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Model Change Bulletin

MCB#2 02/28/2011

AERMET (dated 11059)

This Model Change Bulletin documents changes made to the AERMETMeteorological Preprocessor for the AERMOD Dispersion Model. A briefdescription of the changes is provided below. Additional details arealso available through comments imbedded in the Fortran source code.Changes to the user instructions associated with this update aredocumented in a separate AERMET User's Guide Addendum.

This revised version of AERMET (dated 11059) includes the followingmodifications relative to the previous version (dated 06341):

-- Bug Fixes:

1. Corrected several problems associated with the processing ofsubhourly ONSITE meteorological data, including:

a) Modified the procedure for calculating hourly averages

to use the "hour-ending" convention in accordance withSection 7.3 of "Meteorological Monitoring Guidance forRegulatory Modeling Applications," EPA-454/R-99-005,February 2000; previous versions used the "hour-beginning"convention.

b) Corrected the procedure for calculating hourly averages

from subhourly values for sigma-theta. Previous versionsused an equation in Section 5.2 of the AERMET User's Guide,but this has been replaced with the equation referenced inTable 8-3 of Appendix W. The equation from Section 5.2 ofthe AERMET User's Guide was also coded incorrectly inprevious versions of AERMET, which resulted in runtime

errors in some cases due to ta ing a square root of anegative number.

c) Corrected the processing of subhourly wind data to properlycode hours as calm when more than half of the samples forthe hour are non-missing, but below the wind threshold.Previous versions coded these hours as missing rather thancalm.

d) Addressed several issues associated with the extraction of

ONSITE data that could result in erroneous values beingassigned to ONSITE variables, and also improved errorhandling and reporting for the processing of ONSITE data.

e) Corrected problems associated with processing of ONSITE

precipitation data, including the fact that subhourlyprecipitation values were averaged rather than summedto determine the hourly value, and that negative valueswere also included in the summed value (before averaging)if the missing data code was not properly specified.

2. The implementation of the SUBNWS option was corrected to

avoid using the BULKRN option using ONSITE delta-T data when

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the ONSITE winds are missing and the reference winds arebased on SURFACE data. Vertical rofiles of ONSITE temperature,sigma-theta, and sigma-w are also s ipped if the referencewinds are based on SURFACE data. These changes are intendedto avoid internal inconsistencies in the characterization ofthe boundary layer that may occur with some combinations ofSURFACE and ONSITE data.

3. Corrected problems with time zone adjustments for cases withsurface and onsite stations in different time zones.

4. Incorporated changes to properly handle ISHD surface files

with the problem addressed on an interim basis by FIXISHDutility, which is no longer required.

-- Enhancements:

1. Enhancements were made to the heirarchy for handling missingstation pressure to ma e full use of other available data andof user-specified station elevations for SURFACE stationsand ONSITE stations, if available.

2. Increased the maximum record length for the runstream input file

from 80 to 132 characters, removed any limit on the record length

for ONSITE data records, increased the maximum field length forfilenames from 48 to 96, and allow for use of double quotes ()as field delimiters to support filenames with embedded spaces.

3. Several enhancements to provide more flexibility in selecting

the most appropriate upper air sounding, including a new optionfor a user-specified window on upper air observation time, andan option for AERMET to automatically select the most appropriatesounding based on local sunrise. These enhancements allow AERMETto better utilize upper air data that occurs more frequentlythan twice-daily, including "pseudo" upper air data derived fromgridded meteorological models or reanalysis data, if deemed tobe appropriate in the future (note that use of gridded model

outputs or reanalysis data in AERMET would be considered asan alternative modeling technique at the present time). Theseenhancements also provide better support for applications ofAERMOD beyond the U.S.

4. Enhancement to allow the use of hourly-averaged winds derived

from 1-minute ASOS wind data (TD-6405), generated by the newAERMINUTE program, which can improve the representativenessof airport wind data by providing hourly averages as well asfilling in for many hours that are missing in the standardairport data archives due to calm or variable winds.

5. Adjustment of ASOS-based wind speeds (including winds derived

from 1-minute ASOS data) by +0.5 not to account for the biasin reported ASOS wind speeds due to winds being truncated(rather than rounded) to whole nots. This includes changesto several subroutines to allow trac ing whether surface dataare ASOS or observer-based across all three stages of dataprocessing, and the inclusion of an ASOS 'flag' in the surfaceextraction and surface QA files. A database of ASOS commissiondates has been included to allow AERMET to determine whetherobservations for a specific surface station are ASOS or not.The surface output file format has also been modified to include

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a character string for each hour identifying whether the windspeeds have been adjusted or not to account for the truncationof the ASOS winds ('ADJ-' or 'NAD-') and whether the referencewind data is based on ONSITE, SURFACE or 1-MIN ASOS data (OS,SFC, or A1).

6. Several enhancements to the error handling and reporting related

to processing ONSITE data, including an option to use FREEformat to read the data and the option to specify missing datacodes and upper/lower bounds for ONSITE data as REAL variables.

7. A new option/requirement incorporated in Stage 3 to specify a

secondary set of surface characteristics for use when NWS windsare substituted for missing on-site winds using the SUBNWS optionon the METPREP pathway. The primary set of surface characteristicsis used when the reference wind is based on ONSITE data, and thesecondary surface characteristics are used when SURFACE data aresubstituted for missing ONSITE data. This ensures that anappropriate surface roughness will be used depending on thelocation of the reference wind data.

8. New eywords, AERSURF and AERSURF2, were added to the METPREP

pathway to allow users to specify an external file to readfor surface characteristics, such as an AERSURFACE output file,

rather than entering the data directly in the Stage 3 inputfile.

9. Enhancements to utilize on-site precipitation and relativehumidity data, if available, in the surface output file.

-- Miscellaneous:

1. The LOCATION eyword is no longer supported on the METPREPpathway in Stage 3. The METPREP LOCATION eyword had been usedas the location for determining sunrise for convective mixingheight calculations. This could result in somewhat differentresults using the same raw input data depending on the METPREP

LOCATION, which are not meaningful. A non-fatal warning messageis issued if the METPREP LOCATION eyword is found, and thelocation used to determine sunrise for convective mixing heightcalculations is based on the primary surface station location,i.e., the ONSITE station if available, or the SURFACE (NWS)station location.

2. Modified several subroutines to adjust the lower limit on the

value of surface roughness length to 0.0001 meters, rather than0.001 meters. This allows more flexibility for applications ofAERMET for overwater or other very low roughness settings.

3. Several subroutines were modified to improve error handling and

reporting for all three processing stages, including adjustmentsand improvements to the order and formatting of information inthe report file.

4. Implemented some restrictions on the use of surface (NWS) data

outside the valid range of dates based on the surface data format,i.e., data that have been reformatted from another file format.These restrictions affect cloud cover for ASOS data in the SCRAMor SAMSON format, since the SAMSON CDs and nearly all SCRAM surfacedata predated the advent of ASOS in 1992. The ASOS cloud covers

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are set to missing for any SURFACE data input in the SCRAM orSAMSON format, since the interpretation and formatting of ASOScloud cover codes cannot be verified in these cases.

****************** IMPORTANT ******************

This program will henceforth be identified by its name and a Julian date,i.e.: AERMET (dated 11059). Future changes to AERMET will be identifiedby an updated Julian date found at the beginning of the source codeand followed by a reference to a SCRAM Model Change Bulletin (MCB#n)which will explain the nature and content of the changes. All Model ChangeBulletins for this program will always be available for bac reference onthis website. In the future, notification of changes (MCBs) to this programwill be made in the SCRAM ALERTS section of this website.