6.2 Comparison of the bora turbulence derived from airborne in-situ measurements with the WRF-ARW simulations

Tuesday, 31 August 2010: 10:45 AM
Alpine Ballroom A (Resort at Squaw Creek)
Zeljko Vecenaj, University of Zagreb, Zagreb, Croatia; and D. Belusic, V. Grubisic, and B. Grisogono

The bora is a severe north-easterly downslope wind off the Dinaric Alps along the eastern Adriatic coast. It is most frequent during winter seasons. Bora mean wind speed can surpass 20 m/s and due to its gustiness it reaches maximum speeds greater than 60 m/s. During such events, the turbulence is strongly developed in the lee of the mountain extending far off-shore above the Adriatic sea. Here we examine the along-coast, off-shore structure of the bora turbulence.

We study an early stage of a bora event over the northern Adriatic that occurred on 7 November 1999 during the Mesoscale Alpine Program (MAP) Intensive Observation Period 15. We use the aircraft data measured along the lee side of the Dinaric Alps over the Adriatic by the NCAR Electra aircraft flying two vertically separated flight legs (lower at ~ 370 m, higher at ~ 680 m). Measurements from six dropsondes released from 4200 m along the legs are used as well.

Using these data, main features of turbulence along the northern Adriatic are derived (spatial distribution of TKE and its dissipation rate). The question of the origin of this turbulence naturally arises; is it a local phenomenon or maybe advection plays a significant role? Since the answer is difficult to obtain only from the measurements, we have simulated this event using WRF-ARW model with five different PBL parameterization schemes. It seems that one of them agrees particularly well in comparison with the measurements.

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