Abstract : A.3
"Zadar calm" during severe Bora

Branka IvančAn-Picek, Vanda Grubisic, Ivana Stiperski, Ming Xiao, Alica Bajic
Meteorological and Hydrological Service

Zadar is situated on the eastern coast of the Adriatic Sea in the region of northern Dalmatia. Due to the vicinity of the southern end of Velebit, the 145 km long coastal mountain range that forms part of the Dinaric Alps, the area less than 20 km inland from Zadar is known for high frequency of severe Bora. In contrast, in the city of Zadar Bora is considerably less frequent and weaker compared to its surroundings. In this study, we investigate small-scale characteristics and spatial variability of severe Bora flow in the wider Zadar area with the objective of identifying reasons for the “Zadar calm”.

A detailed analysis is undertaken of several severe Bora events that occurred in the winter-spring 2004/5 season, during which a new sodar system was deployed at Zadar-Zemunik. The Zadar sodar data provides continuous vertical soundings in the range from 40 to 700 m at temporal resolution of 10 minutes. They reveal a significant variability in the wind speed and direction during the severe Bora events. The maximum Bora speeds were observed in the layer between 300 and 700 m, while the low-level flow was characterized by weak winds. This observational analysis also includes surface data from Zadar and several automatic weather stations located inside the target area as well as the Zadar - Zemunik radio-sounding data.

The observations are compared with results of very high-resolution numerical simulation carried out with the NRL COAMPS model. Initial simulation results indicate that the Bora flow never fully penetrated to the surface near the coast in the Zadar area. The simulation results also suggest that the primary mountain wave could be responsible for low-level flow separation over the steep terrain, leading to weak surface winds over Zadar, downstream of Velebit.