Observations and simulation of katabatic flows during a heatwave in Iceland

Katabatic flows during a heatwave in August 2004 in Iceland are studied using observations and a high-resolution simulation with a numerical atmospheric model. In relation with the very high daytime temperatures, weak synoptic winds and clear skies, a radiative surface cooling in excess of 10-15°C was observed during the night throughout Iceland. The simulations and initial conditions are compared to available observations. Most of the observed winds, including patterns where weak synoptic winds or katabatic flow interact with orography, are reproduced well. They reveal that the katabatic flow in Southern Iceland can be characterized as low Froude number 'tranquil' flow. The simulations also give valuable indications of locations of relatively strong katabatic winds where no observations are currently available and where katabatic flows are presumably of importance for the local wind climate.