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Calibration of SNPP-CrIS for the NASA L1bProduct

L. Larrabee Strow, and Howard Motteler

Department of Physics, JCET, University of Maryland Baltimore County (UMBC)

AIRS STM: October 2015

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Overview

Compare using ∼2200 clear ocean tropical scenes

Calculated radiances use kCARTA (monochromatic) andECMWF

Key issues:

C4 RTA needs instrument responsivity applied

CCAST does not need in-band part of responsivity

Does it matter?

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FOV7 Improvements (Dave Tobin, UW (SSEC )

FOV$7$$Differences)wrt)FOV9))with))MW7)a2)x)1:)

Differences)wrt)FOV9))with))MW7)a2)x)0.70:)

9$

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Calibration EquationsCCAST now supporting many SDR approaches.

Calibration Equation Definitions

For this talk the term “UMBC” denotes the CCAST reference calibration equationwhile “NOAA” refers to the SDR Team equation C4.

UMBC:

rES = F · ·f · SA−1 · f · ES − 〈SP〉〈ICT〉 − 〈SP〉SA−1rICT

NOAA-C4:

rES = rICT

F · f · SA−1 · f ·{∆S1∆S2

· |S2|}

F · f · SA−1 · f · |∆S2|

∆S1 = FIR−1(ES − 〈SP〉), ∆S2 = FIR−1(〈ICT〉 − 〈SP〉)

rES is calibrated earth-scene radiance at the user grid

F is resampling from sensor to user grid

rICT is the expected ICT radiance

f , UMBC: is a raised-cosine bandpass filter with wings at or inside the instrumentresponsivity, NOAA: modified ATBD filter

SA, UMBC: Periodic sinc ILS wrapping at the sensor grid, NOAA: Periodic sincwrapping at the undecimated sensor grid.

Non-linearity corrections are included

〈SP〉 and 〈ICT〉 are averages over 9 scans

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Components Operated on by SA−1

Wavenumber (cm-1)

650 700 750 800 850 900 950 1000 1050 1100 1150

Arb

Un

its

0

0.2

0.4

0.6

0.8

1

1.2

α = ∆ S1 / ∆ S

2

Responsivityα * |S

2|

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Components Operated on by SA−1

NOAA Wavenumber (cm-1)

600 700 800 900 1000 1100 1200

Arb

Un

its

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

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Components Operated on by SA−1

CCAST Wavenumber (cm-1)

650 700 750 800 850 900 950 1000 1050 1100

Ra

dia

nce

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

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Simulations (RTA)

RTA with NOAA approach: apply responsivity tomonochromatic radiance, then divide out responsivity afterthe FFT.

This implies a different forward model, RTA, for eachinstrument.

The instrument response function is no longer a sinc function(some disagree with this statement). An individual channel isnot the spectral space convolution with a sinc...

If the detector filter (part of responsivity) changes (spacecrafttemperature), the SDR will change and the RTA will need tochange. Unlikely, but possible.

The radiance record will be different for the next CrIS sincethe reponsivity will be slightly different.

Next slides: UMBC w/ FILT legend in plots means we arecomparing the NOAA simulated radiance to the UMBC CCASTgenerated observations.

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NOAA, UMBC Observations are DifferentA consequence of the SDR calibration equation.

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

∆ in K

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4UMBC Obs B(T) -NOAA Obs B(T)

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NWP Biases: All FOVsThe NOAA and UMBC biases are very similar.

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

NW

P B

ias in K

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

UMBCNOAA

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NWP Bias Difference between NOAA and UMBCRinging only, quite small.

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Bia

s D

iff in

K

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

UMBC-NOAA

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Ringing DiagnosticsShowing BiasHamming-BiasSinc

UMBC w/ Filt: Btcalc with Responsivity applied before doing the FFT.

Sin

c E

rro

r in

K

-1

0

1UMBC

Sin

c E

rro

r in

K

-1

0

1NOAA

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

∆ B

(T)

in K

-1

0

1UMBC w/ Filt

Mid-wave worse with UMBC w/ Filt.

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NOAA and UMBC Differences: Responsivity FilterNOAA C5 numerator embeds responsivity info into the SDR

∆ B

(T)

in K

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

UMBC - NOAA Obs BT

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

∆ B

(T)

in K

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

UMBC w/ Resp. Filter - UMBC no filter

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NOAA and UMBC Differences: Responsivity FilterNOAA C5 numerator embeds responsivity info into the SDR

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SDR Variability over FOV ID: Sinc ILSNOAA has smaller variability over FOV, esp. in long-wave

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Bia

s S

td o

ve

r F

OV

(K

)

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

UMBCNOAAUMBC-NOAA

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SDR Variability over FOV ID: Hamming ILSWith Hamming ILS, larger UMBC is gone.

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Bia

s S

td o

ve

r F

OV

(K

)

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

UMBCNOAAUMBC-NOAA

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Bias with Scan Direction: Sinc ILSNOAA and UMBC very similar.

Even-O

dd in K

-0.5

0

0.5 FOV5 Scan Direction Bias

UMBCNOAA

Even-O

dd in K

-0.5

0

0.5 Side FOVs Scan Direction Bias

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Even-O

dd in K

-0.5

0

0.5 Corner FOVs Scan Direction Bias

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Bias with Scan Direction: Hamming ILSVery small, DC offsets may be scene differences?

Even -

Odd in K

-0.5

0

0.5

All FOVs Scan Direction Bias

UMBCNOAA

Wavenumber (cm-1)

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

Even -

Odd in K

-0.5

0

0.5

All FOVs Scan Direction Bias: Hamming

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Conclusions

Two reasons for CCAST approach:CrIS RTA does not need complicated in-band responsivity filter forreference truth (good for external users).Differences in CrIS radiance response among different instrumentsminimized using CCAST approach.

Remaining WorkUMBC performance almost identical to C5 in the low end of thelong-wave band, where the UMBC f cutoff matches the responsivitycutoff, hence the out-of-band is handled properly in simulation.Improve? CCAST by modifying the other f filter cutoffs to moreclosely match the responsivity cutoffs.These out-of-band contributions are quite small, and can possiblybe ignored for even climate applications.Examine sensitivity of these proposed calibration equations tochanges in CrIS bands for J2+. Want to keep instrumentinter-calibration to the climate level if possible.

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LW Filters

wavenumber

600 700 800 900 1000 1100 1200

norm

aliz

ed w

eig

ht

0

0.2

0.4

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0.8

1

1.2

1.4LW CrIS Filters

responsivitynumeric filterccast bandpasssensor griduser grid

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MW Filters

wavenumber

1100 1200 1300 1400 1500 1600 1700 1800 1900

norm

aliz

ed w

eig

ht

0

0.2

0.4

0.6

0.8

1

1.2

1.4MW CrIS Filters

responsivitynumeric filterccast bandpasssensor griduser grid

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SW Filters

wavenumber

2000 2100 2200 2300 2400 2500 2600 2700

norm

aliz

ed w

eig

ht

0

0.2

0.4

0.6

0.8

1

1.2SW CrIS Filters

responsivitynumeric filterccast bandpasssensor griduser grid