Standardizing Terminology and Description of Satellite-Derived Land Surface Albedo Products Gabriela Schaepman-Strub European Space Agency (ESA) External.

Standardizing Terminology and Description of Satellite-Derived Land Surface Albedo

Products Gabriela Schaepman-Strub

European Space Agency (ESA) External Fellow

F. Berendse, M. Schaepman, B. van den Hurk, B. Holtslag

Landflux Workshop, Toulouse, May 28-31, 2007

Albedo?

white-sky albedo

black-sky albedo

blue-sky albedo

single scattering albedo

planetary albedo

spectral albedo

broadband albedo

plane albedoplanetary albedo

spherical albedo

true albedo

lunar albedoleaf albedo

surface albedocloud albedo

geometric albedo

radar albedo

photospheric albedo

ground albedo

global albedo

multispectral albedo

visible albedonearinfrared albedo

dust albedo

object albedo

simple albedo

bright-sky albedo apparant albedo

inherent albedoall-sky albedo

“Primarily, the chief trouble in straightening out the matter of albedo has arisen from the fact that the term

itself is used in different senses by different writers and sometimes by the

same writer.”

Louis Bell, 1917

Definitions Applicable to Land Surface Albedo‘Albedo is defined as the ratio of reflected solar shortwave

radiation from a surface to that incident upon it.’ Strugnell and Lucht (2001), J. Climate 14(7), 1360–1376

‘Albedo at some level z of a geophysical system is defined as the ratio between the upward flux density or irradiance exiting that particular level and the downward flux density impinging on that same level z.’

Pinty et al. (2005), J. Atm. Sci. 62, 2580-2591Ei Lr

Current Angular Sampling Concepts

Instantaneous multi-angular viewing (# vz) e.g., MISR (9 cams), Chris (tilting)

Daily composites (# sz)e.g., Meteosat, flight campaigns

Multiple-day composites (#sz, vz)e.g., MODIS (16 days)

ReflectedIncoming

Directional Conical Hemispherical

Directional Bidirectional Directional-conical Directional-hemispherical

Conical Conical-directional Biconical Conical-hemispherical

Hemispherical Hemispherical-directional

Hemispherical-conical

Bihemispherical

Schaepman-Strub et al., RSE (2006). Nicodemus et al. (1977)NASA technical note - standardization (in prep.)

Albedo - Geometrical-Optical Considerations

Satellite observations

BHR‘blue-sky albedo’ambient illum. cond.

DHR‘black-sky albedo’intrinsic property

Physical Description of Reflectance Quantities

Reflectance Terminology – Numerical Implications

MISR - Howland

Site SZ [°] AOD558nm

Mean BHR/ Mean((BHR-DHR)/BHR) [%]

446nm 558nm 672nm 867nm

Howland 27.7 0.10 0.031/ 2.1 0.053/ 1.5 0.028/ 1.1 0.318/ 0.7

Railroad Valley 28.4 0.10 0.095/ 1.7 0.137/ 1.3 0.170/ 0.9 0.238/ 0.7

Mongu 44.6 0.05 0.046/ 0.5 0.078/ 0.3 0.094/ 0.3 0.246/ 0.6

Banizoumbou 24.1 0.31 0.060/ 1.2 0.126/ 1.5 0.176/ 1.4 0.357/ 1.3

41.4 0.11 0.084/ 0.5 0.160/ 0.5 0.261/ 0.6 0.376/ 0.6

Hombori 19.6 0.36 0.108/ 5.1 0.232/ 2.5 0.349/ 1.6 0.412/ 1.2

Avignon 25.2 0.07 0.045/ 2.0 0.075/ 1.4 0.069/ 0.9 0.307/ 0.8

36.9 0.19 0.050/ 0.9 0.081/ 0.9 0.079/ 0.7 0.286/ 0.8

Bordeaux 24.5 0.24 0.059/ 1.5 0.097/ 2.0 0.087/ 1.4 0.320/ 1.2

24.0 0.12 0.048/ 1.8 0.078/ 1.5 0.073/ 1.0 0.304/ 0.9

DHR versus Solar Zenith Angle and BHR(iso) of Snow

BHR

BHRiso BHRiso

Schaepman-Strub et al., RSE (2006)

DHR = BHR = BHRiso~50-57° SZ

From Satellite Observations to Land Surface Albedo Products

r

i r

i

EiLr

Spectral albedo

(BHR, BHRiso, DHR)

Broadband albedo

(BHR, BHRiso, DHR)

Atmospheric correction

Angular modeling

Spectral conversion

Bidirectional reflectance factor

(BRF)

Top-of-atmosphere radiance

• Surface/ atmospheric variability for multiple day composites (e.g., change in snow cover (Davidson and Wang, 2004))

• Cloud cover contamination (Gao et al., 2005)

• Coregistration problems of pixels between several bands, directions, dates of observation

• Missing correction for aerosols / diffuse illumination -> HDRF instead of BRF (e.g., Kriebel, 1976; Asrar and Myneni, 1993; Lewis and Barnsely, 1994; Schaepman-Strub et al., 2006)-> Distortion of BRDF if based on HDRF (Lyapustin and Privette, 1999)

• Assumption of Lambertian reflectors (Hu et al., 1999)

• Limited sampling in view angular domain (Privette et al., 1997; Lucht, 1998)

• BRDF model uncertainties (Weiss et al., 1999)

• Extrapolation to sun angles not included in observations (Privette et al., 1997, Lucht, 1998)

• Limited sampling in spectral domain, uncertainties of spectral conversion(Weiss et al., 1999, Liang et al., 2003)

Spectral albedo

(BHR, BHRiso, DHR)

Broadband albedo

(BHR, BHRiso, DHR)

Atmospheric correctionAtmospheric correction

Angular modelingAngular modeling

Spectral conversionSpectral conversion

Bidirectional reflectance factor

(BRF)

Top-of-atmosphere radiance

• Surface/ atmospheric variability for multiple day composites (e.g., change in snow cover (Davidson and Wang, 2004))

• Cloud cover contamination (Gao et al., 2005)

• Coregistration problems of pixels between several bands, directions, dates of observation

• Missing correction for aerosols / diffuse illumination -> HDRF instead of BRF (e.g., Kriebel, 1976; Asrar and Myneni, 1993; Lewis and Barnsely, 1994; Schaepman-Strub et al., 2006)-> Distortion of BRDF if based on HDRF (Lyapustin and Privette, 1999)

• Assumption of Lambertian reflectors (Hu et al., 1999)

• Limited sampling in view angular domain (Privette et al., 1997; Lucht, 1998)

• BRDF model uncertainties (Weiss et al., 1999)

• Extrapolation to sun angles not included in observations (Privette et al., 1997, Lucht, 1998)

• Limited sampling in spectral domain, uncertainties of spectral conversion(Weiss et al., 1999, Liang et al., 2003)

Standardizing the Description of Satellite Albedo Products Objective

Standardized list for operational albedo products describing observations, processing, a priori information, etc.

Importance Provide relevant product information to user (e.g., ATBD’s

often do not contain all information, are long, not updated). Increase knowledge on albedo products (e.g.,

intercomparison studies, applications (comparison with flux tower measurements)).

Facilitate the selection of adequate albedo product for applications.

Towards a Standardized Product Description

Towards a Standardized Product Description cont.

Schaepman-Strub et al., in prep.

Albedo Changes in the Arctic River Lowlands Determine seasonal variation and longterm albedo

change in the Northeastern Siberian Arctic Tundra and their feedback on vegetation development and permafrost degradation.

Main factors contributing to albedo variation Snow cover (including microscale dynamics) Variation in soil moisture Thawing lakes dynamics Changes in the phenological cycle Vegetation development (e.g., shrub encroachment)

Albedo in Northeastern Siberia – from Potential Changes towards Predictions

Changes per decade in summer atmospheric heating (latent plus sensible heat flux) in Alaskan tundra.

Chapin et al., Science 310, 2005

Effect of Structural Complexity on Land-Surface Energy Exchange

Thompson et al., JVS (15), 2004

Flux Tower Site Chokurdah (NE Siberia)

Courtesy: K. v. Huisteden, Free University of Amsterdam

Selected Area of Interest

First Analysis 2000– MODIS Product Quality Flags

Full inversion Magnitude inv.

0

10

20

30

40

50

60

70

80

90

100

49 57 65 73 81 89 97 105

113

121

129

137

145

153

161

169

177

185

193

201

209

217

225

233

241

249

257

265

DOY 2000

MO

DIS

43B

QA

=0 [%

]

Day J AN F E B MAR AP R MAY J UN J UL AUG S E P OC T NOV DE C 1 1 32 61 92 122 153 183 214 245 275 306 3362 2 33 62 93 123 154 184 215 246 276 307 3373 3 34 63 94 124 155 185 216 247 277 308 3384 4 35 64 95 125 156 186 217 248 278 309 3395 5 36 65 96 126 157 187 218 249 279 310 3406 6 37 66 97 127 158 188 219 250 280 311 3417 7 38 67 98 128 159 189 220 251 281 312 3428 8 39 68 99 129 160 190 221 252 282 313 3439 9 40 69 100 130 161 191 222 253 283 314 344

10 10 41 70 101 131 162 192 223 254 284 315 34511 11 42 71 102 132 163 193 224 255 285 316 34612 12 43 72 103 133 164 194 225 256 286 317 34713 13 44 73 104 134 165 195 226 257 287 318 34814 14 45 74 105 135 166 196 227 258 288 319 34915 15 46 75 106 136 167 197 228 259 289 320 35016 16 47 76 107 137 168 198 229 260 290 321 35117 17 48 77 108 138 169 199 230 261 291 322 35218 18 49 78 109 139 170 200 231 262 292 323 35319 19 50 79 110 140 171 201 232 263 293 324 35420 20 51 80 111 141 172 202 233 264 294 325 35521 21 52 81 112 142 173 203 234 265 295 326 35622 22 53 82 113 143 174 204 235 266 296 327 35723 23 54 83 114 144 175 205 236 267 297 328 35824 24 55 84 115 145 176 206 237 268 298 329 35925 25 56 85 116 146 177 207 238 269 299 330 36026 26 57 86 117 147 178 208 239 270 300 331 36127 27 58 87 118 148 179 209 240 271 301 332 36228 28 59 88 119 149 180 210 241 272 302 333 36329 29 60 89 120 150 181 211 242 273 303 334 36430 30 90 121 151 182 212 243 274 304 335 36531 31 91 152 213 244 305 366

First Results – MODIS Black-Sky Albedo (DHR) - 2000

100 120 140 160 180 200 220 240 2600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1green

BS

A

DOY

100 120 140 160 180 200 220 240 2600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1NIR

BS

A

DOY

100 120 140 160 180 200 220 240 2600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1red

BS

A

DOY

100 120 140 160 180 200 220 240 2600

0.1

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0.3

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0.9

1Shortwave

BS

A

DOY

MO

DIS

DH

R (

NIR

)

MO

DIS

DH

R (

bro

adb

an

d)

MO

DIS

DH

R (

gre

en)

100 120 140 160 180 200 220 240 2600

5

10

15

20

25

30

35

40

45

50snow height

DOY

Sn

ow

heig

ht

[cm

]

MO

DIS

DH

R (

red)

Arctic Land Surface Albedo in IPY (endorsed only) Trischenko, Fernandes, CCRS (PI)

‘Surface Albedo Feedback and Energy Budget in the Arctic (SAFE-Arctic)’ - restriction to the Western arctic

Oerbaek (PI), Norwegian Polar Institute‘Network for ARCtic Climate and Biological DIVersity Studies’8 selected sites (Europe, Ca, USA, Russia)

Williams (PI), Univ. of Edinburgh‘Arctic Biosphere-Atmosphere Coupling across multiple Scales’ (Sweden, Finland)

Developments and Challenges Intercomparison of albedo products of different

sensors Spatially continuous fields (multiple-sensor

approach?) Long-term continuity Identification of adequate spectral resolution of

albedo in e.g., climate models and corresponding implementation

-> CEOS/LPV albedo workshop planned in 2008 -> Vegetation development (e.g., shrub encroachment)

is a transitional effect - satellite-derived albedo is a more adequate description than albedo assignment based on discrete land cover classes.

From Prediction to Mitigation Strategies….

Time for Discussion!

MODIS DHR versus BHRiso