Modelled contrast in the response of the surface energy balance to heatwaves for forest and grassland

Observations have shown that differences in surface energy fluxes over grasslands and forests are amplified during heat waves. The role of land atmosphere feedbacks in this process in still uncertain. In this study, we use a single-column model (SCM) to investigate the difference between forest and grassland in their energy response to heat waves. Three simulations for the period 2005-2011 were carried out: a control run using vegetation characteristics for Cabauw (the Netherlands), a run where the vegetation is changed to 100% forest, and a run with 100% short grass as vegetation. A surface evaporation tendency equation is used to analyse the impact of the land atmosphere feedbacks on evapotranspiration and sensible heat release under normal summer and heatwave conditions with excessive shortwave radiation.
Land atmosphere feedbacks modify the contrast in surface energy fluxes between forest and grass, particularly during heat wave conditions. The surface resistance feedback has the largest positive impact, while boundary layer feedbacks generally tend to reduce the contrast. Overall forest give higher air temperatures and drier atmospheres. In offline land surface model simulations the difference between forest and grassland during heat waves cannot be diagnosed adequately owing to the absence of boundary layer feedbacks.