Evolution and Structure of Two Severe Cyclonic Bora Events: Contrast between the Northern and Southern Adriatic

While statistical analyses and observations show that severe bora with maximum gusts exceeding 40 m s−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. Identification of the main differences in the sequence of mesoscale and macroscale events leading to the onset of bora in the northern and southern parts of the eastern Adriatic is of fundamental importance for its forecasting. To this end, presented here is a comparative analysis of the evolution and structure of two typical severe cyclonic bora events—one “northern” (7–8 November 1999) and one “southern” (6–7 May 2005) event. The analysis utilizes airborne, radiosonde, and ground-based observations, as well as the hydrostatic Aire Limitée Adaptation Dynamique Developement International (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. For severe bora in the northern Adriatic, these conditions include a strong forcing of the northeasterly low-level jet and pronounced discontinuities in the upstreamflow structure that promote layering, such as lower- to midtropospheric 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 the 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 the southern Balkan peninsula. The upstream atmospheric layering is shown to strongly modulate 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. Furthermore, the presence of a mountain-parallel upper-level jet aloft appears to impede severe bora development in both the northern and southern Adriatic.