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Understanding the USEPA’s AERMOD Modeling System for Environmental Managers

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## Understanding the USEPA’s AERMOD Modeling System for Environmental Managers

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Understanding the USEPA’s AERMOD Modeling System for Environmental Managers

Ashok Kumar

Abhilash Vijayan

Kanwar Siddharth Bhardwaj

University of Toledo

akumar@utnet.utoledo.edu

Meteorological Data

Meteorological Input Data Preprocessor

- Requires a preprocessor that organizes and processes meteorological data and estimates the necessary boundary layer parameters for dispersion calculations
- Uses AERMET as a preprocessor for this purpose

Type of Meteorological Data for AERMET

- Uses hourly-surface observations data, twice daily upper air soundings data, and onsite data
- Processes all available meteorological data or selected data in the specified input files
- Processes the available hourly surface observations and twice daily upper air soundings data in three stages

Three Stages for Processing Meteorological Data

- First Stage: Extracts meteorological data from the specified files and performs quality assessment checks
- Second Stage: Merges all 24-hour period data and saves in a separate file in the second stage
- Reads the merged meteorological data and estimates the necessary boundary layer parameters for use by AERMOD in the third stage

AERMET Output

- Development of two files:

* A file of hourly boundary layer parameter estimates, and

* a file of multiple-level observations of wind speed and direction, temperature, and standard deviation of the fluctuating components of the wind

- These files are available to AERMOD in an acceptable format.

Output Options

The basic types of printed output files available with AERMOD are:

- Summaries of high values (highest,second highest, etc.) by receptor for each averaging period and source group combination
- Summaries of overall maximum values ( for example, the max 50) for each averaging period and source group combination
- Tables of concurrent values summarized by receptor for each averaging period and source group combination
- These output may also be sent to an unformatted (binary) file

AERMET

- Calculates boundary layer parameters for use by AERMOD and generates profiles of the needed meteorological variables.
- Provides the following surface parameters:
- Surface heat flux, H
- Monin-Obukhov length, L
- Surface friction velocity, u*
- Surface roughness length, z0
- Convective scaling velocity, w*
- Convective mixed layer height, zic
- Mechanical mixed layer height, zim
- Stability of layer

- H > 0 convective layer

- H < 0 stable layer

Calculations for Surface Sensible Heat Flux using Observed Net Radiation

Where:

H = Sensible heat flux

Rn = Net radiation

Bo = Bowen ratio (an indicator for the available surface moisture)

- Note: Use of the energy balance to derive this equation.

Estimation of Net Radiation

If Rn is not available, use Holtslag and Van Ulden method:

use n=0.5 if no data available

Where:

Rn = Net radiation

Tref = Ambient air temperature at reference height for temperature

c1 = 5.31x10-13 W m-2 oK-6

c2 = 60 W m-2

c3 = 0.12

σSB = Stefan Boltzman Constant (5.67x10-8 W m-2 oK-4)

Albedo =r{Ф} = r´ + (1- r´)exp[a Ф + b]

Where: a = -0.1, b = -0.5 (1-r´)2

r´ = r{Ф = 900}

Ф = Solar elevation angle

Calculations for Solar Radiation

R= Ro (1-0.75n3.4)

Where:

R = Solar radiation

Ro = Clear sky insolation (W m-2)

n = Fractional cloud cover {0.0 – 1.0}

Ro = 990 sin Ф – 30

Where:

tp = previous hour

t = present hour

Ф = Solar elevation angle

Transition Point between CBL and SBL(day to night)

- Set Ro = 0
- Compute ФCritical
- Transition Point Ф = ФCritical
- General values of ФCritical
- Overcast conditions = 23o
- Clear and partly cloudy =13o

Friction Velocity

Where:

k= von Karman constant = 0.4

uref = wind speed at reference height

u* = friction velocity

zref = reference height for wind

zo = roughness height

L = Monin Obhukov length

Ψ = Stability term

Monin-Obukhov Length (L)

Where:

g = acceleration due to gravity

cp = specific heat of air at constant pressure

ρ= density of air

k = 0.4; von Karman’s constant

Tref = Reference Temperature of the surface layer

H = Sensible heat flux

Procedure:

Step 1: Calculate assuming neutral conditions (Ψ = 0).

Step 2: Calculate initial estimate of L.

Step 3: Recalculate using equations for u*, Ψm and L.

Step 4: Continue until the value of L changes by less than 1%.

Convective Velocity Scale

- Large turbulent eddies in the CBL have velocities proportional to the w*

Z ic is the connective mixing height

Convective mixing height (zic)

Where:

θ = Potential temperature

A = 0.2 (Deardorff, 1980)

t = Hour after sunrise

Note: Use of early morning potential temperature sounding ( prior to sunrise)

Time evolution of mechanical mixing height

βτ = 2.0 Note: u* = f (time)

Where: t + ∆t = current hour

t = previous hour

AERMOD MODEL

- Modeling system consists of two preprocessors and a dispersion model
- AERMET, The meteorological preprocessor
- AERMAP, The terrain preprocessor that characterizes the terrain, generates receptor grids and facilitates the generation of hill height scales
- Dispersion model AERMOD, uses meteorological data from AERMET and terrain as well as receptor data from AERMAP to produce output files

Friction Velocity in the SBL(cloud cover not available)

Solve by first assuming neutral condition ( θ*=0)

Sensible Heat Flux in SBL

- After finding the values of u* and *

Recompute U* if U* θ* > 0.05ms-1k

Complete L using U* and H

Monin-Obukhov Length

- The Monin Obukhov Length (L) is calculated from the equation given earlierusing the sensible heat flux given in the previous slideand u* from the equation.

- MECHANICAL MIXING HEIGHT (zim ) IN THE SBL
- The mixing height in the SBL results exclusively from mechanical (or shear induced) turbulence. The value of zim is calculated from the equation given earlier.

Vertical Profiles of Meteorological Variables

- Uses similarity relationships, with boundary layer parameters, measured meteorological data and other site specific information provided by AERMET to compute vertical profiles of
- Wind direction
- Wind speed
- Vertical potential temperature gradient
- Vertical turbulence
- Horizontal turbulence

Procedures for Computing Vertical Profiles

- Compares each height at which a meteorological variable must be calculated with the heights at which observations were made.
- If below the lowest measurement or above the highest measurement, the routines compute an appropriate value from selected PBL similarity profiling relationships.
- If data, available both above and below a given height, an interpolation is performed which is based on both the measured data and the shape of computed profile.

Vertical Wind Speed Profile

- At least one wind speed measurement in the surface layer is required for each simulation with AERMOD.
- The equation for wind speed is given below.

Stability Parameter Ψm for Vertical Wind Speed Profile in CBL and SBL

Note: For small z/L (<<1) Ψm =-5 z/L

Wind Directions Profiles

- Wind direction assumed to be constant with height both above the highest and below the lowest measurements for both the CBL & SBL
- Linear interpolation between measurements for intermediate heights

Profiles of the Potential Temperature Gradient

- Potential temperature gradient, an important factor for determining the potential for buoyant plume penetration into and above PBL
- Gradient in the stable interfacial layer just above the mixed layer is taken from morning temperature sounding

Potential Temperature for plume rise calculations

- Computes the potential temperature at the reference height for temperature (i.e., zTref) and from the reference temperature corrected to sea level pressure

Potential Temperature for CBL and SBL

Where is the average potential temperature gradient over the layer Δz

Note : For

Vertical turbulence calculations

- Equations for Vertical turbulence

Lateral turbulence

- Equations for lateral turbulence

AERMAP-Height Scale

- Assumptions in finding the Height scale
- The effect of surrounding terrain on the flow near the receptor decreases with increasing distance
- The effect increases with increasing elevation of that terrain

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