2009 clarreo meeting at larc 1 information content of satellite remote-sensing measurements:...

2009 CLARREO Meeting at LaRC 1

Information content of satellite remote-sensing measurements:

photopolarimetric vs intensity-only


Why the CLARREO solar instrument needs to be an SI-traceable photopolarimeter with high radiometric accuracy/precision high polarization accuracy/precision thorough multi-angle coverage broad multispectral coverage


Do we need to detect global climate change with CLARREO?

http://data.giss.nasa.gov/gistemp/2007/Fig1_2007annual.gif


CLARREO objective

CLARREO measurements must be fully loaded with information content sufficient to attribute climate change and to constrain models.

Measurements need to be sensitive to specific model parameters (remote-sensing justification of CLARREO measurements) in order to constrain climate models and attribute climate change.

CLARREO measurements must be capable of improving the retrieval accuracy of operational instruments.

CLARREO measurements must be capable of improving the accuracy of measurements with operational instruments.

To obtain and archive a benchmark climate record that is on-orbit SI traceable, and can also serve as a calibration reference for operational satellites


CLARREO science objectives

Aerosol direct effect, its long-term trends and attribution

Aerosol semi-direct effect, its long-term trends and attribution

Land albedo change, its long-term trends and attribution

Aerosol indirect effect, its long-term trends and attribution

↓

The short-wave CLARREO instrument must be a self-sufficient climate-monitoring instrument.


CLARREO science objectives

Recalibration of short-wave radiance-only instruments will not solve the climate-monitoring problem


retrievals

A MODIS-Terra level-2 aerosol pixel and a MISR level-2 aerosol pixel are defined as “fully compatible” if they

are located within the narrower MISR swath;

have been collocated spatially to ±3.3 km and temporally to ±3 min;

have been determined to be “cloud-free” by both cloud-screening procedures; and

have been identified as suitable for aerosol retrieval and have been taken through the standard MODIS and MISR retrieval routines, thereby resulting in specific AOT and AE values.


retrievals

Scatter plots of fully compatible MISR vs. MODIS-Terra AOTs for January 2006. The straight dotted line depicts the 1-to-1 perfect agreement.

J. Quant. Spectrosc. Radiat. Transfer 109, 2376 (2008)


retrievals

J. Quant. Spectrosc. Radiat. Transfer 109, 2376 (2008)

MISR vs. MODIS-Terra Ångström exponents (AEs). The straight dotted lines depict the 1-to-1 perfect agreement.


retrievals

J. Quant. Spectrosc. Radiat. Transfer 110, in press (2009)


retrievals

What makes MODIS and MISR mediocre Vis/SWIR remote sensing instruments is not poor radiance calibration but their inherent insensitivity to aerosol and surface properties.

J. Quant. Spectrosc. Radiat. Transfer 110, in press (2009)


Retrievals with aircraft prototype of APS

Spectral AOT values retrieved from precise polarimetric measurements agree exceedingly well with those measured by ground-based sunphotometers over an AOT range from 0.05 to more than 1.

The absence of spectrally-dependent biases demonstrates the reliability of the size distribution estimate for both small and large modes of a bimodal aerosol distribution.

In situ and retrieved size distributions also agree extremely well (difference in aerosol effective radius of less than 0.04 µm).


Polarized radiances

A beam of light is fully characterized by four polarized radiances (Stokes parameters) having the same dimension of W m–2 sr–1 :

I

−I ≤ Q ≤ I

−I ≤ U ≤ I

−I ≤ V ≤ I

V is usually small and difficult to measure and carries little information.


)(/)( ΘIΘQ

Polarization is very sensitive to aerosol particle size and refractive index


Q and U can be measured in much the same way as I. However, |Q| and |U| are often (much) smaller than I, which often causes a problem.

Example:

I = 1 ΔI = ±0.02

Q = 0.02 ΔQ = ±0.02

U = –0.01 ΔU = ±0.02

Q and U cannot be measured.

Polarimetry


However, the ratios

−1 ≤ Q/I ≤ 1

−1 ≤ U/I ≤ 1

can be measured extremely accurately. Astrophysicists can measure them to ±0.000001. The Glory APS will measure them to ±0.001.

Example:

I = 1 ΔI = ±0.02

Q = 0.02 ΔQ = ±0.001

U = –0.01 ΔU = ±0.001

Q and U can be measured quite accurately despite their small absolute values.

Polarimetry


A high-radiometric-accuracy polarimeter is a high-polarization-accuracy polarimeter.

Example:

I = 1 ΔI = ±0.002

Q = 0.02 ΔQ = ±0.002

U = –0.01 ΔU = ±0.002

Q and U are measured with useful accuracy.

Polarimetry


“Must haves” of an SI-traceable self-sufficient SW photopolarimeter

Precise polarimetry particle size distribution, refractive index, and shape (~0.001):

Wide scattering particle size distribution, refractive index, and shape angle range (i + p):

Multiple (~60) (i) cloud particle size via rainbow angleangles (i + p): (ii) particle size, refractive index, and shape

(iii) ocean surface roughness(iv) aerosol retrievals in cloud-contaminated pixels(v) aerosol retrievals above clouds (semi-direct effect)

Wide spectral range (i) separation of submicron and supermicron particles(i + p): (ii) spectral refractive index chemical composition

1370 nm (i + p): characterization of thin cirrus clouds

2200 nm (p): (i) characterization of the land surface contribution at visible wavelengths(ii) cloud particle sizing


Role of polarization in retrieval cross-calibration


Multi-angle photopolarimetry will provide:

1. Better choice of BRDF model based on polarimetric retrievals

2. Thorough quantitative checks of CERES BRDF models

IPCC, 2007

SI-traceable conversion of radiances into fluxes


SI-traceable conversion of radiances into fluxes


Conclusions


A high-accuracy multi-angle multispectral photopolarimeter is the logical instrument with fully tested capability to provide the detailed information content necessary to attribute climate change and constrain models.

This instrument is necessary to improve the retrieval accuracy of operational SW instruments.


Nadir-looking imaging spectrometer: (i) ~200 (600?) units of information per pixel(ii) nonzero swath(iii) is not a stand-alone climate instrument(iv) cannot be used to convert radiances into fluxes(v) cannot be used to cross-calibrate retrievals

Multiangle filter polarimeter (i) ~250 9 3 = 6,750 units of information per pixel(ii) zero swath for science measurement(iii) is a stand-alone climate instrument(iv) can be used to convert radiances into fluxes(v) can be used to cross-calibrate retrievals

Multiangle spectropolarimeter: (i) ~250 200 3 = 150,000 units of information per pixel(ii) zero swath for science measurement(iii) is a stand-alone climate instrument(iv) can be used to convert radiances into fluxes(v) can be used to cross-calibrate retrievals

Multiangle imaging (i) ~250 200 3 = 150,000 units of information per pixelspectropolarimeter: (ii) nonzero swath

(iii) is a stand-alone climate instrument(iv) can be used to convert radiances into fluxes(v) can be used to cross-calibrate retrievals

Trade space


Role of polarization accuracy

Range of acceptable retrievals for different levels of polarimetric and radiometric accuracy.



– Low-altitude reflectance and polarized reflectance

– Measurements of two different surface types at 410, 470, 555, 670, 865, 1590 and 2250 nm (blue, mauve, turquoise, green, red, magenta, black) with different viewing geometries.

– Solid lines are bare soil, dashed lines are vegetation. These are single aggregated scans of a single pixel. Imperfection are primarily due to yaw.

– Reflectances of different surface types show significant variations in color. Polarized reflectance of different surface types is dominated by geometry.

bare soil

vegetation



Polarized observations of clouds are sensitive to the cloud droplet size distribution (rainbow), the cloud top pressure (side scattering in the blue UV) and aerosols above the cloud (side scattering in the red/NIR/SWIR).



BEFLUX is the ground based estimate of the total solar directional hemispheric reflectance at the DoE ARM SGP CF. The RSP estimate of DHR comes from a single snapshot (i.e. instantaneous) while the MODIS processing stream uses sixteen days of data to reduce the effects of aerosols, clouds and increase angular sampling.


Three critical problems:

1. The need to cross-calibrate polarimeters

2. Polarization sensitivity of the CLARREO SW instrument

3. Polarization sensitivity of operational radiometers

Role of polarization in cross-calibrationof measurements


To cross-calibrate polarimeters, the CLARREO SW instrument should have ~0.2% radiance and ~0.1% polarization accuracy

Role of polarization in cross-calibration


Role of polarization in radiance calibration and radiance calibration transfer

The voltage response of any radiometer can be expressed as follows:

Since Vinc is usually very small, we can simplify this formula as follows:

The CALARREO SW instrument must be a polarimeter in order to be

an SI-traceable radiance cross-calibrator.

VMUMQMIMI 4321

UMQMIMI 321



Assume that M12 is known.

M12 Q/I = M12 q must be known to ~0.001.

If M12 is 0.1 then q must be known to 0.01.





Assume that M12 is unknown.

M12Q/I = M12 q must be known to ~0.001.

Since q can be 100%, M12 must be known to ~0.001

This means that Δq = 0.001q/M12

If M12 = 0.1 and q = 0.2 then Δq = 0.002

If M12 = 0.05 and q = 0.2 then Δq = 0.004

If M12 = 0.01 and q = 0.2 then Δq = 0.02


Role of polarization in radiance calibration transfer

The CLARREO photopolarimeter can provide Qinc and Uinc even when the view angles of the two instruments are different

UMQMIMI 321


Nadir-looking imaging spectropolarimeter: (i) contiguous spectral coverage(ii) is not a stand-alone climate instrument(iii) cannot be used to convert radiances into fluxes(iv) cannot be used to cross-calibrate retrievals

Multiangle filter polarimeter: (i) narrow-filter instrument(ii) is a stand-alone climate instrument(iii) can be used to convert radiances into fluxes(iv) can be used to cross-calibrate retrievals

Multiangle spectropolarimeter: (i) contiguous spectral coverage (ii) is a stand-alone climate instrument(iii) can be used to convert radiances into fluxes(v) can be used to cross-calibrate retrievals

Multiangle imaging (i) contiguous spectral coverage

spectropolarimeter: (ii) is a stand-alone climate instrument(iii) can be used to convert radiances into fluxes(iv) can be used to cross-calibrate retrievals

Trade space


Conclusions


A high-accuracy multi-angle multispectral photopolarimeter is the logical instrument with fully tested capability to provide the detailed information content necessary to attribute climate change and constrain models.

This instrument is necessary to improve the retrieval accuracy of operational SW instruments.

This instrument is necessary to improve the radiometric accuracy of operational satellite measurements.


The CLARREO solar instrument does indeed need to be an SI-traceable photopolarimeter with high radiometric accuracy/precision high polarization accuracy/precision thorough multi-angle coverage broad multispectral coverage

Conclusion


The right strategy: fully exploit the information content of the reflected sunlight

),(

),(

),(

),(

V

U

Q

IClassification of passive remote sensing techniques by1. Spectral range2. Scattering geometry range3. Number of Stokes parameters

Hierarchy of existing/planned instruments: AVHRR MODIS, MISR, VIIRS Glory APS

The measurement approach developed for the Glory mission is to use multi-angle multi-spectral polarimetric measurements because:

• Polarization is a relative measurement that can be made extremely accurately. • Polarimetric measurements can be accurately and stably calibrated on orbit.• The variation of polarization with scattering angle and wavelength allows aerosol and cloud

particle size, refractive index and shape to be determined.• Appropriate analysis tools are available.

2009 clarreo meeting at larc 1 information content of satellite remote-sensing measurements:...

Documents

larc2009 clarreo meeting

clarreo solar instrument

misr level

comparisons of modis

modisterra level

compatible misr

standard modis

climate models