Roger W. Brode U.S. EPA/OAQPS/AQAD Air Quality Modeling Group AERMAP Training NESCAUM Permit Modeling Committee Annual Meeting New London, Connecticut May 31, 2007

Presentation Outline AERMOD Terrain Algorithm Types of Terrain Data Terrain Processing Issues Recent AERMAP Updates Unresolved Issues/Planned Updates

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

AERMOD Terrain Treatment

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:

AERMOD Terrain Treatment

AERMOD Hill Height Scale

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

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

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 spacing – Uniform horizontal datum (NAD83) – Currently not supported directly by AERMAP, but NED data may be convertible to DEM-equivalent data for input to AERMAP (needs investigation)

1-Degree DEM Data

7.5-Minute DEM Data

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

Inconsistent Reference Datums

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

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

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

Recent AERMAP Update: Bug Fixes Significant changes (overhaul) of datum (NAD) conversion process – Previous version did not account for shift due to UTM-Lat/Lon conversion – Error typically about 200m in Northing coordinate – Incorporated NAD conversion for DOMAIN extent – Clarified and simplified treatment of DEM file gaps Corrected bug resulting in erroneous elevation for first receptor or source Corrected bug with method to optimize hill height scale calculation by skipping adjacent DEM files

Recent AERMAP Update: Enhancements Enhanced debug output file options with additional user control Optimized runtime by using limited “check” on DEM file structure – added optional parameter to perform full check Additional optimizations for calculating critical hill height scales Added “Input Options Summary” page, similar to AERMOD

Unresolved AERMAP Issues/ Planned Updates Resolve issues with portability of code to Linux Problems with AERMAP for Alaska DEM data Cross UTM zone bug Allocatable arrays Modify AERMAP to support National Elevation Dataset (NED)?

Questions