ATLID L1 product validation needs.
D.P. Donovan. G-J van Zadelhoff (KNMI)
M. Haarig, U. Wandinger (TROPOS)
M. Eisinger (ESA)•2nd ESA EarthCARE Validation Workshop•ESA UNCLASSIFIED – For ESA Official Use Only

25-28 May 2021 (online)Outline• Quick overview of ATLID.
• ATLID measurement process (Photons èL1 products)• Primary L1 products• Complications arising from spectral-polarization cross-talk.

• What data required for Validation.
• Introduce the idea that direct comparison of terrestrial-vs-space based ATBs
will not be enough.

ATLID: A 355nm HSRL lidar for Cloud + Aerosol sensing
Expected Sensitivity: Variable in practice as it depends on theatmospheric circumstances, but roughly speaking:Qualitative:e.g. Detection of features with b Mie about 1´10-3 1/km
at the 50-100 km scale.
Quantitative:e.g. Useful retrieval of extinction lower limit on the order of1´10-2 1/km at the 50-100 km scale.
51 Hz è 285 m for 2 shots accumulated on-board.
Figure: Airbus France.

Figure: Airbus France.

Figure: Airbus France.

Primary L1 Products• Co-polar `Mie’ ATB• Co-polar Rayleigh ATB• Cross-polar (Total è Mie and Rayleigh combined)

andtheir respective decomposed associated error estimates !

We have well-established ideas on
how the L1(L2) data should look-like.
Simulated ATLID Mie
X-talk corrected ATB.

6/9/21E3SIM
applied to
ECCC GEM
high resmodel data

Model Extinction Field

Simulated ATLID Ray
X-talk corrected ATB.

Retrieved Feature mask

Simulated ATLID Cro
X-talk corrected ATB.

Retrieved Cloud and Aerosol Extinction

White contours
correspond to“true” extinction=5.0e-3 1/km.

L1 products are not directly measured !
The instrument response functions and cross-talkmust be accounted for !

Figure: Airbus France.

This is the forward model and…
Figure: Airbus France.

Spectral Cross-Talk

Polarization Cross-Talk
The polarization cross-talk is caused by
the receiver optics and is on the order ofseveral percent.

The spectral cross-talk is caused by theFP characteristics and is on the order ofseveral percent between channels

Ratio to the sum of signals

CoCross

MieRayleighPolarization angle [0, 338°]

This is the inverse model a.k.a “The cross-talkcorrection procedure”
Ray çèMie co-polar X –talk factors

Cross çè Co (Ray and Mie) cross-talk factors

We will need to verify the values of these coefficients in flight !
èAccurate Mie and Rayleigh and linear Depol ATBs from terrestrial lidars at 355 nm will be needed !
Note: upward looking ATB profiles and downward looking ATBS can not be directly compared !
(more on this later)

Polarization cross-talk will change the relationshipbetween the true and observation ratios e.g.
Expected to be near linear but slope and intercept will shift:Simultaneous measurements of both low depol. (e.g.
Rayleigh) and (known) high depol targets are desirable.
Note: In flight calibrations will use, in addition, e.g. solar
background under conditions where it should be (at leastnearly) fully depolarized.

Depol ratio as measured by ATLID(after cross-talk correction !)d Ec ºbT ,^Ratio directly derived by dividing the ATLID(total=Mie+Ray) cross-polar and co-polar
Mie attenuated backscattersbM ,!

Neither the volume depol nor theparticle depol ratio !

dM =d Ec 1+dR

(1 + d R )( R - 1)

Relative difference can besignificant for small values of R !

dR(1 + d R )( R - 1)

æbR ,!d R ö
dM º= ç d Ec ÷÷çbM ,! èbM ,! ød M = 0.05

bM ,^Usual definition of Aerosol Linear Depolarization ratio
There is a need to validate the value of the effective Rayleigh Depolarization ratio for ATLID !

d M = 0.3

We need depolarization at 355 nm

355 nm 532 nm

1064 nm•Spectral slope 355 – 532 nm depends onaerosol type•

Good aerosol typing anddata base is necessary to transfer thedepolarization ratio from 532 to 355 nm
à additional uncertainties•

Dp355 is needed for polarization crosstalk calibration.
Recommendation:
If possible, upgrade your lidar with 355 nm
depolarization channel!
Ideal: Depolarization ratio at 355 and 532 nm!

Haarig et al., ACP, 2017a,b, 2018, see also Burton et al., 2015, Hu et al., 2019, Hofer et al., 2020

One important thing to keep in mind for the L1 ATBS.
Even if one had a ground-based ATLID one can not directlycompare the ATBs !
• Even in low-aerosol loading conditions the Rayleigh attenuation at355 is significant !
• A better approach maybe to derive high accuracy extinction andbackscatter from the terrestrial systems and then use this to predict
what ATLID should see (more on this later…).
• This way also allows quantitative radiometric performance evaluation !

Summary• Main challenge of validation of L1 products is validation of expected sensitivity
limits and the spectral-polarization cross-talk correction.
• High quality terrestrial(inc. airborne) lidar observation of extinction, backscatterand linear depolarization ratio profiles at 355nm are central to these tasks !
• Even though this presentation has focused on the cross-talk problem there will
also be several other less-complex but still important issues to deal with andshould not be forgotten about e.g.
• Is the altitude assignment/pointing of the instrument correct ?

¹• ATB(terrestrial)
ATB(space-based) è Using L2 terrestrial inversion results
and simulating what ATLID should see will be necessary at some level.