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This paper reports the impact of applying improved representation of the area-average aerodynamic properties of South American ecosystems on 7-d weather forecasts made with the Brazilian regional forecast model (Eta-SSiB). Two pairs of 7-d forecasts were made, each pair starting from the same initial conditions on I January 1999 and on 24 June 1999, respectively. In one run, the aerodynamic properties of the ecosystem used for each modeled grid square were those of the most common vegetation present. In the second, the grid-average aerodynamic properties of the vegetation were calculated using aggregation rules in combination with a high-resolution land-cover data set. There are marked differences in the aerodynamic properties of the underlying ecosystems in these two runs in some regions, notably around the Amazon River basin, in regions where there has been land-cover change, and in portions of southern Brazil. The 7-d average, predicted ecosystem-surface exchanges, near-surface temperature, and precipitation for the two pairs of runs were compared. There were differences between the forecasts that were most noticeable in regions where the ecosystem\'s aerodynamic properties were most dissimilar, but also elsewhere. Given the limited nature of the experiments reported, these preliminary results should be treated with care. Nonetheless, they suggest the need for more systematic study of the significance of ecosystems on regional weather forecasts in South America

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