William Physick

TAPM: a practical approach to prognostic meteorological and air pollution modelling

Abstract Air pollution predictions for environmental impact assessments are usually generated by Gaussian plume/puff models driven by observationally-based meteorological inputs. An alternative approach is to use prognostic meteorological and air pollution models, which are better founded than the Gaussian approach, but tend to require much expertise and time to set up and large computing resources to run. This paper provides an overview of the science and verification of The Air Pollution Model (TAPM), which was designed to apply the best science in an easy-to-use and fast-to-run model. TAPM is a PC-based, nestable, prognostic meteorological and air pollution model (with photochemistry) driven by a Graphical User Interface, and is a viable tool for year-long simulations. Datasets of the important inputs needed for meteorological simulations (such as terrain and land-use data and synoptic analyses) accompany the model, allowing quick GUI configuration of the model for any region. We present results from the application of TAPM for urban and coastal areas in Australia, for two United States tracer experiments (Kincaid and Indianapolis) used for international model inter-comparison, and for point source dispersion in wind-tunnel building wakes. The meteorological results show that TAPM performs well in coastal, inland, and complex terrain, in sub-tropical to mid-latitude conditions, for both case studies and year-long simulations. The pollution results show that TAPM performs well for a range of important phenomena such as nocturnal inversion break-up fumigation, convective dispersion, shoreline fumigation, building wakes, and general dispersion in complex rural and urban conditions. In particular, the TAPM performance is very good for the prediction of extreme pollution statistics, important for environmental impact assessments, for both non-reactive (tracer) and reactive (nitrogen dioxide, ozone and particulate) pollutants for a variety of sources (e.g. industrial stacks and surface or urban emissions).

A skewed homogeneous lagrangian particle model for convective conditions

Abstract Numerical solution of the Langevin equation for convective conditions usually requires quite small time steps in order to resolve properly the smaller-scale turbulence near the boundaries. This time step restriction can be eased significantly if it is assumed that the turbulence is uniform over the whole depth of the mixed layer. In this paper, we investigate the effect of this homogeneous turbulence assumption on ground-level concentration (glc) by comparing results with (a) the Willis and Deardorff laboratory experiments on dispersion in fully convective conditions and (b) the predictions of a model employing a more realistic inhomogeneous parameterization for the turbulence. As far as the ability to predict the magnitude and location of the near-source maximum glc, and the ability to maintain a well-mixed profile further downwind are concerned, we conclude that the assumption of homogeneous turbulence is quite adequate. We also compare Gaussian and skewed distributions for homogeneous turbulence and show that realistic results in convective conditions can only be obtained with the skewed distribution.

INCORPORATION OF A HIGH-ROUGHNESS LOWER BOUNDARY INTO A MESOSCALE MODEL FOR STUDIES OF DRY DEPOSITION OVER COMPLEX TERRAIN

For flow over natural surfaces, there exists a roughness sublayer within the atmospheric surface layer near the boundary. In this sublayer (typically 50z0 deep in unstable conditions), the Monin-Obukhov (M-O) flux profile relations for homogeneous surfaces cannot be applied. We have incorporated a modified form of the M-O stability functions (Garratt, 1978, 1980, 1983) in a mesoscale model to take account of this roughness sublayer and examined the diurnal variation of the boundary-layer wind and temperature profiles with and without these modifications. We have also investigated the effect of the modified M-O functions on the aerodynamic and laminar-sublayer resistances associated with the transfer of trace gases to vegetation. Our results show that when an observation height or the lowest level in a model is within the roughness sublayer, neglect of the flux-profile modifications leads to an underestimate of resistances by 7% at the most.

A Lagrangian particle model of fumigation by breakdown of the nocturnal inversion

Abstract Several fumigation modelling studies are reviewed and an approach involving a coupled mesoscale grid point model and a Lagrangian particle dispersion model is chosen for further study. A number of particle model formulations are evaluated for their ability to maintain a well-mixed profile under steady-state convective conditions. The importance of using a formulation of the Langevin equation which is compatible with the specified turbulence parameterization is emphasised. Other considerations such as the form of the parameterizations and the magnitude of the timestep must also be taken into account. For prediction of hourly-averaged surface concentrations under fumigation conditions, it is found that a simple homogeneous turbulence formulation is equally effective as more sophisticated parameterizations involving inhomogeneity and skewness. Such a particle model, in conjunction with a mesoscale model for predicting the growth of the mixed layer, is able to reproduce well the results of the Deardorff and Willis (1982, Atmospheric Environment16, 1159–1170) laboratory experiments on fumigation.

The inland boundary layer at low latitudes: II Sea-breeze influences

Two-dimensional mesoscale model results support the claim of evening sea-breeze activity at Daly Waters, 280 km inland from the coast in northern Australia, the site of the Koorin boundary-layer experiment. The sea breeze occurs in conditions of strong onshore and alongshore geostrophic winds, not normally associated with such activity. It manifests itself at Daly Waters and in the model as a cooling in a layer 500–1000 m deep, as an associated surface pressure jump, as strong backing of the wind and, when an offshore low-level wind is present, as a collapse in the inland nocturnal jet.Both observational analysis and model results illustrate the rotational aspects of the deeply penetrating sea breeze; in our analysis this is represented in terms of a surge vector — the vector difference between the post- and pre-frontal low-level winds.There is further evidence to support earlier work that the sea breeze during the afternoon and well into the night — at least for these low-latitude experiments — behaves in many ways as an atmospheric gravity current, and that inland penetrations up to 500 km occur.

The atmospheric boundary layer — advances in knowledge and application

We summarise major activities and advances in boundary-layer knowledge in the 25 years since 1970, with emphasis on the application of this knowledge to surface and boundary-layer parametrisation schemes in numerical models of the atmosphere. Progress in three areas is discussed: (i) the mesoscale modelling of selected phenomena; (ii) numerical weather prediction; and (iii) climate simulations. Future trends are identified, including the incorporation into models of advanced cloud schemes and interactive canopy schemes, and the nesting of high resolution boundary-layer schemes in global climate models.

The enhancement of cold-front temperature contrast by differential cloud cover

Abstract The thermal impact of differential cloud shading across a cold front is evaluated briefly through conceptual, scaling, and numerical-modeling approaches. It is suggested that in summer the shading may enhance the boundary-layer average thermal contrast across the front by as much as 5 K for prolonged shading over the cold sector and with a dry surface in the warm sector. For short shading duration or wet surfaces along the warm sector, the thermal impact of shading reduces significantly. It is concluded that the shading effect may provide a pronounced contribution to frontogenesis for weak or moderate cold fronts.

Lagrangian particle modelling of buoyant point sources: Plume rise and entrapment under convective conditions

This paper describes several aspects of a Lagrangian particle model capable of simulating dispersion from buoyant point sources. The equations of Briggs (American Meteorological Society, 1975) are used to calculate the plume final rise heights, and a skewed homogeneous turbulence parameterization is used within the convective boundary layer. The homogeneous assumption enables an order of magnitude greater time step to be used than is normally the case, and was shown to have minimal effect on hourly averaged ground level concentrations by Hurley and Physick (Atmospheric Environment 25A, 1313–1325, 1991; 27A, 619–624, 1993). By including statistics from both ambient and source-induced (plume) turbulence in the probability density function (pdf) of the Langevin equation, we are able to apply this equation to particles in the plume as it rises from the stack to the final rise height. The model is used here to simulate various plume rise and entrapment laboratory experiments of Willis and Deardorff (Atmospheric Environment 17, 2435–2447, 1983; 21, 1725–1735, 1987) under convective conditions with a capping stable atmosphere. The simulations show that the model can reproduce the results of the laboratory experiments when a 15% enhancement to the entrainment parameter in the mean plume rise equations is used. Justification for this modification can be related to neglect of the effect of ambient turbulence upon entrainment in the plume rise equations, which in free convective turbulence may be significant.

An approach for estimating exposure to ambient concentrations.

The degree of certainty in epidemiological studies is probably limited more by estimates of exposure than by any other component. We present a methodology for computing daily pollutant concentration fields that reduces exposure uncertainty and bias by taking account of spatial variation in air quality. This approach, using elliptical influence functions, involves the optimum blending of observations from a monitoring network with gridded pollution fields predicted by the complex air quality model TAPM. Such fields allow more information to be incorporated in the exposure fields used in epidemiological studies, rather than having to assume that ambient exposure is the same across a whole city and/or that individuals remain at the one location for the duration of a study.

Formulation of the thermal internal boundary layer in a mesoscale model

Mesoscale models using a non-local K-scheme for parameterization of boundary-layer processes require an estimate of the planetary boundary layer (PBL) height zi at all times. In this paper, two-dimensional sea-breeze experiments are carried out to evaluate three different formulations for the advective contribution in the zi prognostic equation of Deardorff (1974).Poor representation of the thermal internal boundary layer in the sea breeze is obtained when zi is advected by the wind at level zi. However, significantly better results are produced if the mean PBL wind is used for the advecting velocity, or if zi is determined simply by checking for the first ‘sufficiently’ stable layer above the ground.A Lagrangian particle model is used to demonstrate the effect of each formulation on plume dispersion by the sea breeze.