The contrasting role of forests and grassland on land-climate interaction during recent European heat waves

Recent European heat waves have raised the interest in the impact of land conditions, in particular soil moisture, on temperature extremes. Observations from an extensive network of flux towers along with earth observation products reveal a contrasting response of forests and crop-/grassland in their water and energy budgets to heat waves. On the short term, forests primarily increase the sensible heat flux into the atmosphere in response to increased available energy at the land surface, in contrast to grasses that show only elevated latent heating. On the long-term, this elevated latent heating accelerates depletion of soil moisture. We employ multi-year earth observations by MODIS aboard Terra and Aqua to characterize albedo and temperature anomalies for pixels with predominant forest or short vegetation cover. Whereas scenes in June 2003 and July 2006 show little difference between temperature anomalies, a scene in Central France in August 2003 reveals an increasing difference in temperature anomalies, with the grasslands being warmer than the forests. These observations are consistent with this hypothesis of a phase transition induced by reduced evaporative cooling over grasslands during long lasting heat waves. Albedo changes are significant, but their impact on the land surface energy balance is an order of magnitude smaller than the impact of Bowen ratio changes.