Raw GTS data

Raw GTS data are archived (by RAL) under mss:/RAPDMG/LDM/ARCHIVE. (e.g. /RAPDMG/LDM/ARCHIVE/2011/0303/20110303.ddp.tar.gz)

KDEN 022353Z 09008KT 10SM FEW080 SCT120 BKN200 14/M12 A2993 RMK AO2 SLP115 T01391117 10150 20056 55004=
RCTP 030000Z 03017KT 8000 SCT010 BKN015 BKN025 14/11 Q1021 NOSIG=
LITTLE_R format

OBSPROC adopts LITTLE_R format as the input format.
OBSPROC should be aware how raw data are re-formatted to fit in the LITTLE_R format.
The quality flags in the input LITTLE_R format have no meanings (as GTS data do not report quality information). However, some add-ons in the obs files (created by programs other than the GTS decoder) such as GPS PW, GPRSREF, QSCAT may have valid QC flags.

Each report in the LITTLE_R Observations Format consists of at least four records (See WRF-ARW V3: USer's Guide 7-13)

  • A report header record (Some of the key fields in the header record are latitude, longitude, id, elevation, slp, psfc, etc.)
  • one or more data records (Each data record needs to have at lease either pressure or height information.)
  • an end data record
  • an end report record

NCAR MPG real-time data processing information is summarized on the web page http://www.mmm.ucar.edu/individual/bresch/data
The notes about surface observation re-formatting to LITTLE_R format are listed here.

About decoding of surface data: METARs (FM-15) never report surface pressure. METARs usually report altimeter setting. The decoder places the altimeter setting into the little_r surface pressure data record. U.S. and Canadian METARs often contain SLP and temperatures in tenths of degree in the remarks section (RMK). These are decoded. For ship data (FM-13 or FM-18), the elevation is missing and surface (or station) pressure is set to 1013.01 hPa. This value is a flag to indicate a suface report (the elevation of a ship or buoy does not have to be at sea level as in the case of Lake Michigan). Similarly, SYNOP reports (FM-12) also have a flag of 1013.01 in the surface pressure field to indicate surface data. However, the SYNOP code does include a station pressure group. The observed station pressure will be decoded and replace 1013.01 if it is available.

WMO documentation

WMO PUBLICATION NO. 306 - MANUAL ON CODES
The documentation on codes are available on the page http://www.wmo.int/pages/prog/www/WMOCodes/VolumeI1.html or click here for the file I have downloaded and attached here

  • Whenever air pressure at mean sea level can be computed with reasonable accuracy, this pressure shall be reported in the 4PPPP group.
    1. For a station situated in a region of normal synoptic network density, the pressure at mean sea level is considered not to be computed with reasonable accuracy when it introduces a deformation into the analysis of the horizontal pressure field which is purely local and recurring.
    2. For a station lying in a data-sparse area of the synoptic network, reasonable accuracy will beobtained when using a reduction method which has proved to be satisfactory in a region of normal network density and under similar geographical conditions.
  • By regional decision, a high-level station which cannot give pressure at mean sea level to a satisfactory degree of accuracy shall report both the station-level pressure group and the geopotential height of an agreed standard isobaric surface. In that case, the group 4PPPP shall be replaced by the group 4a3hhh. (The level chosen for each station is indicated in Volume A of publication WMO – No. 9.)
  • Group 3P0P0P0P0 (station pressure) shall be included in reports for global exchange from land stations, together with either the group 4PPPP or, in accordance with Regulation 12.2.3.4.2, the group 4a3hhh.
observed Station Pressure to reported Sea Level Pressure

National Weather Service Glossary http://www.nws.noaa.gov/glossary/index.php?letter=s

Sea Level Pressure
The sea level pressure is the atmospheric pressure at sea level at a given location. When observed at a reporting station that is not at sea level (nearly all stations), it is a correction of the station pressure to sea level. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in the sea level correction is a twelve hour mean, eliminating diurnal effects. Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.

A brief essay and training exercise on the various types of sea level reduction of pressure in models and analyses
http://www.crh.noaa.gov/unr/?n=mslp

Station Pressure in NCEP PREPBUFR files

The web page below contains the information about how Pstn is calculated when Pstn is not directly available from the observation reports.
http://www.emc.ncep.noaa.gov/mmb/data_processing/prepbufr.doc/table_2.htm
Table 2. Code table for PREPBUFR report types used by Global GFS and GDAS GSI analyses (last revised 10/25/2010)

  • Reported station pressure is used.
  • When altimeter setting is reported (e.g. metar), station pressure is calculated from reported altimeter setting and elevation.
  • When both station pressure and altimeter setting are missing (e.g. synop), station pressure is calculated from reported mean sea-level pressure and elevation via U.S standard atmosphere approximation when elevation is greater than 7.5 meters. (station pressure is set to equal to sea-level pressure when elevation is smaller than 7.5 meters.)
  • When station pressure and mean seal-level pressure are both missing (e.g. buoy), station pressure is set to 1013 mb (with quality marker=3) when elevation is smaller than or equal to 7.5 meters.
Station Tables (ever-changing)

Each center maintains their own station tables.

Comparison of METAR stations from different sources can be seen on http://weather.gladstonefamily.net/cgi-bin/wxdiff.pl, which is compiled by Philip Gladstone.

The two METAR stations in the NCEP PREPBUFR file that are apparently wrong are BGQQ and UWKE (identified by O-B values in the ASR domain).

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