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

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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