Abstract
Bora is a strong or severe, relatively cold, gusty wind that usually blows from the northastern quadrant at the east coast of the Adriatic Sea. In this study bora’s turbulence triplet covariances were analysed, for the first time, for bora flows. The measurements used were obtained from the measuring tower on Pometeno brdo (“Swept-Away Hill”), in the hinterland of the city of Split, Croatia. From April 2010 until June 2011 three components of wind speed and sonic temperature were measured. The measurements were performed on three heights, 10, 22 and 40 m above the ground with the sampling frequency of 5 Hz. During the observed period, total of 60 bora episodes were isolated. We analyse the terms in prognostic equations for turbulence variances. In that respect, the viscous dissipation term was calculated using two approaches: (i) inertial dissipation method (εIDM) and (ii) direct approach from the prognostic equations for variances of turbulence (εEQ). We determine that the direct approach can successfully reproduce the shape of the curve, but the values are for several orders of magnitudes smaller compared to the real data. Further, linear relationship between εIDM and εEQ was obtained. Using the results for εEQ, viscous dissipation rate in longitudinal, transversal and vertical direction was determined. It is shown that viscous dissipation has the greatest impact on bora’s longitudinal direction. The focus is on the turbulence transport term, i.e., the triplet covariance term. For the first time, it is found that turbulence transport is very significant for the intensity of near−surface bora flows. Furthermore, turbulence transport can be both positive and negative, yet intensive. It is mostly negative at the upper levels and positive at the lower levels. Therefore, turbulence transport, in most cases, takes away turbulence variance from the upper levels and brings it down to the lower ones. This is one of the main findings of this study; it adds to the understanding of peculiarities of bora wind, and perhaps some other severe winds.
Highlights
IntroductionStrong-to-severe, relatively cold, very gusty wind that usually blows from the northeastern quadrant at the east coast of the Adriatic Sea (and many other dynamically similar places around the world) is known as bora
Strong-to-severe, relatively cold, very gusty wind that usually blows from the northeastern quadrant at the east coast of the Adriatic Sea is known as bora
This is because w component spectra do not contain large amount of energy at larger, i.e., synoptic scales since bora flows are driven by cyclonic, anticyclonic or frontal systems, where horizontal motions are predominant
Summary
Strong-to-severe, relatively cold, very gusty wind that usually blows from the northeastern quadrant at the east coast of the Adriatic Sea (and many other dynamically similar places around the world) is known as bora. Bora is synoptically caused by collapsing of a relatively cold northeastern air mass across the Dinaric Mountains—a mountain barrier perpendicular to the air flow [1,2,3,4,5] which generates steep, large amplitude mountain waves (mesoscale part of the flow setup). Bora generally achieves a wide range of wind speeds with gusts; the bora wind gusts of more than 60 m s−1 often occur (e.g., [6,10,11]). For such mostly violent circumstances, Lepri et al 2014 and 2017 [12,13]
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