Modeled Wind Fields

The Vermont Air Division Planning Section has generated sets of meteorological data in support of the dispersion modeling efforts necessary to evaluate transport of air pollutants. To date, MESOPAC and CALMET are the meteorological models that have been utilized. Since 1998, when the Interagency Workgroup on Air Quality Modeling Phase II (IWAQM) recommendations were published, CALMET in particular has been utilized to produce spatially varying windfields necessary to evaluate pollutant dispersion in complex terrain or long range transport cases. CALMET is the meteorological processor used within the CALPUFF modeling system. Both CALMET and CALPUFF are flexible tools which offer many options for use, although production of any windfields with spatial variation is an intensive effort within this system. The CALPUFF modeling system may be utilized for most modeling applications except reactive toxic pollutants. In particular, the CALPUFF modeling system is a good tool for regulatory evaluation of sulfur compounds in long range transport and complex terrain scenarios.

CALMET has its own physics and interpolation schemes to produce a windfield from national weather service (NWS), data measurements. Additionally, CALMET can ingest windfields produced by another model, usually a quasi-geostrophic forecasting tool that can better predict flow patterns in the atmosphere above the surface layer. The Vermont APCD Planning Section has used CALMET to produce windfields with both the approaches described above for various applications. In producing these windfields, every effort has been taken to follow EPA guidance for PSD permit review detailing proper usage of CALMET. Because EPA guidance requires several years of meteorological windfields for regulatory analysis, multiple years of windfields have been produced and an evaluation of multiple year variation occurred. To date, the planning section relies on windfields produced for two domains, one for local scale flow and one for long range transport.

The local scale domain has been set up at two kilometer horizontal resolution over a geographic region encompassing a triangle of upper air radiosonde sites in Montreal, QE, Albany, NY, and Portland, ME. For this domain, a denser network of surface observations is desirable. Consequently windfield data sets have been produced for years when the NWS automated surface observing station (ASOS), network has been in place - 1998 through 2000. Studies have been conducted evaluating windfield accuracy by comparing modeled to measured wind vectors for the 2 km resolution domain. The results of this study compare favorably in the surface layer to similar results for the Regional Atmospheric Modeling System (RAMS) modeling system run at 3 km resolution. They also compare favorably to similar results when CALMET was run at a 0.2 km. resolution. Model validation studies for these high resolution domains provide some insight into the CALMET's dynamic handling of terrain effects, especially in the surface layer of the atmosphere. Model validation methods have also been devised to determine the best CALMET option settings to produce windfields in the surface layer.

A second, more regional domain has been established so the CALMET Model will produce windfields over an area extending westward to 84 degrees longitude and southward to 37 degrees latitude and north and eastward to encompass the entire state of Maine in which the windfield is resolved at a 40 kilometer resolution in the horizontal grid. In the vertical, 8 levels are specified to discern meteorological differences in surface level, transition level, and free atmospheric conditions. This domain is represented in a Lambert - Conformal projection because of its large size. The large dimensions of this domain are necessary for any multiple source analysis required in assessing impacts at Federal Class I areas in permit reviews. The domain was established at the 40 kilometer resolution to allow reasonable computational times when running the CALMET and CALPUFF model for long time periods. Usage of the windfields computed for this domain are acceptable for transport scenarios which occur above the surface layers, or, as defined by the EPA, long range transport of greater than 50 kilometers. For the CALMET runs the geographical processing to produce terrain heights and land use represented in the model was done using the programs available as recommended by EPA.

For the regional CALMET domain meteorological data consists of 48 surface stations, 11 radiosonde stations, and 474 precipitation sites. This data has been processed for the years 1990 to1995. In addition, for the year 1995 buoy and hourly ozone data was included. Most recently, MM5 (Mescoscale Model) data for the year 1992 has been processed as well, providing comparative meteorological data sets for the year 1992.

Last Updated: 2/25/03