Bora Flow Characteristics in a Complex Valley Environment

Marija Bervida,Samo Stanič,Klemen Bergant,Griša Močnik,Longlong Wang,Xiaoquan Song

doi:10.3390/rs13214363

Abstract

This paper complements the existing studies of Bora flow properties in the Vipava valley with the study of Bora turbulence in a lower region of the troposphere. The turbulence characteristics of Bora flow were derived from high resolution Doppler wind lidar measurements during eight Bora wind episodes that occurred in November and December 2019. Based on the vertical profiles of wind velocity, from 80 to 180 m above the valley floor, the turbulence intensity related to all three spatial directions and the along-wind integral length scales related to three velocity components were evaluated and compared to the approximations given in international standards. The resulting turbulence characteristics of Bora flow in a deep mountain valley exhibited interesting behaviour, differing from the one expected and suggested by standards. The intensity of turbulence during Bora episodes was found to be quite strong, especially regarding the expected values for that particular category of terrain. The specific relationship between along-wind, lateral and vertical intensity was evaluated as well. The scales of turbulence in the along-wind direction were found to vary widely between different Bora episodes and were rather different from the approximations given by standards, with the most significant deviations observed for the along-wind length scale of the vertical velocity component. Finally, the periodicity of flow structures above the valley was assessed, yielding a wide range of possible periods between 1 and 10 min, thus confirming some of the previous observations from the studies of Bora in the Vipava valley.

Highlights

Wind field measurements are fundamental for an accurate description of atmospheric dynamics, and for the understanding and prediction of weather development [1].In the lower part of the troposphere, these measurements are important for a wide range of scientific studies and engineering applications
The measurements of wind velocity were acquired at 21 different height levels in the lower troposphere, from which, heights between 80 and 180 m were studied, covering a far uninvestigated vertical range of the atmosphere above the valley floor
Based on eight wind episodes related to Bora that occurred during November and December 2019, the turbulence characteristics and periodicity of the flow over a complex valley environment were assessed

Summary

Introduction

Wind field measurements are fundamental for an accurate description of atmospheric dynamics, and for the understanding and prediction of weather development [1]. In the lower part of the troposphere, these measurements are important for a wide range of scientific studies and engineering applications. Measurements of wind speed and direction are further important for assessing the air pollution dispersion in the atmosphere [3] and for other studies, such as for revealing the structure and evolution of atmospheric rotors [4] or for investigating turbulent transport within a valley [5]. An accurate description of the wind field is vital as well for more realistic simulations in computational wind engineering (CWE) and related studies, such as the interactions between air flow and structures on the ground [6].

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