BSRN Validation for GEWEX/ISCCP R. T. Pinker Department of Meteorology University of Maryland...
-
Upload
gyles-sullivan -
Category
Documents
-
view
215 -
download
3
Transcript of BSRN Validation for GEWEX/ISCCP R. T. Pinker Department of Meteorology University of Maryland...
BSRN Validation for GEWEX/ISCCP
R. T. Pinker Department of Meteorology
University of Maryland College Park, MD
CEOS/WGCV Land Product Validation Workshop on Albedo April 27-28, 2005EGU 2005 Vienna
Highlights
What is available on global scale from GEWEX Scale issues-same satellites, different spatial gridding Scale issues-different satellites Evaluation of products
Discussion in context of :
CEOS/WGCV Land Product WorkshopBoston, 23-24 October, 2002
Error budgets o Scaling may be much larger source of error than calibration or narrow to broadband transformation.
What to measure-albedo; reflectance; spectral intervals
o The scientific community has primary interest in the total albedo. Therefore, it is recommended that priority should be given to measurements of this parameter.
o There is also interest in spectral albedo, both broadband and narrow band.
o Broadband intervals of interest are the visible and NIR for which instrumentation of acceptable quality is available.
o High spectral resolution albedo or BRDF are also of interest in building spectral libraries and for evaluating narrow band albedo from satellites.
Global distribution of surface SW radiation at 0.5 degree, Jan 1992Both downward and upward fluxes are computed, allowing derivation of albedo
What we do
Approach to Derive SW
o Start with spectral reference albedo models as boundary conditionso Compute surface upwelling and down- welling fluxes: Clear conditions All conditions
Compute spectrally for:
diffuse total
Albedo: ratio of upwelling to down -welling flux
Narrow to broad band transformations-updates
To derive angularly and seasonally dependent relationships between narrowband reflectance and broadband albedo, under clear sky conditions, as observed from the several different satellites. The simulations will utilize surface types based on the University of Maryland land cover classifications.
Extensive model simulations with MODTRAN 3.7
10 solar zenith angle bins
16 gaussian points in zenith and 8 in zenith direction to obtain the spectral
flux
20 climatological profiles for temperature, water vapor and ozone in 4
seasons
Newly developed spectral surface albedo models
Objective
Narrow to broadband transformations are based on:
Evergreen Needle leaf Forest 0.74 0.39 0.14 ASTER
Evergreen Broadleaf Forest 0.83 0.46 0.18 ASTER
Deciduous Needle leaf Forest 0.74 0.39 0.14 ASTER
Deciduous Broadleaf Forest 0.83 0.46 0.18 ASTER
Mixed Forest 0.79 0.42 0.16 ASTER
Woodlands 0.35 0.25 0.09 ASTER/Bowker
Wooded Grasslands 0.36 0.32 0.12 ASTER/Bowker
Closed Bushland/Shrubland 0.37 0.36 0.13 ASTER/Bowker
Open Shrubland 0.19 0.22 0.07 ASTER/Bowker
Grassland 0.07 0.19 0.03 ASTER
Cropland 0.54 0.51 0.20 ASTER/Bowker
Barren 0.55 1.35 1.13 ASTER/Bowker
Urban ASTER
Updated Surface Spectral Reflectance ModelsCompatible with Current Global Land Cover ClassificationsScaling Factors for the surface types according to UMD land cover classification
Ch1 Ch2 Ch2Scaling Scaling Scaling Source
Surface type factor factor factor(0.3-0.5) (1.3-2) (2-4)
Channel 2
Channel 1
What albedo products are available:*
At global scale
ISCCP D1 UMD GEWEX/SRB, 2.5 deg, 1983-2001 (There are other estimates based on ISCCP D1, such as: LaRC GEWEX/SRB; ISCCP D1-FD) MODIS 1 deg, UMD SRB model, about 3 yearsAt continental scale
ISCCP DX based estimates, 0.5 deg; 1990-2001
At regional scale
GOES based estimates at 0.5 degree for the US; 1996-current Improved above product for 1996-2000 at 0.5 and 1/8 deg resolution GOES based estimates at 0.5 deg, Amazon basin for 1998-2000.* This is not a comprehensive review of everything available
Examples of low resolution albedo products
.20
.19
.18
.17
.16
.15
.14
.13
.12
.11
.10
.09
.08
.07
.06
.05
.04
.20
.19
.18
.17
.16
.15
.14
.13
.12
.11
.10
.09
.08
.07
.06
.05
.04
Based on GOES pixel level data; available for 1998-2000
Manaus:
Reserva Ducke
Fezenda Dimona
Ji-Parana:
Reserva Jaru
Fazenda Nossa Senhora da Apparacida
Maraba:
Reserva Vale do Rio Doce
Fazenda Boa Sorte
• Forest
• Pasture
Annual mean
Variability among sites
Seasonal variability
Forest Obs 0.133 0.122-0.147 Weak, higher in dry, lower in wet season
GOES 0.088 0.079-0.104 Very weak
DX 0.113 0.103-0.129 Lower in Jun/Jul/Aug
Pasture Obs 0.176 0.169-0.193 Stronger,
Lower in dry season
GOES 0.090 0.075-0.100 Very weak
DX 0.116 0.102-0.127 Lower in Jun/Jul/Aug
Analysis based on Berbet, M. et al. (2003); observations made in 1990-1993 (Culf et al., 1996). The observation uncertainty of the order of 0.006 (Wright et al. 1996). GOES and DX estimates are available from 1998/03 – 2001/02, and from 1998/07 – 2001/02, respectively.
• Both observed and satellite derived albedo are generally lower in dry and higher in wet months.
• Satellite driven albedo show smaller difference between the vegetation cover, and weak seasonal variability.
Based on GOES pixel level
1/2 deg
1/8th deg
GOES 1/8th
ISCCP DX, 0.5 deg
Albedo based on MODIS using ratio of surface fluxes
The surface parameters used to calculate spectral surface albedo needed in the inference scheme for SRB were taken from the MODIS Bidirectional Reflectance Distribution Function (BRDF) and Albedo Product (MOD43B) at the 0.25° resolution (Lucht et al., 2000; Schaaf et al., 2002).The three weighting parameters associated with the RossThickLiSparseReciprocal BRDF model that best describes the anisotropy of each pixel are provided for each of the MODIS spectral bands as well as for the three broad bands (0.3-0.7 µm, 0.7-5.0 µm, and 0.3-5.0 µm). For two broad bands (0.3-0.7 µm, 0.7-5.0 µm), these parameters were used with simple polynomials to estimate the white sky albedo and the black sky albedo for the monthly mean solar zenith angle.
Auxiliary information for driving SRB model: Level-3 MODIS Atmosphere Monthly Global Product at 1° x 1° resolution, processed with the latest collection 4 algorithms for: Cloud_Fraction_Total, Cloud_Optical_Thickness_Combined,Optical_Depth_Land_And_Ocean aerosol, Total_Ozone, and Atmospheric_Water_Vapor Missing aerosol optical depths over arid areas were filled from the MODIS-GOCART integrated monthly aerosol optical depth data, School of Earth and Atmosphere Sciences, Georgia Institute of Technology. Missing cloud optical thickness values were replaced by interpolated values.
One degree spatial resolution
From MODIS, V004 product
MODIS swath data at highest resolution
Evaluation of GOES surface albedos against SURFRAD stations during summer and winter, Fort Pack, MT (0.5 degree)
Evaluation os GOES surface albedo against SURFRAD stations during summer and winter Goodwin Creek, MS (0.5 degree)
Comparison between derived broadband albedo for different satellites
Comparison between the derived broadband surface albedo and broadband surface observations - The methodology will be tested with ground observations at the semi-arid USDA-ARS Walnut Gulch Experimental Watershed in Arizona. The surface observations started in July 1999 and continue up to now. Total short-wave, infrared and PAR upward and downward fluxes are measured at 5 minute intervals.
Comparison between the derived broadband albedo from GOES and broadband albedo observed from a satellite with broad band sensor –
Evaluation
Semi-Arid region: Walnut Gulch, AZ
Ongoing observations in sub-Sahel
Need for ground truth on surface albedo
No information on savannah type vegetation
Summary
o Global products of spectral and total surface albedos are available
o As yet, not fully evaluated
o Usefulness of ground observations for evaluation of low resolution products not obvious-preferably, consistency among satellites important
o Ground observations of albedo important, but should not be expected to match the satellite based albedos
o Documentation of the annual cycle for different surface types important for benchmarking