AERMET's options: what they do & why you'd change them

AERMET's defaults are built for the standard case — a commissioned airport station with complete data. Every option it offers exists because some real data situation breaks that assumption: a site with no cloud observations, a sonic anemometer that never reports calm, a tower with gaps. This page goes option by option: what each does, when you'd use it, and what it means for the results. (The processing pipeline itself is on the AERMET page.)

The stable-boundary-layer pair

Stable, low-wind nights are where dispersion modeling is hardest — and where AERMET offers two documented alternatives to its default calculations (AERMET User's Guide, EPA-454/B-26-005, §3.7.7.4):

ADJ_U* — the low-wind friction-velocity adjustment. AERMET "adjusts the surface friction velocity (U*) under low wind/stable conditions based on Qian and Venkatram (2011)" and "may be used as a regulatory option … with NWS data or with site-specific data that does not include turbulence" (AERMOD User's Guide §3.2.3; see the AERMET page). This is the sanctioned response when calm-night hours drive a failing design value and the concern is the realism of the default stable-hour parameters. The boundary is sharp: pair ADJ_U* with site-specific turbulence measurements and it stops being a regulatory option — "the use of ADJ_U* and turbulence is subject to the alternative model provisions in Section 3.2 of Appendix W" (§3.7.7.4), meaning formal alternative-model approval.

BULKRN — the Bulk Richardson approach. For site-specific programs, a way to compute u* and θ* from temperature differences between two tower heights instead of cloud cover (§3.7.7.4; the tower must report the ΔT variable and its two heights). This is what makes an unattended tower viable: no observer, no cloud data — but two matched thermometers can carry the stable-hour energy balance. Appendix W asks for reviewing-authority consultation before relying on it (§8.4.4.2; see Site-specific towers & prognostic meteorology).

The ASOS wind adjustments

Two options exist purely because of how airport instruments report winds:

ASOS_ADJ (on by default). ASOS archives truncate wind speeds down to whole knots, a systematic low bias; AERMET adds "½ knot (0.26 m/s) to all ASOS-based wind speeds to compensate" (§3.7.7.3). Lower wind speeds mean higher modeled concentrations — so the truncation adjustment, by nudging speeds back up, removes a small artificial conservatism. NO_ADJ turns it off; there is rarely a reason to.

THRESH_1MIN — the sonic-anemometer calm threshold. Older cup anemometers stall below about 2 knots, so light hours reported as calm. Modern sonic sensors "have virtually no starting threshold," so AERMINUTE-derived hourly winds "will not include any calm hours" — including hours with wind too light to trust as a transport direction (§3.7.3). THRESH_1MIN sets a floor below which those hours are treated as calm. The number matters: 0.5 m/s matches "the minimum acceptable wind speed threshold for site-specific meteorological monitoring … under current EPA guidance" (§3.7.3); AERMET warns above 0.5 and refuses to run above 1.0, because every hour below the threshold becomes a calm hour AERMOD cannot model — set it too high and you are quietly deleting the light-wind hours that often control design values.

The substitution family: filling gaps honestly

REFLEVEL SUBNWS — the wind hierarchy. When enabled, AERMET picks each hour's wind from the best available source, in order: "1. ONSITE winds, 2. 1-min ASOS winds; then 3. standard SURFACE winds" (§3.7.7.2). For an NWS-only dataset the keyword is mandatory; for a tower dataset it is the difference between a gap becoming a substituted hour or a missing hour.

CCVR / TEMP substitution — bridging short gaps. AERMET can fill one- and two-hour gaps in cloud cover and temperature by linear interpolation, and the User's Guide explains why that is defensible and why it matters more than it sounds:

…gaps of one or two hours for these parameters near the early morning transition to a convective boundary layer may result in all convective hours for that day being missing since key parameters needed for the convective height processing are not calculated.

Source · AERMET User's Guide (EPA-454/B-26-005) §3.7.7.5.

One missing sunrise observation can erase an entire day's convective hours from the record — which then costs you data completeness, and with it, potentially the usability of the year. Two interpolated hours are the smaller compromise.

Upper-air substitution (new in 26135). AERMET can now read a second upper-air station and substitute its soundings when the primary station's are missing (UG §1.4) — the direct fix for sounding gaps that previously punched holes in a met record (see the AERMET page).

How to think about changing any of these

Every option above is documented in the Stage 2 report AERMET writes, so a reviewer sees exactly what was used. The practical discipline: change an option to match your data situation, not to chase a number — and record the reason in the modeling protocol. An option that happens to lower a design value (ADJ_U* on a calm-night-dominated record) is defensible when the data situation calls for it and agreed with the reviewing authority; the same option chosen because it lowers the number, without that grounding, is the kind of choice review processes exist to catch.

In PlumeSmart

Meteorology builds apply the regulatory defaults and record every option engaged — the processing report travels with the met dataset, so the options behind any surface file are inspectable long after the build.