P.C. Manins

Partial penetration of an elevated inversion layer by chimney plumes

Abstract Laboratory results and a theoretical model are described which quantify the apportionment of buoyant chimney plume material between that trapped and that which penetrates a sharp elevated atmospheric inversion layer under conditions of otherwise neutral lapse rate and uniform cross-wind. Some penetration should occur if P > 0.08, and when corrected for the effects of momentum overshoot of the plume and re-entrainment back into the plume of material already trapped in the inversion, the model predicts that the fraction trapped, f = 0.08P−1 − (P − 0.08), where 0 ≤ f ≤1 and P = F (Ub 1 z 1 2 ) is the penetration parameter. Here F is the buoyancy efflux of the chimney, U is the crosswind speed above chimney top, and b1 is the jump in buoyancy across the sharp inversion which is at a height z1 above chimney top. The model is shown to be a little conservative in estimating the fraction-trapped, and also predicts that the amount of plume material trapped per unit distance in the inversion layer is approximately independent of windspeed, provided only that partial penetration occurs. This is also supported by experiment.

Blocking in periodic valleys

Results are presented from a study of blocked flow (practically stagnant or recirculating light winds) in periodic valleys in thermally stably stratified ambient conditions. Inviscid and turbulent diffusion cases were modelled numerically to clarify the effects of turbulence on the blocking. The reflection of gravity waves from the top boundary of the hydrostatic model atmosphere was avoided by employing the radiation condition given by Klemp and Durran (1983). The dissipative numerical results are compared with new laboratory experiments which utilized the technique of Baines and Hoinka (1985) to simulate a semi-infinitely deep region.A criterion for the occurrence of blocked flow cannot be defined for the inviscid case except when the Froude number, Fr, based on the peak-to-trough ridge amplitude is less than about 0.4: then blocking is clearly identifiable before wave-breaking occurs. Breaking of waves is evident for Fr as large as 0.75, in agreement with analytical results given by Lilly and Klemp (1979).At small Froude number (Fr ≤ 0.5) in the dissipative flow simulations, blocked flow (stagnation) is present in the valleys, but a lee rotor (complete stagnation) is not evident. For order unity Froude numbers, blocking is a wave phenomenon, resulting from wave steepening and overturning or turbulent mixing. A finite thickness is brought to rest or participates in a recirculating flow when it first appears. A strong upward flow appears ahead of the rotor in the valleys, and the downslope wind over the windward side of the valleys is strengthened. Thus the present study shows that conditions for the onset of a rotor, and of stagnant flow, in periodic valleys are different.When blocked flow exists, the amplitudes of gravity waves in the upper layer are only 15% (Fr = 0.3) to 80% (Fr = 1.5) of those given by linear theory; this is supported by observations.

Vertical fluxes in katabatic flows

Katabatic flow is a dynamical process occurring on relatively calm, clear nights above sloping terrain. Its existence is dependent on long-wave radiative transfer, particularly radiative flux divergence within the air itself, for both its generation and (it is concluded here), along with advective warming, for much of its retardation.Utilising sounding data closely spaced in time, a discussion is presented of the importance of surface shear, interfacial shear, advective warming and radiative divergence in a strong katabatic flow. It is concluded that radiative divergence is important in generating static and dynamic instabilities in the flow. The role of radiative cooling in mixing of momentum has largely been ignored so far, and might explain why higher-order models tend to overestimate katabatic speeds on smooth slopes.

Cloud heights and stratospheric injections resulting from a thermonuclear war

Abstract Two consequences of a major thermonuclear war are the injection of fireball material into the atmosphere and the production of vast quantities of dense smoke from fires which are ignited by the blasts. A major concern for assessment of impact on the environment is the height reached by this material. Fireball rise data are presented and a model for the plume rise from large fires in standard ambient conditions is validated with available data. It is concluded that injection of bomb debris into the stratosphere at mid and high latitudes should take place for all explosions with yield greater than approx. 30 kt of TNT equivalent. At low latitudes yields greater than 1 Mt are evidently required. Thus most fireball material would reach into the stratosphere under recently postulated scenarios. Fires would require a power output of 1.5 × 10 7 MW at middle and higher latitudes and 8 × 10 7 MW at low latitudes for significant injection of smoke into the stratosphere in standard conditions. Study of possible fires ignited in a thermonuclear war in rural and urban areas suggests that smoke from rural fires would reach the tropopause but that significant injections into the stratosphere are unlikely. Conflagration of large, medium- and high-density city-centres would, it is predicted, result in much smoke reaching to the tropopause and into the lower stratosphere at higher but not at low latitudes.

APPLICATION OF COUPLED PROGNOSTIC WINDFIELD AND LAGRANGIAN DISPERSION MODELS FOR AIR QUALITY PURPOSES IN A REGION OF COASTAL TERRAIN

High levels of mining, industrial and commercial activities are located in the vicinity of Hunter Valley, which extends inland for more than 150 km from the eastern coastline of Auistralia. The Electricity Commision of New South Wales operates two coal- fired power stations(Bayswater and Liddell)located 3km apart at a distance of 100 km up the valley. The Commission is concerned to assess the worst -case air quality impact of these stations and to know what the impact of possible new stations in the region might be. In this paper, we present a summertime case study in which field measurements of winds and sulfurdioxide copncentrations are compared to simulation results from a state- of-the-art regional air quality model (Physick et. al., 1991)

CCl3F (Freon∗-11) as an indicator of transport processes in an urban atmosphere—a case study in Melbourne

Abstract In view of the lack of surface uniformity, and the importance of relatively uniform ground level area sources of pollution in an urban environment, point source diffusion experiments are of limited usefulness in studying transport processes in an urban atmosphere. While it is not feasible to carry out controlled area source diffusion experiments, we argue here that CCl 3 F is an existing pollutant which for many cities represents a good ground level area source. For cities such as Melbourne where industrial use of CCl 3 F occurs, the effect of a few strong point sources superimposed on the area sources may need to be accounted for. As an example of the use of CCl 3 F as an urban atmospheric tracer, vertical concentration profiles and simultaneous profiles of relevant meteorological parameters were obtained at Aspendale during a very stable June night. The profiles show a marked variation of concentration with height which is explained qualitatively by the change in wind direction with height and the variation with direction of the source strength (as determined by a source inventory) around Aspendale. The futility of estimating pollutant concentrations at height from ground level measurements is clear from our results, as is the importance of a knowledge of the non-uniformity of wind (both vertically and horizontally) due to local effects, such as cold air drainage.

Modelling wind fields in MAQS

Abstract MAQS is the Metropolitan Air Quality Study, a multi-million dollar scientific investigation of photochemical smog and fine particle pollution in the major urban and industrial area of New South Wales on the east coast of Australia. The study area included Newcastle/Hunter Valley region, the Sydney basin, and the Wollongong/Illawarra region. We have extended and applied the detailed prognostic numerical air pollution transport and dispersion model, LADM, to several simulations of local and inter-regional air pollution events for the MAQS region. The investigation highlighted (1) the large uncertainty in specifying surface synoptic meteorological data on high pollution days, (2) the ability of LADM to predict well the local winds and temperatures, even without assimilation of observational data into the model, (3) the need to incorporate observations of surface and upper air winds to give accurate air trajectories, (4) that days conducive to sea-breeze conditions are the key to the meteorology of high pollution in the MAQS region, and (5) that poor dispersion in the Sydney basin also implies inter-regional transport: from the Newcastle/Hunter Valley region to parts of Sydney, or from much of Sydney to the Illawarra.

An inexpensive sequential sampler for gas tracer studies

Abstract A simple, inexpensive automatic air sampler capable of taking five consecutive samples over an adjustable sampling period is described. The main application for the sampler is in the field of gas tracer studies. A feature of the unit is that the sampling period can be set precisely thus enabling concentration (rather than dosage) to be measured directly.

Meteorological Modeling in Sydney Australia: Case Study of a Smog Event

Australia’s large coastal cities experience the most intractable form of air pollution — photochemical smog characterised by the presence in the air of excessive concentrations of ozone gas, a respiratory irritant (Manins et al., 1994) — commonly exacerbated at coastal sites by sea breeze recirculation of pollutants.

Atmospheric Dispersion - Observations and Models: Commentary

Practical problems of atmospheric dispersion cover a vast range of scales and are usually much more complex than can be modelled at present. The principles that have been learnt from observations and model building in more ideal studies, such as described in the lecture by Jeff Weil on ‘Atmospheric Dispersion - Observations and Models’, need to be kept firmly in mind. Then significant solutions to practical problems are possible.