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• NoPiG: The tracer emissions were released directly into <strong>the</strong> CAMx 36 km grid cell<br />

containing <strong>the</strong> tracer release location calculating plume rise using <strong>the</strong> local meteorological<br />

conditions to inject <strong>the</strong> emissions into <strong>the</strong> appropriate vertical layer.<br />

• PiG: Calculate plume rise using local meteorological conditions <strong>and</strong> simulate <strong>the</strong> early<br />

evolution <strong>of</strong> plume dispersion using <strong>the</strong> PiG module.<br />

Vertical Diffusion Coefficients (Kz): The Kz coefficients define <strong>the</strong> rate <strong>of</strong> vertical mixing in a<br />

column <strong>of</strong> grid cells in CAMx. MM5 meteorological model does not directly output Kz, thus <strong>the</strong><br />

MM5CAMx pre‐processor has several different algorithms for diagnosing <strong>the</strong> Kz coefficients.<br />

Four different Kz algorithms were evaluated in <strong>the</strong> CAMx sensitivity tests in this study:<br />

• OB70: O’Brien 1970 algorithm for calculation Kz values by diagnosing <strong>the</strong>m from <strong>the</strong> MM5<br />

output.<br />

• TKE: The Eta planetary boundary layer (PBL) scheme used in <strong>the</strong> ETEX MM5<br />

meteorological modeling has a Turbulent Kinetic Energy (TKE) formulation. When using a<br />

TKE PBL scheme, MM5CAMx can calculate <strong>the</strong> Kz coefficients directly from <strong>the</strong> TKE values,<br />

ra<strong>the</strong>r than diagnosing <strong>the</strong>m from <strong>the</strong> o<strong>the</strong>r meteorological variables in <strong>the</strong> MM5 output.<br />

• ACM2: The Asymmetric Convective Mixing (ACM2) algorithm has two components: a<br />

st<strong>and</strong>ard Kz scheme that calculates diffusion between two adjacent grid cells in a column;<br />

<strong>and</strong> a non‐local diffusion scheme that can calculate diffusion between grid cells in a<br />

column that are not adjacent. In CAMx, <strong>the</strong> ACM2 scheme will deduce when convective<br />

activity is present in a column <strong>of</strong> grid cells <strong>and</strong> add <strong>the</strong> non‐local diffusion to <strong>the</strong> st<strong>and</strong>ard<br />

local diffusion based on <strong>the</strong> Kz coefficients.<br />

• CMAQ: Use <strong>the</strong> algorithm for calculating Kz from <strong>the</strong> CMAQ modeling system (Byun <strong>and</strong><br />

Ching, 1999).<br />

Horizontal Advection Solver: Horizontal advection (transport) is solved in CAMx using finite<br />

difference algorithms that were explicitly developed for simulating transport <strong>and</strong> limit<br />

numerical diffusion that can artificially reduce concentration peaks. Two horizontal transport<br />

algorithms are implemented in CAMx <strong>and</strong> <strong>the</strong>ir effect on model performance for <strong>the</strong> ETEX<br />

experiment was evaluated:<br />

Bott: The Bott (1989) scheme is a positive definite transport scheme that limits numerical<br />

diffusion.<br />

PPM: The Piecewise Parabolic Method (PPM; Colella <strong>and</strong> Woodward, 1984) is a higher<br />

order positive definite transport scheme that is also designed to limit numerical diffusion.<br />

The configuration <strong>of</strong> CAMx presented in <strong>the</strong> previous sections comparing model performance<br />

against <strong>the</strong> o<strong>the</strong>r four LRT models was a st<strong>and</strong>ard configuration used in many regional model<br />

applications:<br />

• Don’t use PiG subgrid‐scale puff module (NoPiG)<br />

• Use <strong>of</strong> CMAQ‐like Kz vertical diffusion coefficients (CMAQ)<br />

• Use <strong>of</strong> PPM horizontal advection solver (PPM)<br />

6.4.3.1 NoPiG CAMx Sensitivity Tests<br />

Figure 6‐17 displays <strong>the</strong> CAMx spatial model performance statistics for <strong>the</strong> sensitivity tests that<br />

were run without using <strong>the</strong> PiG subgrid‐scale puff module. For <strong>the</strong> FMS statistic, <strong>the</strong> CMAQ Kz<br />

<strong>and</strong> PPM horizontal transport sensitivity test (CMAQ/PPM) is performing <strong>the</strong> best with a FMS<br />

value <strong>of</strong> 51.8% followed by CMAQ/Bott (50.9%) <strong>and</strong> ACM2/PPM (50.8%). Vertical diffusion has<br />

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