OBSERVATIONS AND SIMULATIONS OF KATABATIC FLOWS DURING A HEATWAVE IN ICELAND

Katabatic flows during the night of 11–12 August 2004 in Iceland are studied using observations and a numeri- cal weather prediction model. During this period, there was a heatwave in Iceland. In relation with the very high daytime temperatures, weak synoptic winds and clear skies, a radiative surface cooling in excess of 10–15◦Cwasobservedduringthe night at many locations throughout Iceland. The situation has been simulated with the Meso-NH model. The results indicate that katabatic winds develop in the stable nocturnal boundary layer when the cold and heavy air descends from the highlands and mountains down to lower lying areas. The simulations are made at a high horizontal and vertical resolution, forced with boundaries and initial conditions from the ECMWF. The simulations and initial conditions are compared to available ground based observations and satellite images showing the surface radiative temperature. The simulations seem to reproduce well most of the observed winds, including patterns where weak synoptic winds or katabatic flow interact with orography. The simulations also give valuable indications of locations of relatively strong katabatic winds, where no observations are currently available. A similar simulation has been performed with the MM5 atmospheric model and an intercomparison is underway.