Abstract : 2G.13
Sensitivity of Bora to Mediterranean cyclone parameters through an ensemble of numerical cyclone realizations

Kristian Horvath, Branka IvančAn-Picek
horvath@cirus.dhz.hr
Meteorological and Hydrological Service

Bora that happened on 14 November 2004 was one of the most severe ever measured, causing significant human, economic and infrastructural losses. Except for its intensity, the particularities of this event were the severe Bora longevity (~24hr) and simultaneous coexistence on Northern and Southern Adriatic. The synoptic setting was characterized with a high over the Eastern Atlantic spreading over the Central Europe and a deep North-west African cyclone in the Southern Adriatic. This created a strong bipolar macroscale pressure environment that resulted in the built-up of the strong pressure gradient across the Dinaric Alps. The numerical analysis illuminates the influence of the cyclone on the pressure build-up through the ensemble of simulations. Overall, the ensemble results showed that the cyclone influence on Bora intensity diminished from south the north. Nevertheless, is seems that the cyclone centre intensity and Bora strength can not be straightforwardly correlated, due to importance of the sub-synoptic tongue of the low-pressure field inside the cyclone, which induced the circulation that superimposed on Bora flow as well as modified the integral background Froude number. It has been proposed that such sub-synoptic processes acted to break-up the upstream inversion and increase the upstream Bora layer depth. In addition, such a pressure distribution contributed to pressure gradient build-up across the mountain as well as changed thermal properties above the sea, what might contribute to the modified Bora behaviour, postponed hydraulic jump and stronger offshore Bora winds, through the influence on the maintenance of the supercritical downstream flow.