
                        Model Change Bulletin

                                MCB#2                       02/28/2011

                        AERMET (dated 11059)

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

This revised version of AERMET (dated 11059) includes the following
modifications relative to the previous version (dated 06341):                                                       
                                                                        
-- Bug Fixes:                                                           

   1.  Corrected several problems associated with the processing of
       subhourly ONSITE meteorological data, including:
       
       a) Modified the procedure for calculating hourly averages
          to use the "hour-ending" convention in accordance with
          Section 7.3 of "Meteorological Monitoring Guidance for 
          Regulatory 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 versions
          used an equation in Section 5.2 of the AERMET User's Guide,
          but this has been replaced with the equation referenced in
          Table 8-3 of Appendix W. The equation from Section 5.2 of 
          the AERMET User's Guide was also coded incorrectly in 
          previous versions of AERMET, which resulted in runtime
          errors in some cases due to taking a square root of a 
          negative number.
          
       c) Corrected the processing of subhourly wind data to properly
          code hours as calm when more than half of the samples for 
          the hour are non-missing, but below the wind threshold.  
          Previous versions coded these hours as missing rather than 
          calm.
          
       d) Addressed several issues associated with the extraction of 
          ONSITE data that could result in erroneous values being 
          assigned to ONSITE variables, and also improved error 
          handling and reporting for the processing of ONSITE data.
          
       e) Corrected problems associated with processing of ONSITE
          precipitation data, including the fact that subhourly 
          precipitation values were averaged rather than summed 
          to determine the hourly value, and that negative values
          were 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 
       the ONSITE winds are missing and the reference winds are 
       based on SURFACE data. Vertical rofiles of ONSITE temperature, 
       sigma-theta, and sigma-w are also skipped if the reference
       winds are based on SURFACE data.  These changes are intended
       to avoid internal inconsistencies in the characterization of
       the boundary layer that may occur with some combinations of
       SURFACE and ONSITE data.

   3.  Corrected problems with time zone adjustments for cases with
       surface 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 FIXISHD 
       utility, which is no longer required.
          
-- Enhancements:                                                        

   1.  Enhancements were made to the heirarchy for handling missing 
       station pressure to make full use of other available data and 
       of user-specified station elevations for SURFACE stations 
       and 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 for 
       filenames 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 option
       for a user-specified window on upper air observation time, and
       an option for AERMET to automatically select the most appropriate 
       sounding based on local sunrise. These enhancements allow AERMET 
       to better utilize upper air data that occurs more frequently 
       than twice-daily, including "pseudo" upper air data derived from 
       gridded meteorological models or reanalysis data, if deemed to 
       be appropriate in the future (note that use of gridded model 
       outputs or reanalysis data in AERMET would be considered as 
       an alternative modeling technique at the present time).  These 
       enhancements also provide better support for applications of 
       AERMOD 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 new
       AERMINUTE program, which can improve the representativeness 
       of airport wind data by providing hourly averages as well as
       filling in for many hours that are missing in the standard
       airport 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 knot to account for the bias 
       in reported ASOS wind speeds due to winds being truncated 
       (rather than rounded) to whole knots.  This includes changes
       to several subroutines to allow tracking whether surface data 
       are ASOS or observer-based across all three stages of data 
       processing, and the inclusion of an ASOS 'flag' in the surface
       extraction and surface QA files.  A database of ASOS commission
       dates has been included to allow AERMET to determine whether 
       observations for a specific surface station are ASOS or not. 
       The surface output file format has also been modified to include 
       a character string for each hour identifying whether the wind 
       speeds have been adjusted or not to account for the truncation 
       of the ASOS winds ('ADJ-' or 'NAD-') and whether the reference 
       wind 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 ‘FREE’ 
       format to read the data and the option to specify missing data 
       codes 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 winds 
       are substituted for missing on-site winds using the SUBNWS option
       on the METPREP pathway.  The primary set of surface characteristics 
       is used when the reference wind is based on ONSITE data, and the 
       secondary surface characteristics are used when SURFACE data are 
       substituted for missing ONSITE data.  This ensures that an 
       appropriate surface roughness will be used depending on the 
       location of the reference wind data.
       
   8.  New keywords, AERSURF and AERSURF2, were added to the METPREP 
       pathway to allow users to specify an external file to read
       for surface characteristics, such as an AERSURFACE output file,
       rather than entering the data directly in the Stage 3 input
       file.
       
   9.  Enhancements to utilize on-site precipitation and relative 
       humidity data, if available, in the surface output file.

-- Miscellaneous:                                                       
   
   1.  The LOCATION keyword is no longer supported on the METPREP 
       pathway in Stage 3.  The METPREP LOCATION keyword had been used
       as the location for determining sunrise for convective mixing
       height calculations.  This could result in somewhat different
       results using the same raw input data depending on the METPREP
       LOCATION, which are not meaningful.  A non-fatal warning message
       is issued if the METPREP LOCATION keyword is found, and the
       location used to determine sunrise for convective mixing height
       calculations 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 than
       0.001 meters.  This allows more flexibility for applications of
       AERMET 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 adjustments 
       and improvements to the order and formatting of information in 
       the 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 SCRAM 
       or SAMSON format, since the SAMSON CDs and nearly all SCRAM surface 
       data predated the advent of ASOS in 1992.  The ASOS cloud covers 
       are set to missing for any SURFACE data input in the SCRAM or 
       SAMSON format, since the interpretation and formatting of ASOS
       cloud cover codes cannot be verified in these cases.


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