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Observational and numerical analysis of pulsations and turbulence in a bora downslope windstorm event

Turbulent downslope windstorm events are frequent phenomena over complex terrain of the eastern Adriatic coast. While severe northern-Adriatic downslope windstorms are since long in the focus of interest, strong bora winds in the hinterland of middle Adriatic coast are much less studied, yet frequent and equally severe phenomena. The predictability of these events is considerably lower than for its northern counterpart due to the inflow complexity induced by the upwind chain of secondary orographic steep mountain sub-ranges and deep valleys. The aims of this work are first to study sub-mesoscale pulsations embedded in the bora flow and second, to study sources and sinks of bora turbulence through the analysis of the turbulence kinetic energy (TKE) budget. The analyzed event was a strong late-winter anticyclonic bora (28 Apr 2010) in the very complex terrain, characterized by three-dimensional flow, shallow bora layer, a pronounced directional vertical wind shear and interaction with valley circulations in deep valleys. Observational analysis, performed with the use of ultrasonic anemometers at Pometeno brdo hill at 10, 22 and 40 m AGL, suggested that two distinct regimes of pulsations exist: i) Regime A – pulsations observed predominantly during the night and morning hours with periods of 3 – 8 minutes and ii) Regime B – pulsations observed predominantly during the afternoon with periods of 8-11 minutes. Numerical analysis of the event used the WRF model with realistic initial and boundary conditions and multiple nested computational domains in two configurations. The first used a mesoscale model setup at a grid spacing reaching 333 m in the highest resolution domain and a Mellor-Yamada type of the Planetary Boundary Layer (PBL) scheme while the second used a multiscale setup at a grid spacing as fine as 37 m using explicit simulation of large turbulent eddies. The strongest simulated wind speed pulsations were of comparable periods as in the observations and originated beneath the primary mountain gravity wave. These pulsations propagated farther away from the point of origin during the daytime convective PBL than those during the statically stable nighttime conditions. The analysis of sensitivity simulations suggested that pulsations originate beneath the primary mountain wave due to KelvinHelmholtz instability. Additionally, during daytime they are also found in the upstream flow. While mechanical production and dissipation of TKE are the dominant terms of the TKE budget, other terms such as turbulent transport and advection play an important role for the TKE budget. Finally, main differences in the bora subtle structure over the middle and northern Adriatic coastal areas, the latter pertaining to more known bora cases, are pointed out, as well as main differences between results in mesoscale and multiscale simulations.

bora, pulsations, observations, numerical simulations

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