The boundary layer & the parameters that drive dispersion
AERMOD never reads raw weather data. Between the observations and the model sits AERMET, which converts winds, temperatures, cloud cover, and a morning balloon sounding into a small set of boundary-layer parameters — and those parameters, not the weather itself, are what the model disperses plumes with. This page is a lesson on what each parameter physically means. The companion pages cover the preprocessors that produce them.
The planetary boundary layer
The EPA's model formulation document defines the arena:
In most air quality applications, one is concerned with dispersion in the PBL, the turbulent air layer next to the earth's surface that is controlled by the surface heating and friction, and the overlying stratification. The PBL typically ranges from a few hundred meters in depth at night to 1 - 2 km during the day.
Source · AERMOD Model Formulation (EPA-454/B-26-003) §2 — captured verbatim in the source library.
Two forces make this layer turbulent: heating (the sun warms the ground, the ground warms the air, warm air rises as thermals) and friction (wind dragging over a rough surface generates mechanical eddies). The daily rhythm of the first force splits the modeling day in two:
…due to the diurnal cycle of surface heating and cooling, the atmosphere is generally convective during daytime hours and stable during the nighttime.
Source · EPA-454/B-26-003 §2.
The dividing line between the two regimes is exactly one number — the sign of the surface heat flux: AERMET "defines the stability of the PBL by the sign of H (convective for H > 0 and stable for H < 0)" (§3). The two regimes give a plume very different atmospheres to disperse into: a daytime layer 1–2 km deep stirred by thermals, versus a nighttime layer a few hundred meters deep whose only turbulence is shear-driven.
The parameter tour
The formulation document names the complete hand-off:
The surface parameters provided by AERMET are the Monin-Obukhov Length (L), surface friction velocity (u*), surface roughness length (zo), surface heat flux (H), and the convective scaling velocity (w*). AERMET also provides estimates of the convective and mechanical mixed layer heights, zic and zim, respectively.
Source · EPA-454/B-26-003 §3.
What each one means physically:
| Parameter | What it is | Why dispersion cares |
|---|---|---|
| H — sensible heat flux (W/m²) | Heat the surface pumps into the air | Its sign sets the stability regime (§3); its magnitude drives w* and the convective mixing height |
| u* — friction velocity (m/s) | The velocity scale of mechanical (shear-induced) turbulence, computed from wind speed, roughness length, and stability (§3.2.1, eq. 6) | The mechanical part of vertical turbulence scales directly on it (σ<sub>wml</sub> = 1.3 u* at the surface, §4 eq. 37), and so does the mechanical mixing height (§3.4.2) |
| w* — convective velocity scale (m/s) | "the large turbulent eddies in the CBL have velocities proportional to the convective velocity scale (w*)" (§3.2.2) | It is how AERMOD characterizes "the convective portion of the turbulence in the CBL" (§3.2.2) |
| L — Monin-Obukhov length (m) | A stability scale built from u* and H: L = −ρc<sub>p</sub>T·u*³/(kgH) (§3.2.1, eq. 8) — negative in convective hours, positive in stable hours | The stability corrections in the wind-profile relations are functions of z/L (eq. 6–7) — L is where "how stable is this hour" enters the math |
| z<sub>ic</sub>, z<sub>im</sub> — convective & mechanical mixing heights (m) | The lid on vertical mixing (§3.4) | Sets how deep a layer emissions are diluted into; the CBL lid is the larger of the two so early-morning mechanical mixing is not underestimated (§3.4.2) |
| θ* — temperature scale (K) | Scaling for the near-surface temperature profile (§3.3) | Used with u* to compute the stable-hour heat flux: H = −ρc<sub>p</sub>u*θ* (§3.3.3, eq. 20) |
Because the mixing lid has two different generators, it is assembled regime-by-regime:
The mixing height (zi) in the CBL depends on both mechanical and convective processes and is assumed to be the larger of a mechanical mixing height (zim) and a convective mixing height (zic). Whereas, in the SBL, the mixing height results exclusively from mechanical (or shear induced) turbulence and therefore is identically equal to zim.
Source · EPA-454/B-26-003 §3.4.
Where the surface characteristics come in
Three land-surface properties shape everything above, which is why they are inputs to AERMET rather than afterthoughts:
The depth of this layer and the dispersion of pollutants within it are influenced on a local scale by surface characteristics such as surface roughness, reflectivity (albedo), and the availability of surface moisture.
Source · EPA-454/B-26-003 §3.
- Roughness length z₀ — how aerodynamically rough the surface is (forest vs. water). Directly sets how much mechanical turbulence a given wind produces, hence u*.
- Albedo — how much sunlight the surface reflects instead of absorbing. Feeds the net-radiation estimate behind H.
- Bowen ratio — how the surface splits its available energy between heating the air and evaporating water. In AERMET's daytime energy balance the sensible heat flux is H = 0.9·Rₙ / (1 + 1/B₀) (§3.1, eq. 2): a wet surface (low B₀) spends its energy on evaporation and produces weak thermals; a dry one (high B₀) heats the air aggressively.
These three values are exactly what AERSURFACE derives from land-cover data — covered on its own page.
From parameters to model-ready profiles
The hand-off in the formulation document's own words:
Surface characteristics in the form of albedo, surface roughness, and Bowen ratio, plus standard meteorological observations (wind speed, wind direction, temperature, and cloud cover), are input to AERMET. AERMET then calculates the PBL parameters: friction velocity (u*), Monin-Obukhov length (L), convective velocity scale (w*), temperature scale (θ*), mixing height (zi), and surface heat flux (H). These parameters are then passed to the INTERFACE (which is within AERMOD) where similarity expressions (in conjunction with measurements) are used to calculate vertical profiles of wind speed (u), lateral and vertical turbulent fluctuations (σv, σw), potential temperature gradient (dθ/dz), and potential temperature (θ).
Source · EPA-454/B-26-003 §2 (Figure 1 narrative).
This is the payoff: with the PBL characterized by a handful of scaling parameters, AERMOD can reconstruct wind, turbulence, and temperature at any height a plume reaches — not just at the 10-meter anemometer. And the dispersion physics keyed to the two regimes differs fundamentally:
In the stable boundary layer (SBL), it assumes the concentration distribution to be Gaussian in both the vertical and horizontal. In the convective boundary layer (CBL), the horizontal distribution is also assumed to be Gaussian, but the vertical distribution is described with a bi-Gaussian probability density function (pdf).
Source · EPA-454/B-26-003 §2. The same section covers CBL "plume lofting" and penetration of the elevated stable layer — see How AERMOD models dispersion.
What observations does all this require?
Remarkably few — which is what makes NWS-data-driven modeling possible:
AERMOD requires only a single surface measurement of wind speed (measured between 7zo and 100m – where zo is the surface roughness height), wind direction and ambient temperature. AERMOD also needs observed cloud cover. … A full morning upper air sounding (rawinsonde) is required to calculate the convective mixing height throughout the day. Surface characteristics (surface roughness, Bowen ratio, and albedo) are also needed to construct similarity profiles of the relevant PBL parameters.
Source · EPA-454/B-26-003 §2.
Note the two easily-missed requirements: cloud cover (it drives the net-radiation estimate behind H) and the morning sounding (the day's convective mixing height is grown from it). This is why AERMET always pairs a surface station with an upper-air station — and why the pairing itself is a representativeness decision (see Meteorological data & representativeness).
In PlumeSmart
The met workspace produces AERMET-processed surface and profile files and exposes the record behind them — station pairing, years of record, and completeness — so the parameters described here enter a run already in the form the model requires.