1. Roger W. Brode & James Thurman U.S. EPA/OAQPS/AQAD Air Quality Modeling Group AERMAP Training NESCAUM Permit Modeling Committee Annual Meeting Mystic, Connecticut June 24, 2009

2. Presentation Outline AERMOD Terrain Algorithm Types of Terrain Data Terrain Processing Issues Recent AERMAP Updates

3. AERMOD Terrain Treatment Uses CDTMPLUS dividing streamline height concept – Portion of plume above dividing streamline goes over the hill (terrain-responding plume) – Portion of plume below dividing streamline impacts or goes around hill (horizontal plume) Requires representative “Hill Height Scale” for each receptor AERMAP determines “Hill Height Scale” based on highest terrain above a 10:1 slope

4. AERMOD Terrain Treatment

5. u H crit KE < PE KE > PE /Z/Z h hill Critical dividing streamline height, H crit, defined as streamline at which kinetic energy (KE) and potential energy (PE) balance, i.e. Froude number = 1:

6. AERMOD Terrain Treatment

7. AERMOD Hill Height Scale

8. AERMAP Receptor Elevations Based on terrain formulation, AERMAP receptor elevation should reflect “best estimate” of actual elevation at receptor location – AERMAP uses simple 2-dimensional, bilinear interpolation based on four closest terrain “nodes” ISCST3 method of using maximum elevation within “grid cell” containing receptor not appropriate, may not be conservative Focus should be on using adequate receptor resolution to capture significant terrain features

9. Types of Terrain Data 1-Degree Digital Elevation Model (DEM) Data – USGS files cover 1-degree latitude by 1-degree longitude domain – one-half of 1:250,000 scale topographic map – Terrain data nodes spaced every 3 arc-seconds (approximately 90 meters) 7.5-Minute DEM Data – USGS files cover 7.5-minute by 7.5-minute domain – one 1:24,000 scale topographic map – Terrain data nodes spaced every 30 meters or every 10 meters

10. Types of Terrain Data 15-Minute DEM Data – Available for Alaska – spacing varies with latitude National Elevation Dataset (NED) – Seamless dataset for full U.S. – Uniform 30-meter (1-arcsecond) or 10-meter (1/3 rd - arcsecond) spacing – Uniform horizontal datum (NAD83) – Supported by AERMAP (version 09040)

11. 1-Degree DEM Data

12. 7.5-Minute DEM Data

13. National Elevation Dataset (NED)

14. Terrain Processing Issues Horizontal reference datum conversions – NAD27 vs. NAD83 reference datum – Use of ANCHORXY keyword to specify reference datum of source and receptor coordinates Use of DEM data with inconsistent reference datums – May result in gaps between adjacent DEM files or overlapping DEM files

15. Inconsistent Reference Datums

16. Terrain Processing Issues Suggestions for unknown reference datum – First try to ascertain reference datum for sources – If coordinates are from previous application that dates back several years (mid-1990’s or earlier) or have been derived from printed USGS 7.5-minute (1:24,000 scale) topographic maps, then best to assume NAD27 as the reference datum – If coordinates are from a recent application and are likely to have been derived from a global positioning system (GPS) device or a geographic information system (GIS), then the reference datum is more likely to be NAD83

17. Terrain Processing Issues Quality/accuracy of DEM data – Accuracy of DEM data should not be accepted blindly – some independent verification of important elevations should be made – Cases of 7.5-minute DEM elevations reflecting tops of buildings and/or trees – More recent 10-meter resolution 7.5-minute data appears to be more reliable – Data quality of NED data may also be more reliable since it is continually updated

18. Terrain Processing Issues Use of AERMAP for source elevations – Accuracy of DEM elevations generally more important for source elevations, so caution should be used – Relative elevations between sources and buildings and between sources and nearby receptors are most important – discrepancies in relative elevations could have significant impact on results – May be better to use plant survey data or topographic maps for source elevations to avoid potential problems – In any case, method used to estimate elevations should be clearly documented

19. Recent AERMAP Update: Version 09040 Fixed problems with processing AK DEM files (1-deg, 15-min and 7.5 min data; non-uniform longitude spacing of nodes Support for National Elevation Dataset (NED), available from USGS Seamless Data Server in GeoTIFF format Allow “mixed” DEM files (1-deg, 7.5-min, and 15-min for AK); can be used to fill gaps in 7.5-min coverage, such as over water grids; however, no support for mixed DEM and NED data Domain Keywords (DOMAINXY/DOMAINLL) are now optional; will use all available data if omitted Support for INCLUDED keyword on RE and SO pathways Improved handling and reporting of non-standard (non-USGS) terrain data, including identification of receptors located in “gaps” between terrain files or inside terrain files

20. Recent AERMAP Update Continued Allocatable array storage at runtime Modified to use standard convention of negative for West longitude – Simplifies code structure and inputs – Consistency with other tools (e.g., AERSURFACE) – Improves applicability beyond the U.S. – Code included to convert DOMAINLL inputs to new convention for backward compatibility Update to AERMOD Implementation Guide with new sections to discuss issues regarding sources of terrain data (DEM, NED, SRTM, etc.) and use of AERMAP to estimate source elevations, posted on SCRAM March 19, 2009

21. Example Cross UTM zone Gaps and overlapping files due to use of non-standard DEM files

23. UTM zones

24. Receptor in gap between overlapping files File 1File 2 Receptor

25. Receptor in gap between overlapping files Gap Overlap Receptor Preliminary elevation: 2059 m When gap found for file #1 Final elevation: 2060.7 m Using data from file #2 File 1 File 2

26. Non-standard file

27. Standard file

28. Non-standard DEM files

29. Questions