Statistical investigations of flow structures in different regimes of the stable boundary layer

A combination of methods originating from non-stationary timeseries analysis is applied to two datasets of near surface turbulence in order to gain insights on the non-stationary enhancement mechanism of intermittent turbulence in the stable atmospheric boundary layer (SBL). We identify regimes of SBL turbulence for which the range of timescales of turbulence and submeso motions, and hence their scale separation (or lack of separation) differs. Ubiquitous flow structures, or events, are extracted from the turbulence data in each flow regime. We relate flow regimes characterised by very stable stratification but different scales activity to a signature of flow structures thought to be submeso motions.Comment: Accepted for publication in Boundary Layer Meteorolog

A stochastic stability equation for unsteady turbulence in the stable boundary layer

The atmospheric boundary layer is particularly challenging to model in conditions of stable stratification, which can be associated with intermittent or unsteady turbulence. We develop a modelling approach to represent unsteady mixing possibly associated with turbulence intermittency and with unresolved fluid motions, called sub-mesoscale motions. This approach introduces a stochastic parametrisation by randomising the stability correction used in the classical Monin–Obhukov similarity theory. This randomisation alters the turbulent momentum diffusion and accounts for sporadic events that cause unsteady mixing. A data-driven stability correction equation is developed, parametrised, and validated with the goal to be modular and easily combined with existing Reynolds-averaged Navier–Stokes models. Field measurements are processed using a statistical model-based clustering technique, which simultaneously models and classifies the non-stationary stable boundary layer. The stochastic stability correction obtained includes the effect of the static stability of the flow on the resolved flow variables, and additionally includes random perturbations that account for localised intermittent bursts of turbulence. The approach is general and effectively accounts for the stochastic mixing effects of unresolved processes of possibly unknown origin

On the motion of hairpin filaments in the atmospheric boundary layer

A recent work of Harikrishnan et al. [“Geometry and organization of coherent structures in stably stratified atmospheric boundary layers,” arXiv:2110.02253 (2021)] has revealed an abundance of hairpin-like vortex structures, oriented in a similar direction, in the turbulent patches of a stably stratified Ekman flow. In this study, hairpin-like structures are investigated by treating them as slender vortex filaments, i.e., a vortex filament whose diameter d is small when compared to its radius of curvature R. The corrected thin-tube model of Klein and Knio [“Asymptotic vorticity structure and numerical simulation of slender vortex filaments,” J. Fluid Mech. 284, 275 (1995)] is used to compute the motion of these filaments with the atmospheric boundary layer as a background flow. Our results suggest that the orientation of the hairpin filament in the spanwise direction is linked to its initial starting height under stable stratification, whereas no such dependency can be observed with the neutrally stratified background flow. An improved feature tracking scheme based on spatial overlap for tracking Q-criterion vortex structures on the direct numerical simulation data is also developed. It overcomes the limitation of using a constant threshold in time by dynamically adjusting the thresholds to accommodate the growth or deterioration of a feature. A comparison between the feature tracking and the filament simulation reveals qualitatively similar temporal developments. Finally, an extension of the asymptotic analysis of Callegari and Ting [“Motion of a curved vortex filament with decaying vortical core and axial velocity,” J. Appl. Math. 35, 148–175 (1978)] is carried out to include the effect of gravity. The results show that, in the regime considered here, a contribution from the gravity term occurs only when the tail of an infinitely long filament is tilted at an angle relative to the wall

On the motion of hairpin filaments in the atmospheric boundary layer

A recent work of Harikrishnan et al. [arXiv:2110.02253 (2021)] has revealed an abundance of hairpin-like vortex structures, oriented in a similar direction, in the turbulent patches of a stably stratified Ekman flow. The Ekman flow over a smooth wall is a simplified configuration of the Atmospheric Boundary Layer (ABL) where effects of both stratification and rotation are present. In this study, hairpin-like structures are investigated by treating them as slender vortex filaments, i.e., a vortex filament whose diameter $d$ is small when compared to its radius of curvature $R$. The corrected thin-tube model of Klein and Knio [J. Fluid Mech. (1995)] is used to compute the motion of these filaments with the ABL as a background flow. The influence of the mean background flow on the filaments is studied for two stably stratified cases and a neutrally stratified case. Our results suggest that the orientation of the hairpin filament in the spanwise direction is linked to its initial starting height under stable stratification whereas no such dependency can be observed with the neutrally stratified background flow. An improved feature tracking scheme based on spatial overlap for tracking $Q$-criterion vortex structures on the Direct Numerical Simulation (DNS) data is also developed. It overcomes the limitation of using a constant threshold in time by dynamically adjusting the thresholds to accommodate the growth or deterioration of a feature. A comparison between the feature tracking and the filament simulation reveals qualitatively similar temporal developments. Finally, an extension of the asymptotic analysis of Callegari and Ting [J. App. Math (1978)] is carried out to include the effect of gravity. The results show that, in the regime considered here, a contribution from the gravity term occurs only when the tail of an infinitely long filament is tilted at an angle relative to the wall

FLAVOR PREFERENCE AND EFFICACY OF VARIABLE DOSE ONTARIO RABIES VACCINE BAIT (ONRAB) DELIVERY IN STRIPED SKUNKS (\u3ci\u3eMEPHITIS MEPHITIS\u3c/i\u3e)

In North America, terrestrial wildlife rabies control is achieved by oral rabies vaccination programs that principally target mesocarnivores. Success at rabies control in striped skunks (Mephitis mephitis) has been more limited and may require additional enhancements to existing bait products or novel bait designs and attractants. We evaluated preference among captive striped skunks for six different flavors of placebo Ontario Rabies Vaccine Bait (ONRAB®) ‘‘Ultralite’’ Baits (Artemis Technologies, Guelph, Ontario, Canada). Different doses and delivery methods of ONRAB vaccine were tested for efficacy in a subsequent experiment with the same skunks. Cheese-, egg-, and chicken flavored baits were preferred over plain-flavored baits, but a strong preference for a singular flavor was not observed. Vaccine efficacy of 80–100% was observed among skunks challenged at 335 d postvaccination across a log range of doses tested by a direct instillation into the oral cavity route, respectively (109.3 - 1010.2 median tissue culture infective doses), in contrast to more-limited efficacy by bait delivery. Our results extended the duration of ONRAB vaccine efficacy in skunks and suggested that there may be limited flexibility to alter vaccine titer and volume in novel bait designs for skunks

FLAVOR PREFERENCE AND EFFICACY OF VARIABLE DOSE ONTARIO RABIES VACCINE BAIT (ONRAB) DELIVERY IN STRIPED SKUNKS (\u3ci\u3eMEPHITIS MEPHITIS\u3c/i\u3e)

In North America, terrestrial wildlife rabies control is achieved by oral rabies vaccination programs that principally target mesocarnivores. Success at rabies control in striped skunks (Mephitis mephitis) has been more limited and may require additional enhancements to existing bait products or novel bait designs and attractants. We evaluated preference among captive striped skunks for six different flavors of placebo Ontario Rabies Vaccine Bait (ONRAB®) ‘‘Ultralite’’ Baits (Artemis Technologies, Guelph, Ontario, Canada). Different doses and delivery methods of ONRAB vaccine were tested for efficacy in a subsequent experiment with the same skunks. Cheese-, egg-, and chicken flavored baits were preferred over plain-flavored baits, but a strong preference for a singular flavor was not observed. Vaccine efficacy of 80–100% was observed among skunks challenged at 335 d postvaccination across a log range of doses tested by a direct instillation into the oral cavity route, respectively (109.3 - 1010.2 median tissue culture infective doses), in contrast to more-limited efficacy by bait delivery. Our results extended the duration of ONRAB vaccine efficacy in skunks and suggested that there may be limited flexibility to alter vaccine titer and volume in novel bait designs for skunks

Efficacy of Ontario Rabies Vaccine Baits (ONRAB) against rabies infection in raccoons

In the US, rabies lyssavirus (RABV) only circulates in wildlife species and the most significant reservoir from a public and animal health perspective is the raccoon (Procyon lotor). Management of wildlife rabies relies principally on oral rabies vaccination (ORV) strategies using vaccine-laden bait delivery to freeranging target hosts, in order to reduce the susceptible population to prevent the spread of and eliminate RABV circulation. Our objective was to evaluate efficacy of the Ontario Rabies Vaccine Bait (ONRAB) against a lethal RABV challenge in captive raccoons. Sham or live vaccine baits were offered to 50 raccoons and efficacy was evaluated in 46, split into two trials of 17 and 29 raccoons. Raccoons were challenged with a lethal dose of RABV 180 days post-vaccination and observed for 90 days post-infection. Raccoon bait interactions were assigned increasing integer scores for approach, oral manipulation, puncture, and consumption behaviors. Higher bait interaction scores were observed in the fall compared to the spring trial, indicating that more raccoons consumed baits in the fall. Although animal age did not explain variation in bait interaction scores, the geometric mean rabies virus antibody titers among juvenile vaccinates were higher than adults at all pre-challenge time points. The prevented fraction associated with ONRAB delivery was 0.73 (8/11, 95% CI 0.39–0.94) in the spring trial and 0.91 (21/23, 95% CI 0.72–0.99) in the fall trial. All sham-vaccinated raccoons (12/12) succumbed to rabies infection, in contrast to 15% (5/34) mortality among vaccinated raccoons. Our results indicate a high efficacy of ONRAB bait vaccination in protecting adult and juvenile raccoons against RABV infection for a minimum of six months. These data complement experimental field trials that have also demonstrated the potential of ONRAB for the control and prevention of RABV circulation in free-ranging raccoon populations in the US

Subgrid-Scale Dynamics of Water Vapour, Heat, and Momentum over a Lake

We examine the dynamics of turbulence subgrid (or sub-filter) scales over a lake surface and the implications for large-eddy simulations (LES) of the atmospheric boundary layer. The analysis is based on measurements obtained during the Lake-Atmosphere Turbulent EXchange (LATEX) field campaign (August-October, 2006) over Lake Geneva, Switzerland. Wind velocity, temperature and humidity profiles were measured at 20Hz using a vertical array of four sonic anemometers and open-path gas analyzers. The results indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces, suggesting that the effect of the lake waves on surface-layer turbulence during LATEX is small. The measurements allowed, for the first time, the study of subgrid-scale turbulent transport of water vapour, which is found to be well correlated with the transport of heat, suggesting that the subgrid-scale modelling of the two scalars may be coupled to save computational resources during LE

Scale dependence of subgrid-scale model coefficients: An a priori study

Dynamic subgrid-scale models require an a priori assumption about the variation in the model coefficients with filter scale. The standard dynamic model assumes independence of scale while the scale dependent model assumes power-law dependence. In this paper, we use field experimental data to investigate the dependence of model coefficients on filter scale for the Smagorinsky and the nonlinear models. The results indicate that the assumption of a power-law dependence, which is often used in scale dependent dynamic models, holds very well for the Smagorinsky model. For the nonlinear model, the power-law assumption seems less robust but still adequat