REMOTE SENSING - RETRIEVAL OF THE SURFACE ALBEDO

1. REMOTE SENSING - RETRIEVAL OF THE SURFACE ALBEDO Karthaus, September 2005 Wouter Greuell IMAU, Utrecht, NL • Why? • Cloud masking • Retrieval method • An application: estimate surface mass balance from satellite data

2. WHY ? (Surface) albedo = fraction of short-wave radiation reflected by the surface Important for surface mass balance: Net short-wave radiation dominates energy balance Large variations Positive feedback between albedo and mass balance Spatial variations cannot be measured at surface Satellite data But: conversion of measured signals into albedo can cause considerable errors

3. SATELLITE NARROWBANDS Spectral response: no block shape !

4. DATA PROCESSING • geolocation • cloud mask • conversion of signal into albedo

5. CLOUD MASK Difficult over snow and ice Reason: albedo of clouds is similar to that of the surface temperature of low clouds is similar to that of the surface Conclusion: sensor with band around 1.64 µm most suitable But: no such sensor available for AVHRR and MISR Method: use (multiple) thresholds

6. CALIBRATION OF SATELLITE SENSORS Why? Because of post-launch degradation of sensors. Two methods 1) On-board: by measurement of solar radiation reflected from a panel with known reflectance 2) Ground target by measurement of radiance reflected by a ground target with constant optical properties (e.g. desert, dry-snow on polar ice sheets)

7. UNCERTAINTY IN CALIBRATION

8. ABSORPTION AND SCATTERING - ATMOSPHERE Relevant processes: Absorption: mainly by ozone and water vapour; little by aerosols Scattering: gas molecules (Rayleigh scattering), aerosols

9. ATMOSPHERIC CORRECTION Aim: calculate the surface albedo (as) as a function of the planetary albedo (ap) is a function of: aerosols, H2O, O3 solar zenith angle (qs) view zenith angle (qv) elevation (z) Usual approach: calculate ap with a radiative transfer model for a (large) number of as, aerosol loads, H2O amounts, etc. Store results in a Look Up Table (LUT), which is used during the retrieval.

10. REFLECTION VARIES WITH VIEW ANGLE

11. ANISOTROPIC REFLECTION AT THE SURFACE isotropic reflection: intensity of reflected radiation does not vary with the direction of reflection Lambertian surface reflects radiation isotropically Snow and ice reflect radiation anisotropically Why important ? Satellite measures reflected radiation from a single direction (radiance), but we are interested in all the reflected radiation (flux) Correction needed! Directional distribution of the reflected radiation is described by functions called Bi-directional Reflectance Distribution Functions (BRDFs)

12. CO-ORDINATE SYSTEM BRDF s solarzenith angle  zenith angle  azimuth angle

13. EXAMPLE OF A MEASURED BRDF TM2 (green light); solar zenith angle =50˚; albedo = 0.52 Zenith view angle of 75˚ (90˚ = horizontal)  = 180˚ forward scattering  = 0˚ Azimuth direction of sun = backward scattering Nadir = Straight down

14. SPECTRAL ALBEDOS AND NARROWBANDS

15. NARROWBAND TO BROADBAND (NTB) CONVERSION narrowband: spectral band in which the satellite sensor is sensitive to radiation broadband: entire solar spectrum would not be necessary if the albedo was constant with wavelength would have been easy if the narrowbands covered the entire broadband: : broadband albedo i: the narrowband albedo in band i Si: incoming solar radiative flux in band i.

16. BAND ALBEDO MEASUREMENTS BAND ALBEDO MEASUREMENTS

17. EQUATION FOR NTB CONVERSION bb = 0.539 TM2 + 0.166 TM4 ( 1 + TM4 ) Solution: develop equations from simultaneous and coincident measurements of narrowband and broadband albedos

18. RETRIEVAL METHOD converts satellite counts into surface albedo

19. ACCURACY RETRIEVAL METHOD Weakest processing steps retrieval method: insufficient knowledge BRDFs of snow and ice uncertainty in calibration coefficients More accurate processing steps of the retrieval method: NTB conversion 2) atmospheric correction in relatively dry and clean polar and high-mountains atmosphere

20. VALIDATION OF SATELLITE-DERIVED ALBEDO

21. VARIOUS SATELLITE SENSORS

22. SURFACE ALBEDO GREENLAND Surface albedo derived from AVHRR data western margin of the Greenland ice sheet 16 July 1995

23. ESTIMATE SURFACE MASS BALANCE FROM SATELLITE-DERIVED ALBEDO

24. BASIC IDEA OF THE METHOD B: mean annual surface mass balance <Qabs,sw>: potential absorbed short-wave radiative flux I0: extraterrestrial incoming short-wave radiative flux : satellite-derived surface albedo (glacier mean) day 1 - day 2: beginning and end of ablation season

25. WHY COULD METHOD BE SUCCESSFUL? • lower albedo  more melt; absorbed short-wave radiation largest contribution to melt • more melt  lower albedo • less accumulation  lower albedo

26. CONVERSION INTO ENERGY BALANCE EQUATION AND THEN INTO MASS Add transmission of the atmosphere (tatm): Add long-wave and turbulent fluxes (Q0): Convert energy into mass (Lf is latent heat of fusion):

27. ABSORBED RADIATION 1995

28. RELATION SATELLITE-DERIVED WITH MEASURED MASS BALANCE Mean 13 years: Measured: -1202 mm w.e. Satellite: -1142 mm w.e. Interannual variability (standard deviation): Measured: 366 mm w.e. Satellite: 395 mm w.e.

29. SUM UP • Why • Cloud masking • Retrieval method • An application: estimate surface mass balance from satellite data

30. CLOUD MASK (2) Scheme developed for AVHRR data Antarctica: multiple tests; if cloudy according to one test, then pixel is cloudy

31. CLOUD MASK (3) • Comments: • a single test is not enough • each test has physical basis • threshold values are variable in space and are subjectively chosen • combination of tests is subjectively chosen • - low clouds over snow and ice remain difficult to detect

32. ALBEDO OF A STABLE TARGET FROM AVHRR Dry-snow area at top of Greenland ice sheet NOAA 14 NOAA 11 NOAA 16

33. EXPERIMENTAL SET UP TO MEASURE BRDFs pyranometers measure flux (irradiance) pyrheliometers measure radiance