Along-Coast Features of Bora-Related Turbulence

Abstract

The along-coast, offshore turbulence structure of the Bora flow that occurred on 7 November 1999 during the Mesoscale Alpine Programme (MAP) Intensive Observation Period 15 is examined. In this analysis we employ the aircraft and dropsonde data obtained over the Adriatic Sea, where the turbulence structure is determined by estimating turbulent kinetic energy (TKE) and its dissipation rate along the flight legs. The turbulence characteristics of Bora in the lee of the Dinaric Alps is greatly influenced by the mesoscale Bora flow structure over the Adriatic Sea, which in the cross-wind direction features an interchange of jets and wakes related to mountain gaps and peaks. In order to establish the origin of turbulence, the Weather Research and Forecasting-Advanced Research WRF (WRF-ARW) numerical model is used and its results are compared to the measurements. All five TKE-prediction parametrization schemes available in the model show reasonable agreement with the measured values. Since these parametrization schemes do not have horizontal advection included, they suggest that the along-flight structure of the Bora turbulence is principally generated by the local vertical wind shear. Further evidence is needed to support this hypothesis. Electronic supplementary materialThe online version of this article (

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…Typically, the determination of the appropriate TAS is based on detecting the time scale that matches the mesoscale energy gap, i.e., the low-energy intermediate range in the kinetic energy spectrum of atmospheric motions [69]. Often, no obvious mesoscale gap can be found in the Fourier spectra of measurements from complex terrain [70][71][72]. Therefore, the determination of the TAS has to rely on more complex techniques, such as the ogive method [73], multi-resolution flux decomposition [74], or wavelet analysis [75].…”

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

“…As a result of the persistence of this bora case, the flow conditions may to a good approximation be considered stationary during the 2 h period spanning the three flight legs (Klaić et al, 2003;Grubišić, 2004). The analysis of numerical model results corroborates this conclusion (Večenaj et al, 2012).…”

“…As a result, there are several other peaks in the crosswind wavenumber spectra that may overlap with that of the rolls. This complexity also hinders the determination of relevant scales for calculation of turbulence fluxes, as discussed in Večenaj et al (2012). The bora jets and wakes, while basically being cold-air outbreaks resulting in a thermodynamically generated convective ABL over the warmer sea, are also dynamically generated phenomena that carry the signatures of their upstream generation mechanisms, particularly gravity waves and the resultant downwind variation of ABL height.…”

“…In order to avoid noise at high frequencies, the aircraft data were block-averaged to 5 Hz prior to the analyses. Further details about the data are given in Večenaj et al (2012). As a result of the persistence of this bora case, the flow conditions may to a good approximation be considered stationary during the 2 h period spanning the three flight legs (Klaić et al, 2003;Grubišić, 2004).…”

“…A number of complexities arise because of the terrain inhomogeneity, particularly the mountain gaps and peaks along the Dinaric Alps. The major gaps and peaks result in the formation of bora jets and wakes, respectively (e.g., Jiang and Doyle, 2005;Belušić and Klaić, 2006;Signell et al, 2010), which are then related to potential vorticity banners (e.g., Grubišić, 2004) and varying turbulence structure (Večenaj et al, 2012). A number of other features appear within the bora flow, such as lee rotors of different types (e.g., Gohm et al, 2008;Prtenjak and Belušić, 2009;Stiperski et al, 2012) and quasi-periodic pulsations (e.g., Belušić et al, 2007), but they mostly do not extend far over the sea.…”