MAXIMUM SNOW ALBEDO This maximum snow albedo data set reflects the condition that for a deep snowpack, the surface albedo can depend on vegetation type/coverage, e.g. lower albedo for a conifer forest (more darker trees sticking through the brighter snowpack), compared to higher albedo for a grassland region where the vegetation could be completely covered by the snowpack. This can have important implications for the surface energy budget, and dependent issues. The original database was derived from work described in Robinson and Kukla (1985). We extended their database to global coverage; the steps to do this are described in this readme file. In the NOAH land-surface model (part of the NCEP/EMC mesoscale Eta model), the maximum snow albedo (along with the snow water equivalent, veg type, and snow-free albedo) is used in calculating the actual (dynamic) surface albedo. anonymous ftp server/directory/files: ftp.ncep.noaa.gov/pub/gcp/sfcflds/test/fixed/ maxsnoalb.asc.Z (compressed ascii data file, 17K) README_maxsnowalb.txt (this readme file, 8K) maxsnoalb.gif (globl image file, 29K) Array dimensions: mxsnal(361,180) (wrap-around longitude) temporal domain: fixed annual field spatial resolution: 1-deg x 1-deg global lat: -89.5 to 89.5 lon: -180.0 to 180.0 array element (1,1) corresponds to (lat,lon) of (-89.5,-180.0) (i,j) = (1,180) (i,j) = (361,180) (-180.0, 89.5) (180.0, 89.5) _|_________________________|_ | (NORTH) | | | | (WEST) (EAST) | | | | (SOUTH) | _|_________________________|_ | | (-180.0,-89.5) (180.0,-89.5) (i,j) = (1,1) (i,j) = (361,1) The formulae used to convert I/J coordinates into Lat/Lon coordinates are: Lon = -181.0 + I for I=1,361 Lat = -90.5 + J for J=1,180 To read in the data for each month: DIMENSION mxsnal(361,180) INTEGER mxsnal READ(2,10) ((mxsnal(i,j),i=1,361),j=1,180) 10 FORMAT(361(I2)) The maximum snow albedo represents a fixed annual field. Values are in percentages and represent: 0 : ocean/water flag* 1-99 : maximum snow albedo (percent)+ *land-sea mask identical with Matthews 4-season snowfree albedo database. +max snow albedo values equal to or greater than Matthews snowfree albedo. REFERENCE: Robinson, D. A., and G. Kukla, 1985: Maximum surface albedo of seasonally snow-covered lands in the northern hemisphere. J. Clim. Appl. Meteorol., 24, 402-411. ------- Discussion of steps to create current NCEP/EMC version of the 1-deg x 1-deg maximum snow albedo database: The original 1-deg x 1-deg maximum snow albedo database was generated for 90N-20N (more precisely: 89.5N-20.5N), with a zero for a sea flag. Additionally, there were a number of zeros (sea flag) in obvious land regions (e.g. Mexico), apparently the case when there was an inqdequate data sample size used to evaluate maximum snow albedo at a given location. 1. To populate a global 1-deg x 1-deg max snow albedo database, the region 20N-90S was filled using the SiB vegetation class database in the following manner: the 90N-20N region of the maximum snow albedo database was correlated with the SiB vegetation class over this region to determine any pattern ('binning' max snow albedo for a given veg class then averaging, noting ranges). Presumably the forested regions might have a lower maximum snow albedo (more shading of the snowpack under the canopy) than non-forested regions (more open ground/exposed snowpack). After an inspection of the results, for the 20N-90S region the maximum snow albedo was set according to the SiB vegetation class as follows: max snow SiB vegetation class and description albedo ------ ------------------------------------ 32* 1 Broadleaf-evergreen trees (tropical forest) 58 2 Broadleaf-deciduous tress 53 3 Broadleaf and needleleaf tress (mixed forest) 52 4 Needleleaf-evergreen trees 44 5 Needleleaf-deciduous tress (larch) 45 6 Broadleaf tress with groundcover (savanna) 64 7 Groundcover only (perennial) 69 8 Broadleaf shrubs with perennial groundcover 69 9 Broadleaf shrubs with bare soil 58 10 Dwarf trees and shrubs with groundcover (tundra) 69 11 Bare soil 61 12 Cultivations (The same parameters for the class 7) 75+ 13 Glacial *no max snow albedo values for this veg class, so the value was set to the maximum Matthews snowfree albedo. +set to value corresponding to Matthews glacial 'snowfree' albedo. 2. The max snow albedo database should have an identical land-sea mask as the Matthews 4-season snowfree albedo, with the Matthews albedo the 'master'. (Note: the Matthews 4-season snowfree albedo database is self-consistent, that is, the land-sea mask is identical for all 4 seasons.) Both the SiB veg class land-sea mask and the max snow albedo are not entirely consistent with the Matthews snowfree albedo as follows: L A N D - S E A M A S K S count maximum notes/ snowfree max snow SiB veg snow alb resolution albedo albedo class value ------- -------- ------- ----- --------- ---------- sea sea sea 14079 0 # % A sea * sea 28149 0 * % A sea sea land 370 0 # & A sea * land 120 0 * & A sea land sea 96 0 $ % A sea land land 127 0 $ & A land land land 15401 >0 # % B land land sea 120 >0 # & B land sea land 1638 >0 $ % C land * land 4737 >0 * % C land sea sea 61 >0 $ & D land * sea 82 >0 * & D total count: =361x180 = 64980 notes: * = out of max snow albedo database domain # = consistent with max snow albedo within database domain $ = inconsistent with max snow albedo within database domain % = consistent with SiB veg class & = inconsistent with SiB veg class resolution: A = mxsnal from Matthews snowfree albedo database (sea) B = mxsnal from max snow albedo database (land) C = mxsnal from fct(SiB veg class) from item 1 above (land) D = cannot use 'C' since this corresponds to Matthews=sea, so mxsnal from mxsnal nearest neighbor search (within i+/-1,j+/-1). Remaining 16 isolated points (islands) had to be filled in by hand using a larger subjective search radius: i j lat lon orig max new max snow alb snow alb --- --- ----- ----- -------- -------- 327 15 75.5 146.0 0.0 80.0 328 15 75.5 147.0 0.0 80.0 104 23 67.5 -77.0 0.0 76.0 105 23 67.5 -76.0 0.0 76.0 98 28 62.5 -83.0 0.0 80.0 192 35 55.5 11.0 0.0 60.0 213 56 34.5 32.0 0.0 54.0 103 72 18.5 -78.0 0.0 32.0 114 72 18.5 -67.0 0.0 32.0 305 89 1.5 124.0 0.0 32.0 308 89 1.5 127.0 0.0 32.0 309 90 0.5 128.0 0.0 32.0 336 97 -6.5 155.0 0.0 32.0 306 99 -8.5 125.0 0.0 45.0 307 99 -8.5 126.0 0.0 45.0 359 108 -17.5 178.0 0.0 53.0 An additional 5 points with anomalous values were also changed: i j lat lon orig max new max snow alb snow alb --- --- ----- ----- -------- -------- 182 54 36.5 1.0 15.5 32.0 201 58 32.5 20.0 15.5 32.0 303 90 0.5 122.0 15.0 32.0 304 90 0.5 123.0 15.0 32.0 259 50 40.5 78.0 94.0 74.3 -- Michael Ek NCEP/EMC 07 Dec 2000