DOI: 10.3334/CDIAC/cli.db1003 1. Introduction This file describes the available images and the data used to create the images. Instructions for accessing the images via FTP are found in the file readme.img. Also, the readme.img file contains a note on the availability of a CD-ROM containing the images. Several gridded data sets (surface temperature, precipitation, sea level pressure and 500 millibar heights) have been converted into continuous-tone color anomaly maps. The data have been expressed as anomalies from reference period means (or as percentages, in the case of precipitation). By color-coding the anomalies and displaying all four maps for a given time period simultaneously, the links between the general circulation and anomalies of temperature and precipitation are more easily recognized. All data have been processed to produce monthly, seasonal and annual maps as GIF (Graphics Interchange Format) images using 1024 x 822 pixels and 256 colors. (Graphics Interchange Format copyright of CompuServe Inc.; GIF (sm) is a Service Mark property of CompuServe Inc.) GIF was selected because it uses a highly compressed format (thereby minimizing storage and image retrieval time requirements), enables high resolution and good quality color to be utilized, and is portable across many types of computer systems (Carlson 1991). Each GIF image represents the data with color contours and shading on world and hemispheric maps. Three types of images have been produced, one using a cylindrical equidistant projection, the second a polar orthographic projection (displaying both North and South polar projections) and the third a location map of the actual data points. The polar projection allows data in high latitudes to be examined without the distortion of a cylindrical projection. All composites display maps of temperature, precipitation and pressure for an individual month, season or year (where data exists). 2. Data Various global gridded datasets have been used in creating this atlas, as summarized in the table below. Global gridded datasets used to construct the images. ----------------------------------------------------------------------------- Source Latitude Longitude Latitude Longitude Resolution Resolution Range Range ----------------------------------------------------------------------------- 1. Jones et al. (1991) 5 deg 5 deg 87.5N-87.5S 2.5E-2.5W 2. Eischeid et al. (1991) 4 deg 5 deg 86N-62S 0E-5W 3. NCAR 5 deg 5 deg 85N-15N 0E-5W 4. NCAR 5 deg 5 deg 10S-85S 0E-5W 5. NCAR 5 deg 5 deg 85N-15N 0E-5W 6. NCAR 5 deg 5 deg 10S-85S 0E-5W 7. U. of Wash./NCAR (1990) NMC N. Hemisphere equidistant grid Global gridded datasets used to construct the images (cont.). -------------------------------------------------------------------------- Date Range Data Type -------------------------------------------------------------------------- 1. 1854-1991 Monthly surface temperature anomalies (C) 2. 1851-1989 Seasonal and annual percents of mean precipitation 3. 1899-1991 Monthly sea level pressure (mb) 4. 1973-1989 Monthly sea level pressure (mb) 5. 1989-1991 Monthly 500 mb heights (m) 6. 1973-1989 Monthly 500 mb heights (m) 7. 1946-1989 Monthly 500 mb heights (m) Data set 1 is an unpublished version of the gridded set described in Jones et al., 1991; this version is available from the National Center for Atmospheric Research, Boulder, CO. Data set 2 is available from the Carbon Dioxide Information Analysis Center, Oak Ridge, TN. Data sets 3-6 are available from the National Center for Atmospheric Research, Boulder, CO. Data set 7 is available on CD-ROM from the Department of Atmospheric Sciences, University of Washington, Seattle, WA. Monthly, seasonal and annual maps were produced from the original monthly data (Winter = December, January, February; Spring = March, April, May; Summer = June, July, August; Fall = September, October, November; Annual = January through December). For precipitation there were no monthly gridded data, and thus only seasonal and annual maps were produced. a. Temperature Data The temperature data are distributed on a 5 degree latitude by 5 degree longitude grid, as described in the table above. There are 2592 (36 by 72) points in the grid. The data are in the form of monthly, seasonal and annual anomalies to .01 C expressed as departures from a 1950-1979 reference period mean. The original sources of data for the temperature grids are: 1. land-based monthly station surface air temperatures with data from January, 1854 through December, 1991 and with gross errors and inhomogeneities corrected. This is a version of the data described in Jones et al. (1991), regridded onto a 5 degree by 5 degree grid. 2. the Comprehensive Ocean-Atmosphere Data Set (COADS) gridded (2 degree latitude by 2 degree longitude) monthly sea surface temperatures with data from January, 1854 through December, 1986 (Woodruff et al. 1987). 3. the United Kingdom Meteorological Office (UKMO) gridded (1 degree latitude by 1 degree longitude) monthly sea surface temperature data set with data from January, 1987 through December, 1991 (Bottomley et al. 1990). In both 2. and 3. above, the data are surface water temperatures from bucket and/or ship intake valve water samples, as discussed by Jones et al. (1991). In each data compilation, corrections and adjustments were made for assumed problems in measurement techniques (as described in the sources referenced above); no further adjustments to the gridded data were made in producing this atlas. Data from all three datasets were expressed as anomalies, merged and interpolated onto the 5 degree latitude by 5 degree longitude grid. b. Precipitation Data The global precipitation data are distributed on a 4 degree latitude by 5 degree longitude grid, as described in the table above. There are 2736 (38 by 72) points in the grid. The data are in the form of seasonal and annual percentages of the reference period (1951-1970) mean precipitation interpolated onto the grid. The original source of data for the precipitation grids is monthly station precipitation records (Eischeid et al. 1991). In this compilation, stations were included only if they had at least 15 years of data in the period 1951- 1970, and outliers were flagged and checked against other stations in the same area in order to eliminate clearly erroneous values. In addition, for stations with apparent data inhomogeneities, all or part of their records were eliminated. Nevertheless, it is very likely that precipitation data from regions where snowfall is important are underestimates of actual amounts, and may have significant inhomogeneities due to changes in gage type and recoding protocols (Groisman 1991). We note that considerable research is currently in progress on this matter (e.g. Groisman 1993) and may enable adjustments to be made in a later version of the Atlas. c. Sea Level Pressure Data The sea level pressure data are distributed on a 5 degree latitude by 5 degree longitude grid, as described in the table above. There are 2520 (35 by 72) points in the grid. The data are in the form of monthly, seasonal and annual anomalies to 0.1 mb. The anomalies are calculated as departures from a 1951-1980 reference period mean for the Northern Hemisphere and a 1974-1989 reference period mean for the Southern Hemisphere. The shorter reference period for the Southern Hemisphere is due to the shorter period of record of Southern Hemisphere data. There are no sea level pressure data between 15 degrees N and 10 degrees S. The original source of the sea level pressure grids is the Data Support Section, NCAR (Jenne 1975). d. 500 mb Geopotential Height Data The 500 mb height data are distributed on a 5 degree latitude by 5 degree longitude grid, as described in the table above. There are 2520 (35 by 72) points in the grid. The data are in the form of monthly, seasonal and annual anomalies to 1 meter. The anomalies are calculated as departures from a 1951-1980 reference period mean for the Northern Hemisphere and a 1974-1989 reference period mean for the Southern Hemisphere. The shorter reference period for the Southern Hemisphere is due to the shorter period of record of Southern Hemisphere data. There are no height data between 15 degrees N and 10 degrees S. The original sources of data for the 500 mb height grids are: 1. National Meteorological Center (NMC) Northern Hemisphere octagonal grid data (Jenne 1975) retrieved from a compact disc produced jointly by the Department of Atmospheric Sciences, University of Washington and the Data Support Section, National Center for Atmospheric Research (University of Washington and National Center for Atmospheric Research 1990). The NMC grid is a 1977 point grid whose points are equally spaced when viewed on a polar stereographic projection. The compact disc contained data from January, 1946 through June, 1989. 2. Files of Northern Hemisphere and Southern Hemisphere gridded 500 mb heights (5 degree latitude by 5 degree longitude) from the Data Support Section, NCAR. The files contained data from April, 1973 through December, 1991. The NMC data was transposed onto a 5 degree latitude by 5 degree longitude grid and combined with the NCAR data to form the data grid described above. 3. Procedures The data described in section 2 have been processed to produce monthly, seasonal and annual maps as GIF images. Three types of images have been produced for each time period, one using a cylindrical equidistant projection, the second a polar orthographic projection (displaying both North and South polar projections) and the third a location map of the actual data points. Each image is composed of up to four maps showing temperature, precipitation, sea level pressure and 500 mb height anomalies for a particular time. The table below indicates the contour range and contour intervals used in each case. Contour range and contour interval for each map type. ---------------------------------------------------------------------------------- Map Type Contour Range Contour Interval ---------------------------------------------------------------------------------- Temperature +/-4C 0.1C Precipitation 0-300% 10% Sea level pressure (annual) +/-28mb 1mb Sea level pressure (seasonal) +/-30mb 2mb Sea level pressure (monthly) +/-28mb 4mb 500mb heights (annual) +/-280m 10m 500mb heights (seasonal) +/-300m 20m 500mb heights (monthly) +/-280m 40m All of the maps were produced by importing the data into FORTRAN programs that utilized NCAR Graphics Version 3.00 (Clare et al. 1989). NCAR Graphics routines were used for contouring, map projections and color filling. During initial work on data visualization, it was noticed that some data were being omitted by the contouring procedure. This was due to the fact that, using the NCAR graphics contouring routines, no contours were drawn in any box of the grid with a missing value at one or more of its four corners. Because of the spotty nature of some of the data, and the desire to visualize all the data, an algorithm was written to fill missing data values with an averaged sum of all immediately surrounding data values (a maximum of eight values). The net result of this procedure is to both infill the area of missing data and to expand (by one grid point) the outer boundary of an area with data, enabling the visualization of every data point. The procedure provides a more visually coherent picture of anomalies, in effect carrying out explicitly the procedure that a viewer would undertake implicitly by mentally interpolating over areas with intermittent data coverage. We feel that the net result provides an objective means of dealing with the recurrent missing data problem for the purpose of obtaining an overview of large-scale patterns. However, users must be cognizant of the fact that some individual data points may not be supported directly by recorded data. For those users who are concerned with the actual gridded data coverage, location maps have been prepared, showing the location of the actual data for all images in the data sets. 4. Questions Questions on any of the above material should be submitted to cdp@stc10.ctd.ornl.gov or frank@climate1.geo.umass.edu References Bottomley, M., C. K. Folland, J. Hsiung, R. E. Newell, and D. E. Parker, 1990: Global Ocean Surface Temperature Atlas. MIT Press, 20 pp. plus 313 plates. Carlson, W. E. , 1991. A survey of computer graphics image encoding and storage formats. Computer Graphics, 25, 67-75. Clare, F., and D. Kennison, 1989: NCAR Graphics Guide To New Utilites, Version 3.00. National Center for Atmospheric Research, 574 pp. Eischeid, J. K., H. F. Diaz, R. S. Bradley, and P. D. Jones, 1991: A comprehensive precipitation data set for global land areas, DOE Technical Report No. TR051. U.S. Department of Energy, Carbon Dioxide Research Program, 82 pp. Groisman, P. Ya., 1991: Unbiased estimates of precipitation change in the Northern Hemisphere extratropics. Proceedings of the Fifth Conference on Climate Variations. Denver, 42-45. Groisman, P. Ya., 1993: Towards unbiased estimates of North American precipitation, Proceedings of the Eighth Symposium on Meteorological Observations and Instrumentation. Anaheim, Amer. Meteor. Soc., j43- j47. Jenne, R. L., 1975: Data sets for meteorological research. NCAR Technical Note NCAR-TN/IA-111. National Center for Atmospheric Research, 194 pp. Jones, P. D., S. C. B. Raper, B. S. G. Cherry, C. M. Goodess, T. M. L. Wigley, B. Santer, P. M. Kelly, R. S. Bradley, and H. F. Diaz, 1991: An updated global grid point surface air temperature anomaly data set: 1851-1990, NDP020/R1. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, 413 pp. Jones, P. D., T. M. L. Wigley, and G. Farmer, 1991: Marine and land temperature data sets: a comparison and a look at recent trends. In Greenhouse-Gas-Induced Climatic Change: A Critical Appraisal of Simulations and Observations, M. E. Schlesinger, Ed., Elsevier, 153- 172. University of Washington and National Center for Atmospheric Research, 1990: Grid Point Data Set Version II: Gridded NMC Analyses for the Northern Hemisphere (CD-ROM). Department of Atmospheric Sciences, University of Washington and Data Support Section, National Center for Atmospheric Research. Woodruff, S. D., R. J. Slutz, R. J. Jenne, and P. M. Steurer, 1987: A comprehensive ocean-atmosphere data set. Bull. Amer. Meteor. Soc., 68, 1239-1250.