engleski

Contrasting (sub-)mesoscale bura structures

Depending on future large-scale flow, affected by the current climate change, one shall witness possible structural changes in bura flows, or at least, variations in bura preferential statistics. Two different bura cases are addressed in this work: a moderate or strong, and a severe bura. Satellite TerraSAR-X (TS-X) images are supported by the Weather Research and Forecasting (WRF) numerical simulations and a few regular measurements. The moderate or strong case (Case 1) belongs to a deep anti-cyclonic bura setup, and the severe case (Case 2) is a shallow and predominantly cyclonic bura type, though the other two synoptic scenarios part ially occur too due to the effects of a front and high-pressure system. In Case 1, the upstream (vertical) Froude number, Fr (or the inverse of dimensionless mountain height), obtained from Zagreb sounding was about 0.7 < Fr < 1.1, while in Case 2 it was about 0.6 < Fr < 0.7. High resolution TS-X image processing displays very fine Bura spatial structures at Lx 1 km, i.e., at the sub-mesoscale, that has not been shown in the previous bura research. For the severe Case 2, the Adriatic sea-surface convective heat fluxes induced by the vigorous surface temperature differences between the sea and the overflowing very cool fast air are responsible for those fine scale structures (plus the obvious complex coastal orography and geometry). The WRF simulations suggest that the extremely cold upstream airflow reinforced the mountain-wave breaking by enhancing the corresponding mesoscale pressure gradient over the mountains. The satellite images show for Case 2, characterized by the more vigorous cross-mountain flow, secondary and tertiary low-level jets of bura in the lee of Velebit mountain. Yet, we do not know which bura type of bura cases shall be dominant in the future.

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