Observational and numerical case study of the Adriatic bora

Two Adriatic bora events to the lee of the Dinaric Alps occurring in spring 2002 are studied using observations and numerical simulations. The key instrument was an aerosol backscatter lidar operated in a nadir-pointing mode on board the German Aerospace Center's (DLR) Falcon aircraft.

The flight strategy was chosen in order to elucidate the structure of aerosol layers in the lower troposphere of the flow across the Dinaric Alps. The behavior of streamlines across mountain massifs could be traced with the lidar backscatter signal returned from the top of such aerosol layers and from the top of stratus clouds. Additional information came from radiosoundings and weather stations. High-resolution numerical simulations performed with the mesoscale model RAMS (with a horizontal mesh size of 267 m in the innermost model domain) elucidated further flow features.

This presentation focuses on the first bora case of 28 March 2002. Compared with previously published studies, this is a weak event with sustained wind speeds observed near crest level of 15 m/s. The lidar data show the fine-scale structure of orographic gravity waves and the vertical separation of the descending bora flow from the lee slope before it actually reaches the coastline of the Adriatic Sea. The model is essentially able to reproduce this behavior. Further, the simulations and observations give evidence that the strongest bora winds emanated as jet flows from gaps in the main crest line. These gap winds were found, for example, at the topographic incisions of Vratnik Pass (Senj), Delnice (Rijeka), and Baske Ostarije (Karlobag). Associated with these jets are shear lines and therefore anomalies of potential vorticity which originate as banners at the gaps and extend downstream over the Adriatic Sea.