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Surface Data Conversion

METAR and SAO format

The surface observation data conversion program called sacvt parses all station identifiers in the hourly file that are in METAR and SAO formats (obsolete). It will handle automated AMOS and ASOS reports. The following information is decoded:

  • ID -- station identifier
  • TYPE -- type of report (hourly, special, etc)
  • TEMP -- temperature
  • DEWPT -- dewpoint
  • WIND -- wind speed, direction and gust
  • ALT -- altimeter setting
  • SLP -- sea level pressure
  • VIS -- visibility
  • CLD -- cloud layer information including height and coverage
  • WX -- present weather
  • PTEND -- 3 hour pressure tendency and type indicator
  • PREC -- 3, 6, 12 and 24 hour precipitation totals
  • TMAX -- 6 and 24 hour maximum temperature
  • TMIN -- 6 and 24 hour minimum temperature
  • SNOW -- snow cover
  • CTYPE -- low, middle and high cloud types
  • SOLAR -- equivalent solar radiation (SAO only)
  • Any special comments that are needed to clarify current conditions.

The decoding process takes each report and decodes as much of the information as possible. If the report is malformed or corrupted in transmission, it is likely the decoder won't completely decode the product.  The undecoded portion of the report goes into the comment section that is saved to the output data file. All of the reports are decoded but not all of them make it to the output file.  By default, only standard observations are saved (no specials) and only those reports that were observed from 15 minutes before the hour to 10 minutes after the hour.  If duplicate reports exist, they are all copied, by default, to the output file.  Some useful parameters to set are:

  • spec - save specials as well
  • range:start:end - specifies time range for reports.  The default is -15:10.  This can be used to extract second and third hourly reports from those stations that report more than once an hour.
  • dup - remove all duplicates
  • update - remove duplicates and update existing reports with corrections

In addition, there is the ability to decode multiple hour data files if reports are spread out over several hours.  To decode more than one hour, use the num_hour resource to specify how many hours before or after the current file to read.  The decode_hour resource is helpful if the range of hours is both ahead and behind the desired hour.   For example, you need to decode 12Z and files used are between 10-14Z. The current hour would be 14Z, the number of hours is -4 and the decode hour is 12.

Once decoded, the output is put into either an ASCII or netCDF format.  NetCDF is preferable for data interchange to non-WXP applications.  The ASCII format is preferable for WXP applications even though WXP will read the netCDF format.

For further information about WXP, email technical-support@weather.unisys.com
Last updated by Dan Vietor on May 30, 1998