multisatellite Precipitation Analysis over different tropical regions. Three main results are 1) the QPF skill was improved substantially with GDM in comparison to GD; 2) the increase in the horizontal resolution without any ad hoc tuning improves the variance of precipitation over continents with complex orography, such as Africa and South America, whereas over oceans there are no significant differences; and 3) the systematic errors (dry or wet biases) remain virtually unchanged for 5-day forecasts. Despite improvements in the tropical precipitation forecasts, especially over southeastern Brazil, dry biases over the Amazon and La Plata remain in BAM. Improving the precipitation forecasts over these regions remains a challenge for the future development of the model to be used not only for numerical weather prediction over South America but also for global climate simulations.

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