meteorological site representativeness and aersurface issues

Meteorological Site Representativeness and AERSURFACE Issues

Margaret ValisLeon Sedefian

Outline

• AERSURFACE Review• Met Site Representativeness Case Study• AERSURFACE Issues• Summary

Review• AERMOD designed to accept same meteorological data

as ISCST3: NWS surface and upper air data

• AERMOD also designed to accept more robust on-site meteorological data, including multi-level profiles of wind, temperature and turbulence– However, more advanced boundary layer algorithms in AERMOD

require user-specified surface characteristics:AlbedoBowen ratioSurface roughness

• Sensitivity to surface characteristics is one of the main implementation issues with AERMOD

Review• AERSURFACE is a tool designed to assist with

determining surface characteristics data (albedo, Bowen ratio & surface roughness) for use in AERMET and/or AERSCREEN

• Initial version of AERSURFACE was released on SCRAM on January 11, 2008

• AERSURFACE is not currently considered part of the AERMOD regulatory modeling system

AERSURFACE(08009) Design– Revisions to calculating area-weighted averages for surface

roughness: Inverse-distance weighting to adjust for increased width/area of sector with distance

– Revisions to calculation methods: Averaging ln(zo) for surface roughness (= geometric mean)

– Revisions to default domain/distances:1km radius for roughness

– Geometric mean for Bowen ratio, arithmetic mean for albedo

– 10x10km domain for Bowen ratio and albedo –no sector or distance dependency

Notes from AERMOD Implementation Guide (dated Jan. 9, 2008)

• The degree to which predicted pollutant concentrations are influenced by surface parameter differences between the application site and the meteorological measurement site depends on the nature of the application (i.e., release height, plume buoyancy, terrain influences, downwash considerations, design metric, etc.).

• If the reviewing agency is uncertain as to the representativeness of a meteorological measurement site, a site-specific sensitivity analysis may be needed in order to quantify, in terms of expected changes in the design concentration, the significance of the differences in each of the surface characteristics.

Representativeness Case Study:

• Nearest NWS station is Albany,located about 25 km north of thefacility

• Figure shows 10x10km domain forBowen ratio and albedo, and 1kmradius for roughness

• Aerial photos and 1992 NLCD plotsfor each site are also provided

• Estimates of surface roughness,albedo and Bowen ratio based onAERSURFACE are provided for1992 and 2001 NLCD

AERSURFACE Roughness Estimates for ALB Airport and Case Study Facility

0

0.2

0.4

0.6

0.8

1

1 2 3 4 5 6 7 8 9 10 11 12

Sector

Sur

face

Rou

ghne

ss (m

)

ALBFacility

AERSURFACE Weighting Issue

• If AERSURFACE (08009) is run for a location that is very close to the center of aland cover cell, the sector average Zo may be skewed if Zo for the land covercategory of this cell is significantly different than the other land cover typeswithin the sector (e.g. forest versus grasslands) due to inverse distanceweighting; will be addressed in next version.

Met tower location:+Land cover cell center:+

Comparison of Zo for Different Sector Choices

Season

Direction 0-30 30-60 60-90 90-120 120-150 150-180 180-210 210-240 240-270 270-300 300-330 330-360

Zo (12 sect) 0.146 0.141 0.131 0.125 0.194 0.189 0.224 0.028 0.155 0.154 0.167 0.194Zo(3 sect)

Season

Direction 0-30 30-60 60-90 90-120 120-150 150-180 180-210 210-240 240-270 270-300 300-330 330-360

Zo (12 sect) 0.17 0.185 0.187 0.176 0.25 0.214 0.268 0.055 0.177 0.191 0.206 0.232Zo(3 sect)

Season

Direction 0-30 30-60 60-90 90-120 120-150 150-180 180-210 210-240 240-270 270-300 300-330 330-360

Zo (12 sect) 0.192 0.213 0.213 0.226 0.285 0.231 0.288 0.066 0.187 0.206 0.224 0.255Zo(3 sect)

Season

Direction 0-30 30-60 60-90 90-120 120-150 150-180 180-210 210-240 240-270 270-300 300-330 330-360

Zo (12 sect) 0.189 0.204 0.199 0.219 0.279 0.229 0.282 0.056 0.183 0.199 0.215 0.249Zo(3 sect) 0.113 0.219

0.225 0.124 0.227

0.195

0.218

WINTER

SPRING

AUTUMN

SUMMER

0.152 0.72 0.171

0.11 0.209

Potential AERSURFACE Issues

• Uncertainties regarding ASOS met station locations –a key AERSURFACE input

– Use of erroneous station locations in AERSURFACE could invalidate results

• NLCD categories (both 1992 and 2001) are not ideal for estimating roughness at airports

– 1992 Commercial/Industrial/Transportation category includes runways, as well as terminal buildings and parking garages; spans full range of roughness values

– 2001 “Developed” categories also cover runways to buildings, plus grassy areas around runways (“Developed, Open Space” in 2001, formerly “Urban/Recreational Grasses” in 1992)

– Recent 1992 vs. 2001 comparisons also highlight that 2001 developed categories do not reflect presence of trees that were accounted for in 1992 NLCD

AERSURFACE Land Use Misclassification Issue

Low Density Residential (pink) found in area between runway

Summary• Should consider the issue of met data

representativeness in the full context of a project and not as just a “roughness length” issue.

• The profiling of wind, temperature and turbulence from met site is a connected, but separate issue. Each land use parameter should be reviewed to determine its appropriateness.

• Blind application of AERSURFACE/land use data can lead to errors not detected otherwise.