Regional-scale weather patterns and wildland fires in central Portugal

Abstract

The characteristic evolution of the synoptic- and meso-scale wind, temperature and humidity pattern during wildland fire events in Portugal was determined by lagged covariances for the period 1980 to 2001. The daily burnt area was chosen as the parameter to be correlated with atmospheric fields provided by the European Centre for Medium-Range Weather Forecasts Reanalysis (ERA40) data. The related time series consisted of wildland fires with a daily burnt area of more than 500 ha in central Portugal. Five days in advance of a fire event, a strong positive anomaly existed in the surface pressure and in the 500-hPa geopotential field, both appearing to the west of the Iberian Peninsula and moving towards Brittany until lag zero. In advance of the fire event, the flow above Portugal came from the north, turning to easterlies at lag time zero and finally coming from the south-east during the post-event phase. Surface wind statistics taken at Castelo Branco supported these results. Smoke plumes from wildland fires detected by satellites indicated a similar flow structure, which was quite different to the averaged summertime flow above Portugal. Cross-covariance regression between the Iberian thermal low and burnt area showed that the peak amount of burnt area occurred up to 3 days after the appearance of a thermal low. This suggested that in the pre-phase of a wildland fire, heated air is transported from the peninsula’s centre towards Portugal.