Adaptation and Application of the CMAQ 

Modeling System for Real-time Air Quality 

Forecasting During the Summer of 2004

R. Mathur, J. Pleim, T. Otte, K. Schere, J. Young, G. Pouliot, B. Eder

Atmospheric Sciences Modeling Division, ARL/NOAA, NERL/U.S. EPA

D. Kang, S. Yu, H.-M. Lin

Science and Technology Corporation

J. McQueen

National Centers for Environmental Prediction

P. Lee, M. Tsidulko

Science Applications International Corporation

D. Wong

Lockheed Martin Information Technology

Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy
Eta-CMAQ AQF System

Eta-12

CMAQ

Eta Post

PRDGEN

PREMAQ

AQF Post

Verification Tools

Vertical interpolation from eta to sigma

Horizontal interpolation to Lambert grid

CMAQ-ready meteorology and emissions

Gridded ozone files for users

Chemistry model

Meteorology model

Performance feedback for users and developers
CMAQ Configuration

• Structural

– Netcdf replaced with binary IOAPI

• Advection

– Horizontal: Piecewise Parabolic Method

– Vertical: Upstream with rediagnosed vertical velocity 

to satisfy mass conservation

• Turbulent Mixing

– K-theory; PBL height from Eta

– New scheme for specification of minimum Kz
CMAQ Configuration (contd.)

• Gas phase chemistry

– CB4 mechanism with EBI solver

• Cloud Processes

– Mixing and aqueous chemistry: following the scheme in RADM 

• Deposition

– Dry : M3dry modified to use Eta land surface parameters

– Wet

• Aerosols

– 2004 release version
268 grid cells

259

grid

cells

Northeast

“1x” Domain

East “3x” Domain

CMAQ Modeling Domains

Ozone forecasts on 3x and 1x Experimental PM forecasts on 3x
Lateral Boundary Condition Specification

A key uncertainty in long term modeling over 

limited area domains

– Determines “model background”

• Default profiles

– “Clean” tropospheric background values

– Used in 1x

• Seasonal Profiles

– Derived from continental CMAQ simulations for 2001

– Used in 3x
Lateral Boundary Conditions (contd.)

• Ozone profiles from NCEP’s Global Forecast 

System (GFS)

– O3is a 3-d prognostic variable

– Initialized with Solar Backscatter Ultra-Violet (SBUV2) satellite observations

– Motivation

• Simulating varying dynamical conditions 

• Improve model representation of O3in the free 

troposphere

– Effects associated with intrusions

– Study FT-BL exchange mechanisms
Specification of Minimum Kz

• Minimum value of Kz allowed to vary spatially 

depending on urban fraction (furban)

• Kz = 0.1 m2/s, furban = 0

• Kz = 2.0 m2/s, furban = 1

– allows min. Kzin rural areas to fall off to lower values 

than urban regions during night-time; mimics urban 

heat island effects

– prevents precursor concentrations (e.g., CO, NOx) in 

urban areas from becoming too large at night 

– lower Kz(and reduced mixing intensity) in non-urban 

areas results in increased night-time O3 titration

• Helps reduce night time over predictions of ozone “regionally”
Summer 2004: Atypical Ozone Season

July 21

Aug. 12

Source: EPA AIRNOW
Model Performance Characteristics: Summer 2004
Bias

Effects of GFS Ozone and Cloud Mixing

O3

Vertical profile Cloud top
GFS Sensitivity Simulations

• Limit GFS use to above a specified altitude

– 6 km

– 10 km

• Motivation: to limit the use of GFS derived O3

profiles to the upper levels of the model, where 

there is greatest confidence in GFS predictions 

and to avoid abnormally high O3 within the 

boundary layer (noticed in early parts of May)

• Default BC (without GFS)
Target Day Stats: May 18, 2004
Impact of GFS 6km and min. Kz change

1x domain

Solid lines-with changes Dashed lines-without changes

Implemented in 1x domain on 7/20/04

Hourly 1 Hr. Max 8 Hr. Max
Comparison of 3x and 1x Performance

At sites within the 1x domain
Comparison of 3x and 1x 

Lateral Boundary Conditions

Solid lines (3x); filled circles and dash line (1x)

CMAQ 2001 Performance

at 3x Western Boundary

Solid lines: seasonal BCs

Dash/dots: Default profile
“Switch-Off” top-down cloud mixing

Tropopause limit

on cloud top

Diagnostic Tests: Cloud Mixing
Diagnostic Tests: Cloud Mixing

Target Day Stats: May 18, 2004

FSIDE: switch-off top-down mixing

TROPLIM: tropopause limit on cloud top
Diagnostic Tests: Cloud effects on Photolysis

O3 (ppb)

Cloud Fraction: Current (average)

Cloud Fraction: Modified (max)
Scale by Radiation reaching the surface

1-(J/Jclear) 

Photolysis Attenuation

August 12, 2004
Effects of Cloud Process Modifications

Maximum Reductions in O3

August 12, 2004

Below cloud attenuation based on

radiation

Below cloud attenuation based on

Radiation + switch-off top-down mixing+

New CFRAC
Effects of Cloud Process Modifications

August 12, 2004

rad_atten: cloud mixing + photolysis attenuation modifications
Comparison with Previous Day Persistence

Max. 1 Hr. Ozone

Persistence Eta-CMAQ

Correlation coefficient plots from S. McKeen
PM2.5 Forecast Comparisons with AIRNOW : Preliminary

Daily Average

August 15 August 16 August 17
Summary

• Lateral boundary conditions play a dominant role in 

regulating modeled O3 background levels

– Critical when ozone levels are relatively low as in the past two 

summers

– Higher O3 BC led to a systematic higher bias in the 3x simulation

• Careful consideration needs to be given in deriving LBCs 

from larger scale models

– Are conditions representative?

• Bias propagation

– Consistent coupling

• Over-predictions at low O3range related to 

representation of cloud processes

– Top-down mixing

– Photolysis attenuation
Looking ahead ....

• Methods to improve coupling between models

– Boundary conditions

• GFS, Eta, CMAQ

• Layer structure and model top to improve representation of 

tropopause dynamics

• Comparison of model (GFS, CMAQ) and observed freetropospheric O3 values 

– Radiation

• Photolysis attenuation

– Boundary layer mixing

• Revisit Eta-Kh

– Transition to WRF

• Advection on E-grid

– Minimize interpolations
Looking ahead ....

• Continue testing alternate formulations over a wider 

range of conditions

– Mixing

– Below cloud washout (low observed O3 during precipitation 

events)

• Assessment of experimental PM forecasts results

– Surface data: AIRNOW, IMPROVE, CASTNET

– Satellites

• AOD