SUR_ALBD.DOC
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.
\DATA\VEGTATN\SUR_ALBD\YyyMmm.sfx
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. |
| colostate.edu |
______________|__________________________________|
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.
ACKNOWLEDGMENTS
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
(1987).
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
classification.
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
enters.
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.
5.2.1.1 Tolerance.
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
calculation.
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
ARRAY(I,J)
I = 1 IS CENTERED AT 179.5W
I INCREASES EASTWARD BY 1 DEGREE
J = 1 IS CENTERED AT 89.5N
J INCREASES SOUTHWARD BY 1 DEGREE
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
ARRAY(360,180)
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
CD-ROM).
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
CD-ROM.
- Leaf area index (LAI), derived from the NDVI data, also on this
CD-ROM.
- 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.
None.
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.
None.
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
Climate.
Sellers, P.J., 1985. Canopy reflectance, photosynthesis and
transpiration. International Journal for Remote Sensing,
6:1335-1372.
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
Internet: daacuso@eosdata.gsfc.nasa.gov
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
Internet: daacuso@eosdata.gsfc.nasa.gov
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: daac.gsfc.nasa.gov
Node number: 192.107.190.139
Login example: telnet daac.gsfc.nasa.gov
Username: daacims
password: gsfcdaac
You will be asked to register your name and address during your first
session.
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. OUTPUT PRODUCTS AND AVAILABILITY
14.1 Tape Products
None.
14.2 Film Products
None.
Not available at this revision.
14.3 Other Products
None.
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)