An observational study of an arctic front during the IPY-THORPEX 2008 campaign

Kurzfassung

The fact that severe weather associated with polar lows and arctic fronts still comes unforeseen and puts human life at risk shows that an effort towards increased understanding of them is required. The observations of an arctic front by dropsondes and Doppler lidar carried onboard a research aircraft during the IPY-THORPEX field campaign offered a rare opportunity to investigate the mesoscale structure of the front and to validate the output from operational numerical weather prediction models. The observations revealed features similar to those of polar fronts such as a relatively steep frontal zone, the presence of a strong low-level jet and an elevated dry slot, making the arctic front appear as a shallow cold front confined to levels below 700 hPa. The dry slot indicated the presence of a downfolding of the tropopause, and together with the observations of an upper-level jet this strongly supports the inclusion of an arctic tropopause fold connected to the arctic jet stream in a conceptual model of the tropopause. A comparison between data from operational numerical weather prediction models and observations obtained during the flight shows that the models simulated the broad features of the frontal zone such as jets, dry slot and the depth of the front fairly well, although parts of the front were slightly misplaced. However, the models failed completely in their simulations of one of the three mesoscale cyclones associated with the front as they located it over the coast of northern Norway while the correct location was over the Greenland Sea according to the observations and analysis.