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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Evolution and structure of two severe Bora events: Contrast between the northern and southern Adriatic

Kristian Horvath (Meteorological and Hydrological Service); Stjepan Ivatek-Sahdan, Branka Ivancan-Picek, Vanda Grubisic

While statistical analyses and observations show that severe Bora with maximum gusts exceeding 40 ms-1 can occur in all parts of the Adriatic, the Bora research to date has been mainly focused on the dynamics and structure of severe Bora in the northern Adriatic. Examined to a significantly lesser degree is a less predictable counterpart in the southern Adriatic, where the Dinaric Alps are higher, broader, and steeper, and where the upwind Bora layer is generally less well defined. A comparative analysis of the evolution and structure of two typical severe Bora events, one "northern" (07-08 Nov 1999) and one "southern" (06-07 May 2005) is performed utilizing airborne, radiosonde, and ground-based observations, as well as the hydrostatic ALADIN/HR mesoscale model simulations.

It is shown that the development of a severe Bora in both the northern and southern Adriatic is critically dependent on the synoptic setting to create an optimal set of environmental conditions. Firstly, the presence of a mountain-parallel upper-level jet aloft appears to impede severe Bora development in both the northern and southern Adriatic. For severe Bora in the northern Adriatic, favorable conditions include a strong forcing of the northeasterly low-level jet and pronounced discontinuities in the upstream flow structure that promote layering, such as lower- to mid-tropospheric inversions and environmental critical levels. The development of severe Bora in the southern Adriatic is crucially dependent on the establishment of a considerably deeper upstream layer that is able to overcome a strong blocking potential of the southern Dinaric Alps. While the upstream layering is less pronounced, it is closely tied to the presence of a cyclone in the southern Adriatic or over southern Balkan peninsula.

The upstream atmospheric layering is shown to strongly modulate the Bora behavior, and different phases of severe Bora, related to the presence or absence of upstream layering, are shown to occur within a single Bora episode.