On Turbulent Fluxes During Strong Winter Bora Wind Events

Abstract

Well known for its severity, the bora downslope windstorms have been extensively studied over the last several decades. This study focuses on the turbulence characteristics of bora at a topographically complex site near the eastern coast of the Adriatic Sea. For this purpose, a 3-month eddy-covariance dataset obtained at three levels (10, 22, 40 m) on a 60-m flux tower is used. After determining a suitable averaging time scale of 15 min using the fast Fourier transform and the ogive method, vertical fluxes of momentum and heat were calculated for 17 bora episodes. Up to a wind speed of 12 m s −1 , typical vertical profiles of momentum and heat were observed. However, for wind speeds >12 m s −1 , several interesting observations arose. First, the nighttime heat flux at the 10-m level was often found to be positive rather negative. Second, vertical profiles of the momentum flux were larger at the 22-m level than at 10-and 40-m levels, mostly during nearly neutral to weakly stable thermal stratification. Third, these momentum flux profiles showed a large dependence on wind direction, with virtually no vertical transport of momentum for the largest observed wind speeds. For the first time, bora coherent structures have been analyzed using the so-called variable-interval time averaging (VITA) method. The method detected coherent structures in all three wind-speed components, with structure topologies similar to those observed over forest canopies. The momentum flux increase at the 22-m level, relative to the 10-and 40-m levels, is further supported by the VITA findings.

Micro-Scale Properties of Different Bora Types

“…This is in accordance with [52] for the nocturnal stable boundary layer in the Croatian lowland (town of Kutina), while Babić et al [30] found an energy gap at the 15-min period for bora episodes at Pometeno Brdo, northeast from the city of Split. Hence, all further analyses were performed on block intervals of 30 min length.…”

“…According to some authors [30,48,49], the ultrasonic temperature is a good approximation of the potential temperature.…”

“…Here, we took block intervals with absolute values of the stability parameter less than 0.02 as near-neutral, which is the same criterion that was used in [30].…”

“…These intervals are defined as the intervals during which |ζ| < 0.02 is valid for all three levels [30]. For every interval the profile friction velocity u * p and roughness length z 0 are calculated as follows:…”

Feasibility of Investment in Renewable Energy Systems for Shipyards

“…An important and still open issues are related to wind characteristics around wind turbines placed in topographically complex terrain, [47], and in regions characterized by non-standard transient winds, [48], particularly in case of extreme weather conditions encountered during summer, [49], and winter, [50]. These untypical wind conditions can considerably increase structural fatigue of wind turbines and thus shorten their lifetime, while the power output of wind turbines in these transient wind conditions is smaller than it is the case for quasi-steady flow conditions.…”