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FGGE Forecast Experiments for Amazon Basin Rainfall

Julio BuchmannDepartment of Meteorology, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, 21910 Brazil

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Lawrence E. BujaDepartment of Meteorology, University of Utah, Salt Lake City, UT 84112

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Jan PaegleDepartment of Meteorology, University of Utah, Salt Lake City, UT 84112

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Chi-Dong ZhangDepartment of Meteorology, University of Utah, Salt Lake City, UT 84112

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David P. BaumhefnerNational Center for Atmospheric Research, Boulder, CO 80307

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Abstract

We investigate the pattern of Amazon Basin rainfall forecasts of ten Global Weather Experiment (GWE) cases. Although the computations are based upon a rather crude wavenumber 15 resolution, the control forecasts exhibit a rather fine structure of the rainfall over tropical South America, including enhancements over the interior of the Amazon Basin and suppression on the northeast coast of Brazil. The forecasts appear to be in rather good agreement with climatology. The sensitivity of this model forecast to the presence of anomalous east Pacific heating is investigated through experiments in which a nonadiabatic term is added to the thermodynamic equation. These experiments suggest significant suppression of rainfall over the central Amazon Basin, and especially over the northeast portion of Brazil. This suppression is associated with the downward branch of a Walker circulation whose development is determined by a region of subsidence which propagates eastward from the eastern Pacific at a rate of about 30 m s−1. This evolution, which is consistent with the Kelvin wave contribution to the Walker cell, affects Brazil within about two days of the heating onset.

The evolution of upper-level convergence, implied sinking motion, and suppression of rainfall over tropical South America in the forecasts does not depend sensitively upon the placement of the anomalous tropical Pacific heating. In particular, enhancements of the North Pacific are approximately as effective as those of the South Pacific.

Abstract

We investigate the pattern of Amazon Basin rainfall forecasts of ten Global Weather Experiment (GWE) cases. Although the computations are based upon a rather crude wavenumber 15 resolution, the control forecasts exhibit a rather fine structure of the rainfall over tropical South America, including enhancements over the interior of the Amazon Basin and suppression on the northeast coast of Brazil. The forecasts appear to be in rather good agreement with climatology. The sensitivity of this model forecast to the presence of anomalous east Pacific heating is investigated through experiments in which a nonadiabatic term is added to the thermodynamic equation. These experiments suggest significant suppression of rainfall over the central Amazon Basin, and especially over the northeast portion of Brazil. This suppression is associated with the downward branch of a Walker circulation whose development is determined by a region of subsidence which propagates eastward from the eastern Pacific at a rate of about 30 m s−1. This evolution, which is consistent with the Kelvin wave contribution to the Walker cell, affects Brazil within about two days of the heating onset.

The evolution of upper-level convergence, implied sinking motion, and suppression of rainfall over tropical South America in the forecasts does not depend sensitively upon the placement of the anomalous tropical Pacific heating. In particular, enhancements of the North Pacific are approximately as effective as those of the South Pacific.

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