Estimation of Turbulent Triplet Covariances for Bora Flows

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.

Spatial–Temporal Analysis of a Summer Heat Wave Associated with Downslope Flows in Southern Brazil: Implications in the Atmospheric Boundary Layer

“…The tower is located in a 24 hectare area typical of the Pampa biome, characterized by low herbaceous grassland vegetation ≈8 km south of the terrain discontinuity (Figure 1). Initially, 12 levels of sonic anemometers were installed at different heights: 1.5, 3,5,7,9,11,14,17,20,23,26, and 30 m, and 15 levels to measure air temperature and relative humidity at 0.5, 1, 2.25, 3, 4, 6, 8, 10, 12, 15.5, 18.5, 21.5, 24.5, 28, and 29.5 m. Turbulence observations of wind components and sonic temperature were sampled at a frequency of 10 Hz, while air temperature and relative humidity were sampled each minute. Means, fluxes, variances, and other statistical moments were determined using a 30 min standard time window.…”

“…One manifestation of this type of change in the turbulent field is provoked by topographically induced mesoscale flow [2,3]. Particularly, downslope flows are associated with strong wind and intense turbulence production [2,[4][5][6][7][8][9]. Understanding downslope winds (DWs) and their interaction with turbulence are of great importance for predicting the transport and dispersion of pollutants and for developing parameterizations used in weather and climate numerical models.…”