8A.7
The impact of data assimilation on modeling the inter-regional transport of smoke into the Sydney metropolitan area from prescribed burns
Milton S. Speer, Bureau of Meteorology, Sydney, Australia; and L. M. Leslie and R. Bunker
The main objective of the study was to model the trajectory of air that resulted in a sustained pollution episode over the Sydney metropolitan area. Smoke was transported over inner Sydney from a prescribed burn early in the study period (11-14 April, 1997). Then, later in the period over a major highway just to the southwest of Sydney also from a prescribed burn.
The methodology initially involved replicating the operational procedure as closely as possible by performing a series of high resolution model experiments using a nesting strategy with a mesoscale numerical weather prediction (NWP) model developed by the second author (LML).
It was found that without the addition of observations of surface and low level real-time observations of wind speed and direction near the burn that occurred early in the period, the model boundary conditions from the coarse mesh influenced the forecast of wind speed and direction quickly after initialisation producing a trajectory which was away from the Sydney metropolitan area instead of towards it. However, when the model was re-initialised with additional surface and low level observations in the vicinity of the fire using a data assimilation methodology, there was a dramatic improvement in the wind forecasts.
For the second event which occurred late in the period the model accurately predicted a wind change that transported a combination of smoke and fog over a highway at about the critical time when a multiple vehicle pile-up occurred in extremely poor visibility due smoke and fog. In this case there was no need for additional data to be included in the assimilation since the wind change was induced by a large scale synoptic wind change moving across southeastern Australia and these large scale conditions were successfully captured in the initialisation without extra data as will be shown from the model results. In fact there was very little gain in the forecast timing of the wind change between a model forecast run at 5 km horizontal resolution compared to one at 15 km.
The main conclusion from the results of this study is that in a very light wind regime such as occurred early in the study period the trajectory of air is strongly influenced by local diurnal circulations and hence high resolution forecasts benefit very much by including extra data in the assimilation prior to model initialisation. When the main influence locally is from the large scale conditions as occurred with the wind change late in the period then there is little or no benefit to be gained from increasing the horizontal resolution passed a certain point (in this case 15 km).
The model will become increasingly sophisticated as more components are added. For example, the absorption, chemical change, wet/dry deposition, rainout, re-suspension etc. that is associated with the major outdoor air pollutants in Sydney (particulates, ozone, nitrogen) will form part of a major chemistry component of the model targeted for progressive implementation of real-time prediction of air pollution transport and dispersion in the metropolitan air sheds of Sydney, Los Angeles and Beijing.
Session 8A, The urban environment-meteorology, dispersion, and air quality (Parallel with Session 8B)
Tuesday, 11 January 2000, 4:00 PM-5:45 PM
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