AERSURFACE: surface characteristics from land cover

The boundary-layer physics in AERMET runs on three site properties — surface roughness length, noontime albedo, and daytime Bowen ratio. AERSURFACE is EPA's tool for deriving all three from satellite land-cover data, so they are "realistic and reproducible" rather than hand-picked. Its User's Guide defines the three precisely:

The surface roughness length is related to the height of obstacles to the wind flow and is, in principle, the height at which the mean horizontal wind speed is zero based on a logarithmic profile. The surface roughness length influences the surface shear stress and is an important factor in determining the magnitude of mechanical turbulence and the stability of the boundary layer. The albedo is the fraction of total incident solar radiation reflected by the surface back to space without absorption. The Bowen ratio, an indicator of surface moisture, is the ratio of sensible heat flux to latent heat flux and, together with albedo and other meteorological observations, is used for determining planetary boundary layer parameters for convective conditions driven by the surface sensible heat flux.

Source · AERSURFACE User's Guide (EPA-454/B-26-007), version 26135 §1.0 — captured verbatim in the source library. (The Guideline's own footnote still cites the 2020 edition, EPA-454/B-20-008; the passages quoted here are identical in both, and B-26-007 is the edition EPA currently distributes.)

Its regulatory standing — recommended, not codified

AERSURFACE "is not a regulatory component of the AERMOD Modeling System as listed in Appendix A" (UG §1.1) — but the Guideline leaves little practical room to skip it:

The EPA recommends that the surface characteristics input to AERMET should be representative of the land cover in the vicinity of the meteorological data … Therefore, the model user should apply the latest version AERSURFACE, where applicable, for determining surface characteristics when processing measured land-based meteorological data through AERMET. In areas where it is not possible to use AERSURFACE output, surface characteristics can be determined using techniques that apply the same analysis as AERSURFACE.

Source · 40 CFR 51 App. W §8.4.2(b) — verified against the current eCFR (issue date 2026-08-01).

Note the subject of that sentence: the land cover at the meteorological tower, not at your facility. The characteristics describe the surface the measurements grew out of — that's what lets AERMET back out u* and H from what the tower saw. (For prognostic data the same paragraph directs use of the prognostic model's own grid-cell characteristics, and the AERSURFACE UG notes MMIF output already includes them.)

The data: NLCD at 30-meter resolution

The NLCD identifies the predominant land cover at a resolution of 30 x 30-meter grid cells. AERSURFACE assigns each land cover category within each 30 x 30-meter land cover grid cell seasonal values of albedo, Bowen ratio, and surface roughness. Temporally representative average values (e.g., annual, seasonal, or monthly) are calculated for the area of interest from the seasonal values.

Source · AERSURFACE UG (EPA-454/B-26-007) §2.1. Version 26135 processes "all current and future NLCD years available for download from the MRLC" (§1.2), with percent-impervious and percent-tree-canopy supplements where available — supplements must be concurrent with the land-cover year (§1.3).

Two different spatial domains — on purpose

The three characteristics are not averaged over the same footprint, because they act through different physics:

• Roughness: 1-km radius, direction-resolved, distance-weighted. "The default method in AERSURFACE calculates surface roughness length as an inverse distance weighted geometric mean, based on the land cover within the area around the meteorological tower out to a default fixed radial distance of a 1 kilometer (km) from the tower" (§2.4.1) — computed per user-defined wind sector, because what matters is the roughness of the upwind approach flow that shaped the measured wind. This default is the ZORAD method; the alternative ZOEFF internal-boundary-layer method "is considered research grade" (§1.3).
• Bowen ratio: 10 km × 10 km, unweighted geometric mean of the grid cells "centered on the measurement site … no distance or directional dependency" (§2.4.2).
• Albedo: the same 10 km × 10 km area, unweighted arithmetic mean (§2.4.3).

Monthly values come first; seasonal or annual values are arithmetic means of the months per the control-file settings (§2.4.2–2.4.3).

Seasons are land-surface states, not calendar quarters

AERSURFACE's five "seasons" (Table 2-3) describe what the ground looks like, with default month assignments the user can re-map to the site's climate:

Source · AERSURFACE UG (EPA-454/B-26-007) Table 2-3.

The two winter rows are the point: snow cover flips a surface's albedo and roughness character, so "winter" alone is ambiguous — the user tells AERSURFACE which winter the site actually has.

One more judgment call the tool exposes: ambiguous "Developed" land-cover categories can be treated as airport (runways, paved lots — lower roughness) or non-airport (buildings — higher roughness), decided per sector "by the predominant land use within a kilometer radius of the meteorological tower, giving more weight to land use nearest the tower" (§2.3.2).

In PlumeSmart

When the platform builds meteorology, surface characteristics are derived with AERSURFACE's method at the meteorological station — sectors, seasons, and NLCD vintage recorded — and the resulting values travel with the met dataset so a reviewer can trace every z₀, Bowen ratio, and albedo to its land-cover basis.