Near-ground turbulence of the Bora wind in summertime

Abstract

Bora is a strong, temporally and spatially variable downslope wind that blows along the Eastern Adriatic Coast and many other dynamically similar places around the world. Bora mean velocity rarely exceeds 20m/s, while its strong gusts create significant problems for engineering structures, traffic and agriculture. Previous meteorological and geophysical studies laid foundations on Bora large- and mesoscale motions, but further research is necessary to fully understand and resolve Bora turbulence in a form usable for engineers. In this study, unique high-frequency measurements, carried out simultaneously in three heights on a meteorological tower in the hinterland of the city of Split, Croatia, are analyzed for a single summertime Bora episode. Time histories and vertical profiles are studied for turbulence intensity, Reynolds shear stress and turbulence length scales in comparison with values recommended in major international wind engineering standards. Turbulence intensity and Reynolds shear stress proved to be not that sensitive to meandering of the mean wind velocity, as the observed values remain within the same range during the time record. This trend applies on mean wind velocities larger than 5m/s, as for smaller velocities the spread of values increases considerably. Turbulence length scales are observed to increase with increasing mean wind velocity and vice versa. With increasing height from the ground, turbulence intensity and absolute Reynolds shear stress decrease, turbulence length scales increase, all in agreement with the atmospheric physics. However, while turbulence intensity, Reynolds shear stress and longitudinal turbulence length scales generally agree well with the values recommended in international standards for the respective terrain type, turbulence length scales related to lateral and vertical velocity fluctuations are much larger than the standard values.