1.  TITLE

1.1  Data Set Identification. 

     Calculated snow-free albedo.

     (Monthly ; CSU, NASA/GSFC)

1.2  Data Base Table Name.

     Not applicable.

1.3  CD-ROM File Name. 


     Note: capital letters indicate fixed values that appear on the CD-ROM 
     exactly as shown here, lower case indicates characters (values) that 
     change for each path and file.

     The format used for the filenames is: YyyMmm.sfx, where yy is the last 
     two digits of the year (e.g., Y87=1987), and mm is the month of the year 
     (e.g., M12=December).  The filename extension (.sfx), identifies the data 
     set content for the file (see Section 8.2) and is equal to .ALB for this 
     data set.

1.4  Revision Date Of This Document.

     April 5, 1995.

                             2.  INVESTIGATOR(S)

2.1  Investigator(s) Name And Title.

     Donald A. Dazlich
     Department of Atmospheric Science
     Colorado State University
     Fort Collins CO

2.2  Title Of Investigation.

     Earth Observing System - Inter-Disciplinary Science
     project (Sellers - Mooney)

2.3  Contacts (For Data Production Information). 

              |             Contact 1            |
2.3.1 Name    |Donald A. Dazlich                 |
2.3.2 Address |Department of Atmospheric Science |
              |Colorado State University         |
      City/St.|Fort Collins CO                   |
      Zip Code|80523                             |
2.3.3 Fax     |(303) 491-8428                    |
2.3.4 Email   |dazlich@erehwon.atmos.            |
              |                  |
NOTE: Providing information on these data is not part of my daily routine; 
please read literature and descriptions prior to asking questions. Allow for 
some delay in answering of questions.

2.4  Requested Form of Acknowledgment.

     Please cite the following publications when ever these data are used:

      Sellers, P.J., D.A. Randall, C.J. Collatz, J.A. Berry, C.B. Field, D.A. 
          Dazlich, C. Zhang, and C.D. Collelo, 1995a. A revised land surface 
          parameterization (SiB2) for atmospheric GCMs. Part 1: Model 
          formulation. submitted to Journal of Climate.
      Sellers, P.J., S.O. Los, C.J. Tucker, C.O. Justice, D.A. Dazlich, G.J. 
         Collatz, and D.A. Randall, 1995b. A revised land surface 
         parameterization (SiB2) for atmospheric GCMs. Part 2: The generation 
         of global fields of terrestrial biophysical parameters from satellite 
         data. Submitted to Journal of Climate.


     This research was funded by the NASA Earth Observing System Inter 
     disciplinary science (EOS-IDS) program, Sellers-Mooney team (contract 
     NAS-531732). Sietse Los provided the NDVI derived parameters and biome 
     classification map necessary to make the calculations for this data set.

                            3.  INTRODUCTION

3.1  Objective/Purpose.

     Normalized Difference Vegetation Index (NDVI) derived parameters for the 
     24 months January 1987 through December 1988 were used to calculate 
     monthly mean surface albedos at 1 X 1 degree resolution for vegetated 
     land surfaces (Sellers et al, 1995b). The CSU GCM (Randall et al, 1989) 
     was used to make this calculation by synthesizing the Simple Biosphere 
     model (SiB2) (Sellers et al, 1995a) which used the NDVI based biophysical 
     parameters leaf area index (LAI), and green fraction of vegetation 
     (Greenness), with the radiation parameterization of Harshvardhan et al 

3.2  Summary of Parameters. 

     Snow free surface albedo.

3.3  Discussion.

     The surface albedo is a significant physical parameter controlling the 
     flux of energy at the interface between the Earth's surface and the 
     atmosphere and must be prescribed in a General Circulation Model (GCM).  
     It is a function of the reflectance of the underlying soil surface and of 
     the vegetative canopy above. The extent and nature of the canopy varies 
     widely not only with growing season, but year to year as well as it 
     responds to interannual variations in the climate.

     Satellite data provide a way to describe the seasonal and interannual 
     variations in surface albedo solar radiation in the visible and near-
     infrared wavebands, reflected by the Earth's surface and collected by a 
     remote sensing device, can be combined into a spectral vegetation index 
     such as the Normalized Difference Vegetation Index (NDVI) and related to 
     physical properties of vegetation.  In particular, the physical 
     vegetation parameters leaf area index, and green fraction of vegetation 
     can be derived from NDVI. An updated version of SiB, named SiB2 (Sellers 
     et al, 1995a), can calculate the surface albedo for various spectral 
     intervals (visible and near infrared) and incident beams (direct and 
     diffuse) by combining these NDVI derived parameters with other 
     biophysical parameters that are a function of a prescribed vegetation 

     The total-band all-beam surface albedo is an average of the above four 
     components weighted by the incident radiation in each of those bands.  
     These incident components can vary with cloudiness and with solar zenith 
     angle. Further, the albedo components are themselves strongly a function 
     of solar zenith angle. To get a monthly mean surface albedo requires 
     integrating the various instantaneous surface albedos over all times of 
     day and the likely incident radiation.  This is where the use of the GCM 

     The radiation parameterization of the CSU GCM (Harshvardhan et al, 1987) 
     computes the incident surface radiation in the bands where SiB2 defines 
     the surface albedos. Weighted by the solar zenith angle, this model 
     output can be used to integrate and average the surface albedo components 
     from SiB over the diurnal cycle to produce a monthly surface albedo.

     The CSU GCM (Randall et al, 1989) is a finite difference model with 4x5 
     degree horizontal resolution and 17 vertical levels. Features of the 
     model include parameterized convection, solar and terrestrial radiation 
     with diurnal cycle, planetary boundary layer, and SiB2 (Sellers et al, 
     1995a). The solar radiation fields of the GCM are produced using the 
     parameterization of Harshvardhan et al (1987).

                        4.  THEORY OF MEASUREMENTS

Not available at this revision.

                            5.  EQUIPMENT

5.1  Instrument Description.

     Not applicable.

     5.1.1  Platform.

            Not applicable.

     5.1.2  Mission Objectives.

            Not applicable.

     5.1.3  Key Variables.

            Not applicable.

     5.1.4  Principles of Operation.

            Not applicable.

     5.1.5  Instrument Measurement Geometry .

            Not applicable.

     5.1.6  Manufacturer of Instrument.

            Not applicable.

5.2  Calibration.

     Not applicable.

     5.2.1  Specifications.

            Not applicable.


                     Not applicable.

     5.2.2  Frequency of Calibration.

            Not applicable.

     5.2.3  Other Calibration Information.

            Not applicable.

                             6.  PROCEDURE

6.1  Data Acquisition Methods.

     For a particular month, SiB2 calculated the surface albedo components 
     (reflectance) hourly as a function of the time varying input parameters 
     LAI and Greenness, and permanent physiological parameters that are a 
     function of the defined biome classification, and the solar zenith angle. 
     The surface and canopy are assumed to be snow-free. The albedo components 
     are weighted by the cosine of the solar zenith angle and the fraction of 
     incident radiation in the corresponding band. The albedo components are 
     summed over one month with the one hour time step using daily updated 
     solar declination angles. These component sums are added and normalized 
     to obtain the monthly mean surface albedo for the 1 X 1 degree area.  The 
     full procedure is described in Sellers et al., (1995a,b).

     The incident radiation fields are from a recent 10 year integration of 
     the CSU GCM that includes SiB2 and real monthly Sea Surface Temperature 
     data for the years 1979 through 1988, as described by Randall et al 
     (1995). Ten year means for each month  were used in weighting the albedo 

6.2  Spatial Characteristics. 

     6.2.1  Spatial Coverage. 

            The coverage is global.  Data in each file are ordered from North 
            to South and from West to East beginning at 180 degrees West and 
            90 degrees North.  Point (1,1) represents the grid cell centered 
            at 89.5 N and 179.5 W (see section 8.4).

            The Surface Albedo data value coverage is between latitudes of 75
            degrees North and South.  Latitudes higher than 75 degrees (North
            or South) are filler and equal zero.  Data for these regions were
            not available in the GIMMS continental data set.  The calculation
            of surface albedo is made for all vegetated land points (no
            permanent ice cover), as defined by the biome classification
            scheme (see VEG_CLSS.DOC).

            Permanent land ice points are assigned albedo values based on the 
            characteristics of a the bare soil biome classification, and 
            assumed values of APAR=0.001, LAI=0.01 and canopy greenness=0.141.  
            Regions of polar night are assigned an albedo based on the 
            insolation partition of the last month with sunlight and the next 
            month with sunlight and assuming a zenith angle of 89 degrees for 
            calculation of the direct beam albedos.

     6.2.2  Spatial Resolution. 

            The data are given in an equal-angle lat/long grid that has a 
            spatial resolution of 1 X 1 degree lat/long.

6.3  Temporal Characteristics. 

     6.3.1  Temporal Coverage 

            January 1987 through December 1988. 

            (Data acquisition switched from NOAA 9 to 11 in November 1988).

     6.3.2  Temporal Resolution. 

            Monthly mean.

                           7.  OBSERVATIONS

7.1  Field Notes.

     Not applicable.

                           8.  DATA DESCRIPTION

8.1  Table Definition With Comments.

     Not Applicable.

8.2  Type of Data. 

|                 8.2.1                |              |             |          |
|Parameter/Variable Name               |              |             |          |
|    |               8.2.2             |     8.2.3    |    8.2.4    |  8.2.5   |
|    |Parameter/Variable Description   |Range         |Units        |Source    |
|SF_ALBEDO                             |              |             |          |
|    |Snow free surface albedo         |Min = 0.08 $  |[Unitless]*  |          |
|    |(fraction of incident solar      |Max = 0.4 $   |             |          |
|    |radiation reflected by surface)  |              |             |          |
|    |                                 |              |             |          |
$ The minimum and maxmimum Albedo are for land surfaces, rain forest and 
  desert respectively. The values are approximate. The potential range is 0 
  to 1.
* Albedo Units are non-dimensional, a fraction between 0 and 1.

8.3  Sample Data Base Data Record.

     Not applicable.

8.4  Data Format

     The CD-ROM file format is ASCII, and consists of numerical fields of 
     varying length, which are space delimited and arranged in columns and 
     rows.  Each column contains 180 numerical values and each row contain 360 
     numerical values.  

          Grid arrangement

             I  = 1 IS CENTERED AT 179.5W
             J  = 1 IS CENTERED AT 89.5N

             90N - | - - - | - - - | - - - | - -
                   | (1,1) | (2,1) | (3,1) |
             89N - | - - - | - - - | - - - | - -
                   | (1,2) | (2,2) | (3,2) |
             88N - | - - - | - - - | - - - | - -
                   | (1,3) | (2,3) | (3,3) |
             87N - | - - - | - - - | - - - |
                  180W   179W    178W   177W


8.5  Related Data Sets 

     1 X 1 degree Soil Reflectance data, Donald A Dazlich,(on this CD-ROM). 
     1 X 1 degree normalized difference vegetation index (NDVI) global data 
           (on this CD-ROM).
     1 X 1 degree Fourier based adjustment, solar zenith angle correction, 
           interpolation of missing data and reconstruction of evergreen 
           broadleaf land cover types (tropics), (FASIR-NDVI) data (on this 
     1 X 1 degree fraction of photosynthetic active radiation absorbed by the 
           vegetation canopy (FPAR) monthly global data (on this CD-ROM). 
     1 X 1 degree leaf area index (LAI) global data (on this CD-ROM). 
     1 X 1 degree Greenness global data (on this CD-ROM). 
     1 X 1 degree roughness length (Zo) monthly global data (on this CD-ROM).
     1 X 1 degree vegetation classification map (on this CD-ROM).

     Also see section 8.5 (Related Data Sets) in the NDVI document.

                          9.  DATA MANIPULATIONS

9.1  Formulas.

     9.1.1  Derivation Techniques/Algorithms.

            A two-stream model described in Sellers (1985) and Sellers et al 
            (1995a) was used to calculate surface reflectance.

9.2  Data Processing Sequence 

     A full description of the models and procedures used can be found 
     in Sellers (1985), Sellers et al., (1995a, b) and some results are 
     discussed in Randall et al., (1995).  In summary, the following 
     data are inserted into the two stream radiative transfer model.

     -  Soil/background reflectance, these fields are also on this 

     -  Leaf area index (LAI), derived from the NDVI data, also on this 

     -  Canopy greenness factor, derived from the NDVI data, also on 
        this CD-ROM.

     The LAI and canopy greenness factor data are combined with biome-
     dependent optical and meteorological data in SiB2 to estimate the 
     density, geometrical arrangement and spectral properties of canopy 
     phyto-elements. These and the soil/background information are used 
     by the SiB2 2-stream model to calculate the direct beam and diffuse, 
     visible and near-infrared radiation transfer within the canopy soil 
     system. Canopy reflectance are weighted by the incoming fluxes, as 
     calculated by the model of Harshvardhan et al. (1987), and summed to 
     provide an estimate of surface albedo.

     9.2.1  Processing Steps and Data Sets 

            See section 9.2.

     9.2.2  Processing Changes.


9.3  Calculations.

     See Sellers (1985), Sellers et al., (1995a,b).

     9.3.1  Special Corrections/Adjustments.

            See references in section 9.3.

9.4  Graphs and Plots.

     The performance of the two-stream model (using in situ parameters rather 
     than satellite data inputs) is assessed in Dorman and Sellers (1989) for 
     a few site-specific studies. Sellers et al. (1995b) and Randall et al., 
     (1995) reproduce and discuss global fields.

                                10.  ERRORS

10.1  Sources of Error.

      The calculation procedure assumes that the fraction of incident 
      radiation in each band does not change with time of day. It is also 
      assumed that the magnitude of the incident radiation varies with time of 
      day according to the cosine of the zenith angle. In fact, due to diurnal 
      variations in cloudiness, there can be large diurnal variations in the 
      partition of incident radiation among the bands and large deviations in 
      magnitude from the assumed weighting by the cosine of the zenith angle.

      To evaluate the magnitude of the errors due to these assumptions, the 
      surface albedo calculated for this method at 4x5 resolution using 1987 
      NDVI data where compared to those actually computed from a CSU GCM 
      simulation using the same data. The maximum difference in surface albedo 
      was 3%, the RMS difference was 1%, and there was no systematic error.

      Error is also introduced due to the fact that GCM output is used to 
      prescribe the radiation fields rather than real data. While data is not 
      available to validate the partition of the radiation, the CSU GCM has 
      been used successfully in several studies of the Earth's radiation 
      budget (Randall et al, 1995; Harshvardhan et al, 1987). An attempt will 
      be made in late 1994/early 1995 by Los (see section 2.3) to compare some 
      of the global points against published field data.

10.2  Quality Assessment. 

      The performance of the two-stream model (using in situ parameters rather 
      than satellite data inputs) is assessed in Dorman and Sellers (1989) for 
      a few site-specific studies. Sellers et al., (1995b) and Randall et al., 
      (1995) reproduce and discuss global fields.

      10.2.1  Data Validation by Source. 

              See reference in section 10.2 above.

      10.2.2  Confidence Level/Accuracy Judgment. 

              See reference in section 10.2 above.

      10.2.3  Measurement Error for Parameters and Variables.

              See reference in section 10.2 above.

      10.2.4  Additional Quality Assessment Applied.

              See reference in section 10.2 above.

                                11.  NOTES 

11.1  Known Problems With The Data.

      Experience has shown that the two-stream model tends to overestimate 
      reflectances for tall deciduous canopies, as it does not account well 
      for the effects of radiation trapping by 'holes' in the canopy, see 
      Sellers et al., (1989). This effect probably gives rise to errors on the 
      order of 3% or less absolute. 

11.2  Usage Guidance. 

      The NDVI data set reflects global patterns of vegetation, however, 
      serious errors may be present in the data set that could limit the 
      validity of conclusions, especially for specific locales.

11.3  Other Relevant Information.

      Dorman and Sellers (1989) and Sellers et al., (1989) published results 
      using the two-stream model where all the input parameters were obtained 
      directly or indirectly from in situ measurements, leaf area index, 
      greenness, soil reflectance, vegetation optical properties, etc.

      In the fields on this CD-ROM, the model was forced using satellite data 
      to define leaf area index, canopy greenness factors and soil 
      reflectance, as described in Sellers et al., (1995a,b) and Randall et 
      al., (1995). It is argued in their publications that these albedo 
      fields should be a considerable improvement over the previous ones 
      published in Dorman and Sellers (1989).

                            12.  REFERENCES 

12.1  Satellite/Instrument/Data Processing Documentation. 


12.2  Journal Articles and Study Reports. 

      Dorman, J.L. and P.J. Sellers, 1989. A global climatology of albedo, 
          roughness length and stomatal resistance for atmospheric general 
          circulation models as represented by the simple biosphere model 
          (SiB). J.A.M., 28(9):833-855.
      Harshvardhan, R. Davies, D.A. Randall, and T.G. Corsetti, 1987. A fast 
          radiation parameterization for general circulation models. J. 
          Geophys. Res., 92:1009-1016.
      Los, S.O.,  C.O. Justice, C.J. Tucker, 1994. A global 1 by 1 degree NDVI 
          data set for climate studies derived from the GIMMS continental NDVI 
          data. International Journal of Remote Sensing, 15(17):3493-3518. 
      Randall, D.A., Harshvardhan, D.A. Dazlich, and T.G. Corsetti, 1989. 
          Interactions among radiation, convection, and large-scale dynamics 
          in a general circulation model. J. Atmos. Sci., 46:1943-1970.
      Randall, D.A., P.J. Sellers, J.A. Berry, D.A. Dazlich,  C. Zhang, G.J. 
          Collatz, 1995. A  revised land surface parameterization 
          (SiB2) for atmospheric GCMs. Part 3: The greening of the Colorado 
          State University general circulation model. submitted to Journal of 
      Sellers, P.J., 1985. Canopy reflectance, photosynthesis and 
          transpiration. International Journal for Remote Sensing, 
      Sellers, P.J., J.W. Shuttleworth, J.L. Dorman, A. Dalcher and J.M. 
          Roberts, 1989. Calibrating the simple biosphere model (SiB) for 
          Amazonian tropical forest using field and remote sensing data: Part 
          1, average calibration with field data. J.A.M., 28(8):727-759.
      Sellers P.J., J.A. Berry, G.J. Collatz, C.B. Field and F.G. Hall, 1992. 
          Canopy reflectance, photosynthesis and transpiration, III. A 
          reanalysis using enzyme Kinetics-electron transport models of leaf 
          physiology. Remote Sensing of Environment, 42:187-216.
      Sellers, P.J., D.A. Randall, C.J. Collatz, J.A. Berry, C.B. Field, D.A. 
          Dazlich, C. Zhang, and C.D. Collelo, 1995a. A revised land surface 
          parameterization (SiB2) for atmospheric GCMs. Part 1: Model 
          formulation. Submitted to Journal of Climate.
      Sellers, P.J., S.O. Los, C.J. Tucker, C.O. Justice, D.A. Dazlich, G.J. 
         Collatz, and D.A. Randall, 1995b. A revised land surface 
         parameterization (SiB2) for atmospheric GCMs. Part 2: The generation 
         of global fields of terrestrial biophysical parameters from satellite 
         data. Submitted to Journal of Climate.

12.3  Archive/DBMS Usage Documentation.

      Contact the EOS Distributed Active Archive Center (DAAC) at NASA Goddard 
      Space Flight Center (GSFC), Greenbelt Maryland (see Section 13 below).
      Documentation about using the archive or information about access to the 
      on-line information system is available through the GSFC DAAC User 
      Services Office.

                             13.  DATA ACCESS

13.1  Contacts for Archive/Data Access Information.

      GSFC DAAC User Services
      NASA/Goddard Space Flight Center
      Code 902.2
      Greenbelt, MD 20771

      Phone:     (301) 286-3209
      Fax:       (301) 286-1775

13.2  Archive Identification.

      Goddard Distributed Active Archive Center
      NASA Goddard Space Flight Center
      Code 902.2
      Greenbelt, MD 20771

      Telephone:  (301) 286-3209
      FAX:        (301) 286-1775

13.3  Procedures for Obtaining Data.

      Users may place requests by accessing the on-line system, by sending 
      letters, electronic mail, FAX, telephone, or personal visit.

      Accessing the GSFC DAAC Online System:

      The GSFC DAAC Information Management System (IMS) allows users to 
      ordering data sets stored on-line.  The system is open to the public.

      Access Instructions:

      Node name:
      Node number:
      Login example: telnet
      Username:  daacims
      password:  gsfcdaac

      You will be asked to register your name and address during your first

      Ordering CD-ROMs:

      To order CD-ROMs (available through the Goddard DAAC) users should 
      contact the Goddard DAAC User Support Office (see section 13.2).

13.4  GSFC DAAC Status/Plans.

      The ISLSCP Initiative I  CD-ROM is available from the Goddard DAAC.


14.1  Tape Products


14.2  Film Products 


      Not available at this revision.

14.3  Other Products 


                         15.  GLOSSARY OF ACRONYMS

AVHRR            Advanced Very High Resolution Radiometer
CD-ROM           Compact Disk (optical), Read Only Memory
CSU              Colorado State University
DAAC             Distributed Active Archive Center
EOS              Earth Observing System
GCM              General Circulation Model of the atmosphere
FASIR (NDVI)     Fourier Adjusted, Solar zenith angle correction,
                 Interpolation (of missing data during winter), and
                 Reconstruction of NDVI over tropical forests.
GSFC             Goddard Space Flight Center
IDS              Inter disciplinary Science
ISLSCP           International Satellite Land Surface Climotology Project
LAI              Leaf Area Index
NASA             National Aeronautics and Space Administration
NDVI             Normalized Difference Vegetation Index
NOAA             National Oceanographic and Atmospheric Administration
RMS              Root Mean Square
SiB2             Simple Biosphere model (Sellers et al 1995a)