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A summertime near-ground velocity profile of the Bora wind

  • Lepri, Petra (Meteorological and Hydrological Service) ;
  • Kozmar, Hrvoje (Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb) ;
  • Vecenaj, Zeljko (Department of Geophysics, Faculty of Science, University of Zagreb) ;
  • Grisogono, Branko (Department of Geophysics, Faculty of Science, University of Zagreb)
  • Received : 2014.05.30
  • Accepted : 2014.08.30
  • Published : 2014.11.25

Abstract

While effects of the atmospheric boundary layer flow on engineering infrastructure are more or less known, some local transient winds create difficulties for structures, traffic and human activities. Hence, further research is required to fully elucidate flow characteristics of some of those very unique local winds. In this study, important characteristics of observed vertical velocity profiles along the main wind direction for the gusty Bora wind blowing along the eastern Adriatic coast are presented. Commonly used empirical power-law and the logarithmic-law profiles are compared against unique 3-level high-frequency Bora measurements. The experimental data agree well with the power-law and logarithmic-law approximations. An interesting feature observed is a decrease in the power-law exponent and aerodynamic surface roughness length, and an increase in friction velocity with increasing Bora wind velocity. This indicates an urban-like velocity profile for smaller wind velocities and rural-like velocity profile for larger wind velocities, which is due to a stronger increase in absolute velocity at each of the heights observed as compared to the respective velocity gradient (difference in average velocity among two different heights). The trends observed are similar during both the day and night. The thermal stratification is near neutral due to a strong mechanical mixing. The differences in aerodynamic surface roughness length are negligible for different time averaging periods when using the median. For the friction velocity, the arithmetic mean proved to be independent of the time record length, while for the power-law exponent both the arithmetic mean and the median are not influenced by the time averaging period. Another issue is a large difference in aerodynamic surface roughness length when calculating using the arithmetic mean and the median. This indicates that the more robust median is a more suitable parameter to determine the aerodynamic surface roughness length than the arithmetic mean value. Variations in velocity profiles at the same site during different wind periods are interesting because, in the engineering community, it has been commonly accepted that the aerodynamic characteristics at a particular site remain the same during various wind regimes.

Keywords

Bora;velocity profile;aerodynamic surface roughness length;friction velocity;logarithmic-law;power-law;field measurements

Acknowledgement

Supported by : National Science Foundation

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