On Bora wind driven near surface turbulence over complex terrain

Bora is a gusty wind that blows over the eastern Adriatic coast in the lee of the mountains from the NE quadrant with sustained speeds between 5-20 m s-1 and gusts surpassing 50-70 m s-1 (i.e., a downslope windstorm with hurricane speeds). Similar bora-like flows occur elsewhere in mountainous areas where the flow transcriticality (regarding “vertical” Froude number or, its inverse, dimensionless mountain height) is the dominant mesoscale flow property. Current and future efforts in studying bora gradually moves toward progressively smaller spatio-temporal scales, i.e., toward bora sub-mesoscale (e.g., rotors and mini-wakes) and turbulence.

A three level mast (10, 20 and 40 m) was equipped with Gill WindMaster Pro ultrasonic anemometers measuring 3D wind speed and sonic temperature at 5 Hz sampling rate at the top of the so-called Pometeno brdo (Swept-away Hill, 600 m ASL), a bora upwind site to the city of Split. Measurements lasted from April 2010 to June 2011, thus capturing multiple bora events (some including pulsations) during all annual seasons. For two different bora cases, we calculate all the terms in the simplified turbulent kinetic energy (TKE) budget equation (1D model of the TKE budget, horizontal homogeneity assumed), except the pressure covariance term. Next, we address bora turbulence anisotropy, i.e., the velocity component variance aspect ratio. The enhanced bora anisotropy might explain barely adequate experimental, otherwise fine dimensional scaling between the TKE, dissipation and integral length-scale. A few related bora cases are simulated with WRF model.