PROCESS_ANALYSIS_NOTES - 13 January 2010 The following describes procedures for conducting process analyses with the CMAQ modeling system. Process analysis is a diagnostic tool that can be used by the modeling analyst to quantify contributions of individual science processes to model predicted concentrations. Such information is useful for understanding a model's predictions and for determining why predictions change when the model configuration or when model inputs are changed. Technical details of the CMAQ implementation of process analysis are contained in Chapter 16 of the "Science Algorithms of the EPA Models-3 Community Multiscale Air Quality (CMAQ) Modeling System (EPA-600/R- 99/030), and will not be repeated here. The focus here is on the operational steps necessary to invoke process analysis in the CMAQ system. However, this science document has not been upgraded to reflect modifications to process analysis, required by the new mass-conserving advection scheme now available in the CMAQ chemical transport model (CCTM). The CMAQ build scripts included with this release are set up such that process analysis is de-activated. This is accomplished by including three "no-op" include files that effectively turn off process analysis - PA_CTL.EXT, PA_DAT.EXT and PA_CMN.EXT. Whenever process analysis is to be omitted, these three no-op files should be used when the CCTM is compiled. To activate Process Analysis, the three no-op files must be replaced with three include files that define how the information is to be collected during model simulations. These files are generated by running the Process Analysis Control Program (PACP), for which bldit and run scripts are provided with this release. The steps to invoke process analysis follow. 1. Create an ASCII text file containing process analysis commands. See Chapter 16 of the aforementioned science document for command rules and syntax. New syntax for the mass-conserving advection scheme in the CCTM is discussed in special note d), below. 2. Build the PACP using the bldit script. Note that the CVSROOT directory should point to the PROCAN subdirectory of the source archive (e.g., in the accompanying bldit script, the variable "Project" is set to $M3MODEL/PROCAN). The only change to the release script that may be required involves the chemical mechanism. The script variable "Mechanism" must be set to the chemical mechanism that will be used in the CCTM. 3. Run the PACP using the run script. The input file to be used can be defined using the environment variable PACP_INFILE. (If this environment variable is not set, the PACP will look for an input file named "pa.inp" in the run directory.) The output of the PACP consists of four ASCII files: the aforementioned three include files (PA_CTL.EXT, PA_CMN.EXT, and PA_DAT.EXT), and a PA_REPORT file that is included for informational purposes. All four output files will be placed in the run directory. 4. Use the three include files generated by the PACP program in lieu of the no-op files when the CCTM is built (i.e., change the "ICL_PA" variable in the CCTM bldit script to point to the path of the include files generated by the PACP). The process analysis output files generated by the CCTM will be included with the other CCTM output files in the OUTDIR specied in the run script for the CCTM. See special note b) below regarding setting the output domain before running the CCTM. Special notes: a) When invoking process analysis, be sure to use the same chemical mechanism when building the PACP and the CCTM. At present no consistency check is performed, and the use of inconsistent mechanisms will produce erroneous results. b) As described in the science document, the domain for process analysis can be set smaller than the CCTM computational domain to reduce computer resources requirements. The science document indicates that the process analysis output domain is set using the OUTPUT_DOMAIN command in the PACP input file. That is no longer the case. The process analysis domain is now set in the CMAQ run script via the environment variables PA_BCOL_ECOL, PA_BROW_EROW, and PA_BLEV_ELEV. When process analysis is invoked, these variables must be un- commented and set appropriately. The PACP is now programmed to terminate with an error message if the OUTPUT_DOMAIN command is included in the PACP input file. c) An example PACP input file and corresponding output files are included in the "data" directory of this release, but these files have not been used in the example CCTM run. d) There's a new "base" operator, HADV for the mass-conserving advection scheme that combines the X- and Y-advection. These cannot be simply separated as is the case for PPM advection. All the operators that combine the base operators have been renamed to contain only four characters, like the base operators. This was done to allow a larger, maximum 11 character length of the chemical species names, which get prepended by the operator name (with the additional "_" character) to become the I/O-API variable names (16 character limit) written to the output file(s). New combined operator names (created from base operator names): Diffusion: TDIF = HDIF + VDIF where "TDIF" is total diffusion PPM scheme: ADV2 = XADV + YADV ADV3 = XADV + YADV + ZADV TADV = XADV + YADV + ZADV + ADJC TRAN = XADV + YADV + ZADV + ADJC + HDIF + VDIF where "ADV2" is horizontal advection for the PPM scheme "ADV3" is total advection without mixing adjustment for the PPM scheme "TADV" is total advection for the PPM scheme "TRAN" is total transport for the PPM scheme Mass-conserving scheme: MADV = HADV + ZADV TRNM = HADV + ZADV + HDIF + VDIF where 'MADV" is total advection for the mass-conserving scheme 'TRNM" is total transport for the mass-conserving scheme e) Integrated Reaction Rate (IRR) Analysis. CMAQ includes integrated reaction rate analyses that can be performed to assist the analyst in understanding the underlying reasons for model predictions. In IRR, numerical solutions of the gas-phase chemistry solvers are used to calculate integrated rates of reactions during the model simulations. Special accuracy requirements for these calculations preclude the use of some solvers and may necessitate changes to the convergence tolerances in other cases. Tests have shown that the EBI solver as configured in CMAQ is insufficiently accurate for use in IRR analyses (the inaccuracy here applies to the IRR part, not to the gas- chemistry solution part). Although accuracy could be improved somewhat by adjusting convergence tolerances and reducing the chemistry integration time step, the accompanying loss in computational efficiency would negate any advantage of this solver. Hence, IRR analysis is not implemented in the EBI solver. With respect to ROS3 and SMVGEAR, tests have shown both to be sufficiently accurate for IRR analyses in most cases. However, if IRR results are being computed for fast reacting radicals such as OH and HO2 with the ROS3 solver, then it is recommended that the absolute tolerance be decreased from the default value of 1.0E-07 ppm to 1.0E-09 ppm by setting the environment variable RB_ATOL as described above. The default SMVGEAR convergence tolerances should be sufficient for most applications.