Sea Level Pressure

Monthly mean pressure fields from the NCEP/NCAR Reanalysis Project were used to create decadal and longer-term pressure fields. (For more information on the NCEP/NCAR Global Reanalysis Project, see the section titled "Description of the NCEP/NCAR Global Reanalysis pressure data".) We used Reanalysis products because the Reanalysis project employs a state-of-the-art data assimilation model. Use of a model constrains pressure values to be realistic and consistent with other assimilated data. The Reanalysis project makes use of historical data sources that may include additional data beyond what we have included on this Atlas (for instance, data from the Surface Land Synoptic Data set is included in the Reanalysis). Most of the data on this Atlas are already in the Reanalysis, through the Comprehensive Ocean-Atmosphere Data Set (COADS) and Surface Land Synoptic Data sources.

Data source for pressure fields

The data used for these fields come from the "Selected Monthly Means" from NCAR data set 090.0, NCEP Global Reanalysis Anals, 6-hourly. Data files were obtained from ftp://ncardata.ucar.edu/pub/reanalysis/monthly/PRES.msl.

Method of constructing the pressure field products

To construct the pressure field products, monthly reanalysis sea level pressure data from north of 54 degrees North were converted from WMO Gridded Binary (GRIB) format on a 2.5 degree latitude by 2.5 degree longitude scale, to the Atlas EASE-Grid format. Cubic convolution was used to regrid the data. Decadal and longer-term means were then calculated for each grid cell.

Gridded pressure field products

Gridded pressure field products are:

The browse versions of these files are shown as gif format images with a color bar and contours. These browse files are for the purpose of quickly visualizing the content of the corresponding ASCII data files. The IDL routine used to color map the images gives a smooth and visually pleasing result, but keep in mind that the gridded ASCII files have one value only for every grid cell. The grid cell centers are shown as red dots. For information on the structure of the gridded files, see the section on "EASE-Grid"

Description of NCEP/NCAR Global Reanalysis pressure fields

The NCEP/NCAR Global Reanalysis Project is a collaborative effort of the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) to produce an over 40-year record of global analysis of atmospheric fields. Data are recovered from a variety of sources, quality-controlled and assimilated with a data assimilation scheme that uses first guess fields, optimal interpolation, and an operational forecast model. More information can be found at NCAR (http://www.sdc.ucar.edu/dss/pub/reanalysis/index.html) and from the NOAA Climate Diagnostics Center (http://www.cdc.noaa.gov/). Currently, reanalysis products are available from 1948 through 1998.

The Reanalysis project uses pressure data from Comprehensive Ocean Atmosphere Data Set (COADS). COADS contains data from ships, buoys, and Russian North Pole drifting stations. See the "Description of COADS" in the "Total and low cloud cover" documentation.

Land station pressure data for the Reanalysis come from Surface Land Synoptic Data. This data set is composed of data received over the GTS telecommunication system from 1967 forward. Data from 1967 to 1976 are from the Air Force; data from 1976 to present are from NCEP and are available from NCAR as data set ds464.0, NCEP ADP Global Sfc Obs, daily Jul 1976-on. Prior to 1967, data are from Air Force Tape Deck 13, and are available from NCAR as data set ds467.0, NCDC TD13 Global Sfc Obs, daily 1899-1970. Surface synoptic data from 223 Soviet stations and from 300 US stations for the period 1948 through 1966 are used in the Reanalysis. Some percentage of the data from the 65 Russian stations included in the Coastal Stations section of this Atlas is included in this data source. The percentage varies; for instance, about 30 percent of stations appear in Surface Land Synoptic Data after about 1970, with far less in earlier years.