Dobbelt Midtvejskollokvium – Niels Bohr Institute - University of Copenhagen

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Dobbelt Midtvejskollokvium

Eva Podgrajsed and Emilie Hermansson 

Foehn winds and katabatic winds are topographically dependent phenomena that frequentlly occur in Greenland. When air is cooled on the Greenland ice sheet it will descent down the slope towards the coast line due to gravitational force and thus a katabatic wind is formed. The other topographic wind pushes air up along a mountain and when the air is foreced to descend on the lees side it will get dry and warm compared to the windward side and thus the foehn event is not affected by negative bouyancy. When a pressure system enhances the katabatic flow in Greenland the event is called a Piteraq. The most extrem Piteraq ever to be documented in Greenland occured in Tasiilaq in February 1970, when wind gust up to 72 m/s were measured. The Piteraq event in Tasiilaq and a foehn event that happened on the south west tip of Greenland end of January 2010 are being studied in these two projects. The aim is to compare the DMI-HIRLAM K05 hydrostatic weather prediction system with the non-hydrostatic HARMONIE model in order to see which model best predicts these two topographic events. For the Piteraq event the wind speed is the most crucial parameter to predict compared to the temperature for the foehn event. In both the Piteraq and the foehn cases the two models captured the phenomena but the extreme values gott different results. The predicted wind speeds in the Piteraq got to low values from both models. HARMONIE predicted maximum wind speeds of 33m/s and HIRLAM of 26m/s compared to the observations that had maximum winds speeds of 54m/s. Also the temperatures in the foehn event got to low values from both models. HARMONIE predicted maximum temperature of 8oC and HIRLAM of 4oC compared to the maximum observed temperature of 13.2oC. 

Internal supervisor: Aksel Walløe Hansen, NBI
Eksternal supervisors: Niels Woetmann Nielsen, DMI and Leif Rasmussen, DMI