FILE ds010.0fmt VERSION 1.4 CHANGE TIME 96/11/13 11:09:04 ACCESS TIME 96/11/13 11:10:16 Wilbur M.L. Spangler/Roy L. Jenne Data Support Section Scientific Computing Division National Center for Atmospheric Research Box 3000, Boulder, Colorado 80307 Daily and Monthly Lat-lon Sea Level Pressure Grids (Northern Hemisphere, Jan 1899 to current) General Description of Daily 5-degree Lat-lon SLP Grids: NCAR has daily sea level pressure grids from 1899 to current years from various sources as explained below. All grids are 5-degree lat-lon grids from 15N - 85N and 00E - 355E. There is also a pole value for later grids. Where possible, grid point values missing in the original grids, have been interpolated. Interpolation was along latitude bands, but not over a large number of missing points. There are no daily grids currently available for Dec 1944 - Dec 1945. General Description of Monthly Mean Grids: Monthly mean grids in the same format have been computed for the entire period. Daily grids for all time periods available were used to compute the monthly means; a count of these daily grids is carried along with the mean grid. The mean grid for December 1944 is not available. The mean grids for 1945 were supplied by Scripps Institution of Oceanography. Sources of Data for Daily Grids: Grids from historical maps, Jan 1899 - Jun 1939 (13Z): This data, originated at the National Climatic Center, Asheville, N.C., was sent to the Extended Forecast Lab; from where a copy was sent to the NOAA Environmental Research Laboratories. There, Mr. Cotton and others spent about 1.5 man years comparing each point to the average of surrounding values. They checked each difference over 10 mb with the original maps to see whether a correction was necessary. Errors greater than 10 mb were likely discovered and corrected by this process. Data are missing where historical maps could not be analyzed. Eastern Russia (40-80N, 35-150E) is missing for 1916-1920, for 3 months in 1921, for one month each in 1922 and 1931, and for 6 months each in 1938 and 1939. The grids were on a diamond lat-lon grid (5-degree lat, 10-degree lon), 20N-80N, with no data at 75N. There are no pole values. Grids from MIT, Jul 1939 - Nov 1944 (12Z): MIT read values from charts from the Extended Forecast Lab. The data are for each 5 degrees of latitude and even 10 degrees of longitude for 15N-80N. Data for 85N is present for 10 days. Grids from Navy*, Jan 1946 - Mar 1955 (12Z), Apr 1960 - Jun 1962 (12Z), Jul 1962 - current (00Z,12Z): The grids from the historical series maps for Jan 1946 - Mar 1955, and Apr 1960 - Jun 1962 were digitized with a curve follower at NCC under Navy contract. These data were then analyzed by the Navy's operational objective analysis program that is used on current data. These grids include manual bogus to put in tropical storms. There is no data at 15N. * Although the Navy grids are the primary source for updating this set, occasionally grids from the National Meteorological Center are used until the Navy grids are received. - 2 - Grids from the 433L ESSPO Project, Apr 1955 - Mar 1960 (00Z,12Z): Data for every other point in the NMC octagonal grid were manually read from charts for this period. There is no data at 15N. Format of Grids: The daily grids and monthly grids although in the same format are maintained on separate tapes. There is one 273 word (60-bits each) record in binary packed integer format for each data grid. Identification is included with each record. The format is broken down as follows: Word 1, counting bits from left to right bits 1-6 format number ( = 10) bits 7-17 year, e.g. 1899 bits 18-21 month, e.g. 11 = NOV bits 22-26 day ( = 0 for monthly mean) bits 27-31 hour ( = 31 for monthly means when more than one time period; or when time unknown as for the year 1945) bits 32-41 level ( = 1013 for sea level) bits 42-50 type ( = 6 for sea level pressure) { 3, for Navy data { 4, for ESSPO data bits 51-56 source { 10, for NOAA data { 11, for MIT data { 28, for NMC data when used { 29, for data reworked by Trenberth** bits 57-60 grid type ( = 1) Word 2, bits 1-15 pole value* bits 16-21 number of grids in mean ( = 0 for dailies) bits 22-60 not used Words 3-272 1080 binary integer data values in 15-bit bytes.* Data values are to nearest tenth millibar and are the rightmost 14 bits of each byte. The grid of Y(I,J) is dimensioned Y(72,15). As I goes from 1 to 72, longitude goes from 00E-355E; as J goes from 1 to 15, latitude goes from 15N-85N. Word 273, 60-bit checksum of first 272 words. * The leftmost bit of each packed 15-bit data value is used to distinguish between grid points of original data and grid point values which have been interpolated or are missing. If this bit is 0, the data value is an original value; if this bit is 1 the data value has been derived (non-zero) or is missing (zero). This applies to the pole value also. This bit is 0 (except for 15N) on grids from 1946 on because the data were interpolated from grids with a high degree of resolution. For monthly means a zero grid value indicates there were fewer than 10 daily values and the mean is considered missing. ** Dr. Kevin Trenberth, NCAR, made error and discontinuity corrections to the monthly means grids for the period, 1899-1977. These mean grids are the preferred grids, although the means grids computed from the dailies without the Trenberth corrections are also available.