MONTHLY ALBEDO AND GREEN VEGETATION FRACTION anonymous ftp server/directory/files: ftp.ncep.noaa.gov/pub/gcp/sfcflds/test/fixed/ README_albedo_gfrac.txt (this readme file, 6K) albedo_apr.asc.Z gfrac_apr.asc.Z albedo_aug.asc.Z gfrac_aug.asc.Z albedo_dec.asc.Z gfrac_dec.asc.Z albedo_feb.asc.Z gfrac_feb.asc.Z albedo_jan.asc.Z gfrac_jan.asc.Z albedo_jul.asc.Z gfrac_jul.asc.Z albedo_jun.asc.Z gfrac_jun.asc.Z albedo_mar.asc.Z gfrac_mar.asc.Z albedo_may.asc.Z gfrac_may.asc.Z albedo_nov.asc.Z gfrac_nov.asc.Z albedo_oct.asc.Z gfrac_oct.asc.Z albedo_sep.asc.Z gfrac_sep.asc.Z Monthly 0.144 deg albedo (compressed ascii data files) (/pub/gcp/sfcflds/test/fixed/albedo_###.asc.Z, ###=jan, feb,... dec) Monthly 0.144 deg green vegetation fraction (compressed ascii data files) (/pub/gcp/sfcflds/test/fixed/gfrac_###.asc.Z, ###=jan, feb,... dec) additional files: gfrac_max.asc.Z = maximum annual value gfrac_max_mon.asc.Z = month of maximum value gfrac_min.asc.Z = minimum annual value gfrac_min_mon.asc.Z = month of minimum value Array dimensions: albedo(2500,1250), gfrac(2500,1250) resolution : 0.144 deg lat: -89.928 to 89.928 lon: -180 to 179.856 array element (1,1) corresponds to (lat,lon) of (-89.928,-180.000) (i,j) = (1,1250) (i,j) = (2500,1250) (-180.000, 89.928) (179.856, 89.928) _|_________________________|_ | (NORTH) | | | | (WEST) (EAST) | | | | (SOUTH) | _|_________________________|_ | | (-180.000,-89.928) (179.856,-89.928) (i,j) = (1,1) (i,j) = (2500,1) The formulae used to convert I/J coordinates into Lat/Lon coordinates are: Lon = -180.000 + 0.144*(I-1) for I=1,2500 Lat = -89.928 + 0.144*(J-1) for J=1,1250 To read in the data for each month: DIMENSION albedo(2500,1250),gfrac(2500,1250) INTEGER albedo INTEGER gfrac READ(2,10) ((albedo(i,j),i=1,2500),j=1,1250) READ(3,10) ((gfrac(i,j),i=1,2500),j=1,1250) 10 FORMAT(2500i2) The twelve monthly albedo and green veg fraction fields are assumed to be valid at the 15th of each month. Values are in percentages. albedo values represent: 0 : ocean/water flag (also gfrac=0) 1-69 : surface albedo (percent) 70 : glacial/perpetual snow (percent, with gfrac=1) gfrac (green veg fraction) values represent: 0 : ocean/water (also albedo=0) 1 : bare soil* 2-99 : green vegetation coverage (percent) *gfrac=1 represents a bare soil FLAG, NOT a green vegetation fraction REFERENCES Albedo: Csiszar, I., and Gutman, G., 1999: Mapping global land surface albedo from NOAA AVHRR. J. Geophys. Res., 104, 6215-6228. Ivan Csiszar: icsiszar@nesdis.noaa.gov Greenness: Gutman, G. and A. Ignatov, 1997: The derivation of green vegetation fraction from NOAA/AVHRR data for use in numerical weather prediction models. Int. J. Remote Sensing., 19, 1533-1543. Manuscript available from Garik Gutman at ggutman@nesdis.noaa.gov. ------- DISCUSSION OF STEPS TO CREATE CURRENT NCEP/EMC VERSIONS OF THE NESDIS 0.144 DEG MONTHLY ALBEDO AND GREEN VEGETATION FRACTION: Greenness fraction: the original 0.144 deg monthly greenness fraction files had no greenness for Antarctica (zeros=sea). The region south of 60S was filled with 1 (land) or 0 (sea) using the 1x1 deg vegetation type (Dorman and Sellers, 1989; JAM) (http://www.emc.ncep.noaa.gov/mmb/gcp/sfcimg/vegtype/vegtype_global.gif) since this provided the best 1x1 deg land/sea mask available. Also see 'ftp://ftp.ncep.noaa.gov/pub/gcp/sfcflds/README', item 6. Albedo: the original monthly albedo files covered only 55S to 75N, so the global greenness fraction (including the filled in Antarctic) was used to fill in the regions south of 55S and north of 75N as follows: --------- ------ greenness albedo --------- ------ 1 (land*) 70 (glacial) 0 (sea) 0 (sea) *south of 55S and north of 75N, the only non-zero greenness values=1 Both Greenness and Albedo: The best land/sea mask (coastline) coincides with the points where both albedo=0 and greenness=0. Some points defined as glacial (albedo=70) in the arctic (e.g. Greenland, Canadian Archipelago) had a greenness>1, inconsistent with glacial conditions. The number of points varied from 10K to zero depending on month. The greenness was reset to 1. There were some discrepancies between the albedo and greenness in terms of the land-sea mask: case 1. Albedo>0 (land), greenness=0 (sea), 2465 points (out of 3125000 total, less than 0.08 percent), the same location all months (which makes these corrections easier/more consistent to make). Many of these cases were obviously bare soil conditions, in the Sahara, South African and South American west coastal deserts and several inland locations (less than 0.08 percent of all points). For these points, the albedo was kept, and the greenness reset to 1 (bare soil). case 2. Albedo=0 (sea), greenness>0, 2452 (out of 3125000 total, less than 0.08 percent... actually in these cases, it turned out that greenness=1 always), again the same location all months. These cases were less obvious since they occurred in all regions (not just desert regions), though mostly near the coastlines (i.e. around the Arabian Peninsula, (north)west coast of Africa, a few small regions along the Chinese coast, shoreline of Lake Victoria in central Africa, and a few other areas). For these points, the albedo and greenness were taken as an average of the nearest non-sea neighbors, searching out to a distance of 5 grid points (0.72 deg). There were also a small number of points north of 70N (Greenland, northern Siberia) where albedo=0 (sea), greenness=1; these were reset to glacial: albedo=70, greenness=1. Only 13 exceptions remained after the nearest neighbor process (Aleutian Is/North Pacific-7 points, South Georgia/South Atlantic-6 points, Socotra Is/off Horn of Africa-2 points. Aleutian Is and Socotra Is were filled in manually using nearest neighbors more than 5 grid points away, while nominal values similar to the Aleutian Is, though seasonally reversed, were used for South Georgia. Greenness fraction: greenness values for Nov, Dec, Jan are suspect north of 58N (there is an unrealistic mid-winter max in greenness). Greenness during N,D,J reset to minimun annual greenness value for a particular point north of 58N. --- Michael Ek NCEP/EMC 12 Feb 2002