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F Modeling Studies of the Dispersion of Smoke Plumes from the World Trade Center Fires Summary of a presentation by Alan Huber, NOAA/APL and EPA/ORD The EPA and NOAA-Air Resources Laboratory had begun observational plume modeling studies in lower Manhattan, focusing on air pollution exposure assessment, in early 2001. For these studies, they developed a portable, battery-operated meteorological observing system that utilized a minisodar and a 10-m tower to obtain wind data, and they had collected several months of data from this system. They also compiled a digital model of the building topography of lower Manhattan. Following the September 11, 2001, terrorist attacks, the other applications of this work became immediately obvious. . The CALPUFF model (a Gaussian puff model) was used Pack the dispersion of emissions from the fires at the World Trade Center site for the period September 11 through December 8. They started with an initial assumption of the volume source, tracked the dilution of that source, and integrated the results over time to estimate possible exposure in surrounding neighborhoods. The meteorological data came from wind fields generated by CALMET (a diagnostic model) and from assimilation of surface meteorology and ARPS model data. The CALPUFF-CALMET system was found to perform fairly well and be valuable for making forecasts in a real-time mode. However, to complement this work, EPA is developing a much more sophisticated, finer-scale CFD model and will also be carrying out wind-tunnel physical modeling studies. The information from these dispersion modeling studies will be used to estimate the potential exposure of various populations around ground zero, providing input for epidemiological assessments of possible health impacts resulting from this exposure. Several important lessons were learned from this work: It is important to do routine meteorological observations and modeling in major cities so that you can develop an understanding of the local-scale flow features before an emergency event occurs. · Simple plume models are not sufficient for tracking dispersion in a dense urban area. Buildings and other aspects of the urban environment have a huge effect on flow and dispersion patterns. 85
86 APPENDIX F · Meteorological data collected from many standard observational sites intentionally located in open areas (such as those based at airports) do not necessarily represent conditions occurring in nearby urban areas. · The simple observation of which way the wind is blowing is very important for some purposes, for example, to tell people where they should be monitoring for possible exposure and health impacts.