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

WHY ? (Surface) albedo =fraction of short-wave radiation reflected by the surface Important for surface mass balance: 1)Net short-wave radiation dominates energy balance 2)Large variations 3)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

Spectral response: no block shape ! AVHRR band nr Wavelength range (µm) SATELLITE NARROWBANDS

DATA PROCESSING -geolocation -cloud mask -conversion of signal into albedo

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 CLOUD MASK

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) CALIBRATION OF SATELLITE SENSORS

UNCERTAINTY IN CALIBRATION

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

is a function of: aerosols, H 2 O, O 3 solar zenith angle (  s ) view zenith angle (  v ) elevation (z) Aim:calculate the surface albedo (  s ) as a function of the planetary albedo (  p ) Usual approach: calculate  p with a radiative transfer model for a (large) number of  s, aerosol loads, H 2 O amounts, etc. Store results in a Look Up Table (LUT), which is used during the retrieval. ATMOSPHERIC CORRECTION

REFLECTION VARIES WITH VIEW ANGLE

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) ANISOTROPIC REFLECTION AT THE SURFACE

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

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

SPECTRAL ALBEDOS AND NARROWBANDS

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 S i : incoming solar radiative flux in band i. NARROWBAND TO BROADBAND (NTB) CONVERSION

BAND ALBEDO MEASUREMENTS

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

converts satellite counts into surface albedo RETRIEVAL METHOD

Weakest processing steps retrieval method: 1)insufficient knowledge BRDFs of snow and ice 2)uncertainty in calibration coefficients More accurate processing steps of the retrieval method: 1)NTB conversion 2)atmospheric correction in relatively dry and clean polar and high-mountains atmosphere ACCURACY RETRIEVAL METHOD

VALIDATION OF SATELLITE-DERIVED ALBEDO

SensorTMAVHRRMODISMISRASTER full name Thematic Mapper Advance Very High Resolution Radiometer MODerate resolution Imaging Spectroradio meter Multi-angle Imaging SpectroRadio meter Advanced Spaceborne Thermal Emission and Reflection radiometer resolution at nadir 30 m1.1 km 250 to 1000 m 275 to 1100 m15 to 90 m coverage (equator) once per 16 days once per day every 1 to 2 days once per 9 days sensors are pointable data since 1982 (MSS since 1972) number of channels remarks measurements from 9 angles VARIOUS SATELLITE SENSORS

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

ESTIMATE SURFACE MASS BALANCE FROM SATELLITE-DERIVED ALBEDO

BASIC IDEA OF THE METHOD B:mean annual surface mass balance :potential absorbed short-wave radiative flux I 0 :extraterrestrial incoming short-wave radiative flux  :satellite-derived surface albedo (glacier mean) day 1 - day 2:beginning and end of ablation season

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

CONVERSION INTO ENERGY BALANCE EQUATION AND THEN INTO MASS Add transmission of the atmosphere (  atm ): Add long-wave and turbulent fluxes (Q 0 ): Convert energy into mass (L f is latent heat of fusion):

ABSORBED RADIATION 1995

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

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

CLOUD MASK (2) Scheme developed for AVHRR data Antarctica: multiple tests; if cloudy according to one test, then pixel is cloudy Band(s)Day / night Type of cloudsPhysics 3 (3.75 µ)daylowwater droplets have higher reflectivity than ice / snow 5 (12.0 µ)bothcold, highclouds have lower temperature than ice / snow 4 (10.8 µ)bothuniform clouds over heterogeneous surface and vice versa and bothcirruscloud transmissivity differs from band 4 to nightfog / low emissivity of water droplets differs with ; not for ice / snow 3 - 5nightpartially- covered pixels ?

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

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

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