The Effect of Orography on Bora Wind Turbulence

Turbulence statistics are analysed for Bora episodes that occurred simultaneously in the town of Senj and at the Vratnik pass at the northern Croatian Adriatic coast. The wind record spans over 9 months. The results are compared to address the difference in characteristics of near ground turbulence in the town of Senj, located at the shoreline leeward of the main mountain peaks, and at the Vratnik pass, located at the mountain ridgeline 7.5 km upstream of Senj. The analysis reveals some important findings. In particular, turbulence kinetic energy (TKE) is systematically larger at Senj, approximately twice the value at Vratnik, yet the mean wind speed is higher at Vratnik. Furthermore, the TKE dissipation rate epsilon is nearly identical at the two locations. This is further explored by comparing how the turbulence at the two locations adheres to a simple shear-dissipation model where the standard deviation of the longitudinal velocity component is proportional to the mean longitudinal wind velocity. Vratnik relates to this model better, while the TKE balance in Senj is more complex. The difference in statistics and TKE balance can, in part, be attributed to the relative proximity of the anemometers to the rough surface. Turbulence length scale is substantially larger in Senj. Turbulence intensities are in correspondence with values recommended in engineering codes.