Urban heat islands and extreme climate events: surface temperature variability over Paris during the heat-wave of summer 2003

In summer 2003, a persistent anticyclone over western Europe, blocked rain-bearing depressions from the Atlantic ocean, and advected very hot air northward from south of the Mediterranean. These conditions resulted in a heat-wave of exceptional strength and duration, and a death toll exceeding 30,000. Temperatures anomalies were most extreme in France. In August, the region of Paris experienced nine consecutive days with high maximum and minimum temperatures. Maximum surface air temperatures up to 40 °C were recorded, with minimum temperatures steadily increasing from 20 to 25.5 °C at the peak of the heat wave. However, the network of weather stations is too sparse to estimate diurnal temperature amplitude gradients; furthermore, temperatures at stations in parks such as Montsouris may differ significantly from those in densely built areas. Time series of 125 NOAA-AVHRR satellites images, from July 31 to August 20 2003, were processed to estimate radiant surface temperatures, diurnal variation and the distribution of its amplitude. The landcover classification of the Paris basin was mapped from a high resolution multi-spectral SPOT image. Combined interpretation of surface temperature and landcover shows the effects of buildings density and surface physical properties on sensible heat. During the day, Paris downtown is cooler than industrial suburban districts, whereas during the night, it gives rise to a significant heat island. The negative correlation between afternoon land surface temperature and normalized vegetation index shows the cooling effect of urban parks, even in downtown. Vulnerable areas and temperature thresholds were determined regarding risks of heat stress. Comparison between time-series of co-located satellite images of August 1998, a normal year, and 2003, indicates a large difference in surface temperature amplitude, confirming the impact of anomalous nighttime temperatures in the heat-wave process.

Keywords: Urban climatology, heat-wave, urban heat island, satellite remote sensing.