The role of helicity and fire–atmosphere turbulent energy transport in potential wildfire behaviour

Background. Understanding near-surface fire–atmosphere interactions at turbulence scale is fundamental for predicting fire spread behaviour. Aims. This study aims to investigate the fire–atmosphere interaction and the accompanying energy transport processes within the convective boundary layer. Methods. Three groups of large eddy simulations representing common ranges of convective boundary layer conditions and fire intensities were used to examine how ambient buoyancy-induced atmospheric turbulence impacts fire region energy transport. Key results. In a relatively weak convective boundary layer, the fire-induced buoyancy force could impose substantial changes to the near-surface atmospheric turbulence and cause an anticorrelation of the helicity between the ambient atmosphere and the fire-induced flow. Fire-induced impact became much smaller in a stronger convective environment, with ambient atmospheric flow maintaining coherent structures across the fire heating region. A highefficiency heat transport zone above the fire line was found in all fire cases. The work also found counter-gradient transport zones of both momentum and heat in fire cases in the weak convective boundary layer group. Conclusions. We conclude that fire region energy transport can be affected by convective boundary layer conditions. Implications. Ambient atmospheric turbulence can impact fire behaviour through the energy transport process. The counter-gradient transport might also indicate the existence of strong buoyancy-induced mixing processes

Investigation of grapevine areas under climatic stress using high resolution atmospheric modelling: case studies in South Africa and New Zealand

High-resolution atmospheric simulations (500 m) were used to assess viticultural areas under climatic stress in South Africa and New Zealand. The potential areas in which high daytime temperature stress was likely to affect grapevine photosynthesis and grape composition were identified. Results indicated different diurnal temperature variations within the two areas due to synoptic and local environmental factors, often associated with the influence of terrain

Atmospheric turbulent structures and fire sweeps during shrub fires and implications for flaming zone behaviour

Background. Wildfires propagate through vegetation exhibiting complex spread patterns modulated by ambient atmospheric wind turbulence. Wind gusts at the fire-front extend and intensify flames causing direct convective heating towards unburnt fuels resulting in rapid acceleration of spread. Aims. To characterise ambient and fire turbulence over gorse shrub and explore how this contributes to fire behaviour. Methods. Six experimental burns were carried out in Rakaia, New Zealand under varying meteorological conditions. The ignition process ensured a fire-line propagating through dense gorse bush (1 m high). Two 30-m sonic anemometer towers measured turbulent wind velocity at six different levels above the ground. Visible imagery was captured by cameras mounted on uncrewed aerial vehicles at 200 m AGL. Key results. Using wavelet decomposition, we identified different turbulent time scales that varied between 1 and 128 s relative to height above vegetation. Quadrant analysis identified statistical distributions of atmospheric sweeps (downbursts of turbulence towards vegetation) with sustained events emanating from above the vegetation canopy and impinging at the surface with time scales up to 10 s. Conclusions. Image velocimetry enabled tracking of ‘fire sweeps’ and characterised for the first time their lifetime and dynamics in comparison with overlying atmospheric turbulent structures. Implications. This methodology can provide a comprehensive toolkit when investigating coupled atmosphere–fire interactions

Two-dimensional numerical analysis of a thermally generated mesoscale wind system observed in the MacKenzie Basin, New Zealand

A mesoscale numerical model was used to perform two-dimensional numerical simulations of a thermally driven circulation, known as the Canterbury Plains Breeze, to examine the effect of key physical mechanisms that determine the intensity of this circulation. The mesoscale model has a 2.5 order turbulence closure scheme with a terrain following coordinate system, and has been previously used successfully for numerical studies in mountainous landscapes. The numerical results confirm observational data showing that during settled weather, the Canterbury Plains Breeze is a significant climatological feature of surface airflow in the Mackenzie Basin in the South Island of New Zealand. This circulation is generated because the elevated plateau creates a horizontal temperature gradient between the air inside and outside the basin at the same height. Other forcing factors, such as the gradient in soil moisture and the landsea discontinuity, only enhance the intensity of this mesoscale flow by modifying the horizontal temperature gradient

Prognostic urban-scale air pollution modelling in Australia and New Zealand

This paper reviews research conducted in the past two decades in urban-scale air quality modelling in Australia and New Zealand, with emphasis on prognostic models. With advances in computer technology – especially desktop computers – air pollution dispersion modelling is now a feasible undertaking not only for wellfunded research institutions, but also for air quality consultants. It has been suggested that as prognostic models become more user friendly they will eventually replace Gaussian dispersion models as a tool for urban air quality impact assessment. However, for now, Gaussian models are still widely used. Prognostic dispersion models have been applied to a number of Australian and New Zealand urban regions with relative success. In Australia, the major focus has been in simulating photochemical smog episodes. In contrast, New Zealand studies have mostly dealt with nocturnal dispersion of particulate matter during stagnant weather conditions

Numerical simulations of turbulent flow within and in the wake of a small basin

Small terrain features, such as small valleys, basins, sinkholes, low hills, and outcrops, while generally associated with mountainous regions, can also be found over plains. In this study, we present a numerical investigation of the effect of a small terrain feature (a 30mhigh rim) on the mean and turbulent flows inside and downstream of an enclosed basin it surrounds. Results from high-resolution numerical simulations (10m isotropic spatial resolution) indicate that small terrain features in the proximity of larger ones can induce relatively large modifications to the mean and turbulent flows. The 30mhigh rim is found to have an effect on the mean wind speeds at least 600m upstream from the basin. The main effect is a 10% reduction in wind speed up to 120m above ground level due to the upstream blocking effect of the rim. The presence of the rimcan also double the turbulent kinetic energy (TKE) both inside and downstream of the basin compared to an otherwise identical basin without a rim. The slopes of the basin play an important role in first creating and then defining the wake, and in intermittent wind regimes most of the scalar transport from near the slope of the basin happens through slope roll vortices that define the edge of the downstream wake region of the basin. Inside the basin, the rim acts to limit momentum transfer in the lower half of the basin, which suggests a mechanical forcing effect induced by the rim on lower basin environments that could interact with thermal buoyancy effects in heated or cooled basins. Some of the wake features resemble wind-eroded surfaces in the wakes of Martian craters. Results also reveal a critical height level (43m below the rim height) that acts as the most favored location for TKE production and destruction, which could be important for the top-to-bottomturbulence erosion of basin boundary layers. These results stress the importance of resolving small-scale terrain features, as their effects can be nonlocal