engleski

A numerical analysis of a deep Mediterranean lee cyclone: sensitivity to mesoscale potential vorticity anomalies

A 12-15 November 2004 Atlas lee cyclone and the related occurrence of severe bora along the eastern Adriatic coast were numerically analyzed using the MM5 mesoscale model. Motivated by the fact that sub-synoptic scales are more sensitive to initialization errors and dominate forecast error growth, this study is designed in order to assess the sensitivity of the mesoscale forecast to the intensity of mesoscale potential vorticity (PV) anomalies. Five sensitivity simulations are run after removing the selected anomalies, allowing for the analysis of the cyclone intensity and track, and additionally, the associated severe bora in the Adriatic. The results of the ensemble showed that the cyclone was sensitive to the exact details of the upper-level dynamic forcing, implying a significant influence of mesoscale perturbations on the cyclone predictability. The spread of cyclone intensities was the greatest in the deepest phase of the cyclone lifecycle, due to different cyclone movement speeds towards the Mediterranean. However, the cyclone tracks diffluence appears to be the greatest during the cyclone movement out of the Atlas lee, most likely due to predominant upper-level steering influence and its influence on the thermal anomaly creation in the mountain lee. Furthermore, it is quantitatively shown that the southern Adriatic bora was more sensitive to cyclone presence in the Mediterranean then bora in the northern Adriatic, due to unequal influence of the cyclone on the cross-mountain pressure gradient formation. The orographically induced pressure perturbation was shown to be strongly correlated with bora in the northern and to a lesser extent in the southern Adriatic, implying the existence of additional controlling mechanisms of bora in the southern part of the basin. In addition, the bora intensity in the southern Adriatic appeared to be highly sensitive to the precise sub-synoptic pressure distribution in the cyclone itself, indicating close relationship between the predictabilities of Mediterranean cyclones and bora in the southern Adriatic.

lee cyclone; Bora; mesoscale anomalies; potential vorticity; ensemble

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