engleski

Sea/land breeze development during a summer bora event along the north-eastern Adriatic coast

The interaction of a summer frontal bora and the sea/land breeze along the north-eastern Adriatic coast was investigated by means of numerical simulations and available observations. Available measurements (in-situ, radiosonde, satellite images) provided model validation. The modelled wind field revealed several regions where the summer bora (weaker than 6 m s-1) allowed sea breeze development: in the western parts of the Istrian peninsula and Rijeka Bay and along the north-western coast of the island of Rab. Along the western Istrian coast, the position of the narrow convergence zone that formed highly depended on the balance between the bora jets northward and southward of Istria. In the case of a strong northern (Trieste) bora jet, the westerly Istrian onshore flow presented the superposition of the dominant swirled bora flow and local weak thermal flow. It collided then with the easterly bora flow within the zone. With weakening of the Trieste bora jet, the convergence zone was a result of the pure westerly sea breeze and the easterly bora wind. In general, during a bora event, sea breezes were somewhat later and shorter, with limited horizontal extent. The spatial position of the convergence zone caused by the bora and sea breeze collision was strongly curved. The orientation of the head (of the thermally-induced flow) was more in vertical causing larger horizontal pressure gradients and stronger daytime maximum wind speed than in undisturbed conditions. Except for the island of Rab, other lee-side islands in the investigated area did not provide favourable conditions for the sea breeze formation. Within a bora wake near the island of Krk, onshore flow occurred as well, although not as a sea breeze flow, but as the bottom branch of the lee rotor that was associated with the hydraulic jump-like feature in the lee of the Velika Kapela Mountain.

sea breeze/bora interaction; sea breeze front; convergence zone; lee rotor

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