A case study of a severe Adriatic bora on 28 December 1992

Abstract

Most bora research, until now, considered the appearance of the severe bora only on the northern Adriatic. However, the important topic of this paper is the internationally less known southern Adriatic bora. Preliminary results, including ours, begin to change the traditional view of bora as a local small-scale phenomenon by suggesting that bora-related airflow has a multi-scale nature. Here, we present a rare synoptic situation which was characterized by the strong NE flow through the entire depth of the troposphere, which could be responsible for the appearance of the unusually severe wind over the continental part of Croatia and the bora on the southern Adriatic at 27/29 December 1992. The results of isentropic diagnostics show that the southern Adriatic bora mechanism could not be explained by 2-D hydraulic theory with simpler upstream conditions as was mainly the case on the northern Adriatic. The downstream effects, due to the isallobaric component of the ageostrophic motion, play an essential rôle in the southern Adriatic bora genesis. This does not diminish the importance of upstream effects on the bora flow. From the theoretical approach, it is considered that upstream conditions for the southern Adriatic bora should be based on 3-D theories and numerical simulation which could explain the flow splitting, divergence and steep lowering of isentropic surfaces.