ESA-JAXA Pre-Launch EarthCARE
Science and Validation
Workshop

13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

Characterisation of Dust Aerosol from Space Lidar
ALADIN and CALIOP Measurements
Rui Song1, Adam Povey1,2, Roy G. Grainger1

1. National Center for Earth Observation, Atmospheric, Oceanic and Planetary Physics,
University of Oxford, Oxford, OX1 3PU, UK
2. Now at National Center for Earth Observation, Physics and Astronomy, University ofLeicester, Leicester, LE4 5SP, UKOutlines• Space Lidar ALADIN and CALIOP• AEOLUS mission concept• Comparison between ALADIN/AEOLUS and CALIOP/CALIPSO• Aerosol classification from space Lidar• CALIOP aerosol classification scheme• ALADIN aerosol and cloud discrimination• Case study: 2020 Sahara dust
• Retrievals of aerosol backscatter and extinction coefficients• Extinction coefficients at varying altitude layers• Correction of CALIOP extinction retrievals using ALADIN-measured lidar ratio• Synergy between ALADIN and CALIOP• Conclusions
ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

ALADIN/AEOLUS vs. CALIOP/CALIPSO
ALADINCALIOP

Polar orbits⩗⩗~15 tracks per day⩗⩗Downwards sounding radiation⩗⩗Detect backscattered signals.

⩗⩗

355 nm532/1064 nm

Observation geometry

35°

Near nadir

detection of particulate andmolecular backscattered signals

Split

Joint

Wavelength

detecting aerosol depolarisation

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy⩗CALIOP aerosol classification
V4.1/4.2V3 and earlier

V4.5•

clean marine

Troposphere:Troposphere:

•dust

•

clean marine•

clean marine•

polluted continental

•

dust

•

dust

•

clean continental•

polluted continental/smoke•

polluted continental/smoke•

polluted dust•

clean continental•

clean continental•

and smoke•

polluted dust•

polluted dust•

elevated smoke•

elevated smoke•

dusty marine•

dusty marine

Stratosphere:

Stratosphere:•

PSC aerosol•

PSC aerosol•

volcanic ash•

volcanic ash•

sulfate/other•sulfate/other•elevated smoke•

elevated smoke

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

ALADIN aerosol and cloud discrimination
https://navigator.eumetsat.int/product/EO:EUM:DAT:MSG:CLM• SEVIRI dust RGB (shades of pink to violet denote dust )SEVIRI CLM cloud mask

https://navigator.eumetsat.int/product/EO:EUM:CM:MSG:CMA _SEVIRI _V001

SEVIRI CM SAF cloud mask

SEVIRI dust flag
Ashpole and Washington (2012)

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

Case study: 2020 Sahara dusto The most intense dust event overthe past two decadeso Elevating dust particles to altitude
exceeding 6 km
o Transporting all the way into theAmericas across the tropicalAtlantic Ocean

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

Aerosol backscatter and extinction coefficients
BackscatteroAverage of all retrievals between 14th - 24th June 2020.

oALADIN backscatter coefficient needs co-polar to total conversion.

oBoth instruments indicating that the dust ascended up to 7 km.

Extinction

Discrepancies are attributed to:• Different spectral range; (ALADIN 355 nm, CALIOP 532 nm)• Different overpass time;•Different spatial sampling rate; (ALADIN 87 km, CALIOP 5 km)

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

Extinction coefficients at varying altitude layers
24th June 2020

(a)

(d)

ALADINCALIOP

Collocation:• CALIOP is 4 hours ahead of ALADIN.

2.4 – 3.4 km

(b)

(e)

3.4 – 4.4 km

4.4 – 5.4 km(c)

5.4 – 6.4 km(f)Vertical resolution:• CALIOP retrievals were upscaled from a vertical resolution of 0.03 km tomatch ALADIN’s 1-km resolution.
Dust layer:• Layers beneath 2.4 km are not included due to the known reduction in
accuracy from ALADIN resulting from low SNRs.

19th June 2020

(a)

(d)

ALADINCALIOP

2.4 – 3.4 km

(b)

(e)

3.4 – 4.4 km

4.4 – 5.4 km(c)

5.4 – 6.4 km(f)Intercomparison between ALADIN and CALIOP:o The two extinction retrievals demonstrate good agreement in capturing
dust aerosols’ spatial distribution.
o At bottom layers (e.g. 2.4 – 3.4 km), ALADIN often fails to provide qualitycontrolled retrievals, due to heavy attenuation from layers above.
Discrepancy in extinction coefficients:o Temporal and spatial variability in two Lidar observations.
o Different retrieval method: CALIOP relies on Lidar Ratio, ALADIN does not.

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

Lidar Ratio from two space lidars
𝐿𝑅(Lidra Ratio) =••𝛼 (extinction coef6icient)
𝛽 (backscatter coef6icient)

Elastic backscatter lidar (CALIOP): relies on LR to retrieve
the extinction coefficients
High spectral resolution lidar (ALADIN): directly measuresLR, but extinction retrieval is independent of LR

Collocated MODIS and CALIPSO measurements can be used to evaluate
the total aerosol optical depth (AOD)

Dust lidar ratio:
𝐿𝑅!"#$%& = 43.5 𝑠𝑟
𝐿𝑅"#"'$( = 63.5 𝑠𝑟

CALIOP underestimates AOD (CALIOP measurements with attenuatedbins have been removed)

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), ItalyCorrection of CALIOP extinction coefficients
Multiple experiments with ground-based Raman lidars and airborne
HSRLs have demonstrated no wavelength dependence of dust lidar
ratios at 355 and 532 nm.

From collocated observations, apply ALADIN measured lidar ratio (at355 nm) on CALIOP extinction retrievals (at 532 nm)

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

Synergy between CALIOP and ALADIN••••

Align well in detecting the dust aerosols that ascended
to a height of 4.5 - 6.5 km on the 16th and 17th.
The peak was noted by the end of the 17th, when thelayer AOD surpassed 0.7.
The dust aerosols remained confined within thisregion, and were continuously observed by the twosatellites over the subsequent 5 days.
Consistent with reanalysis data from ECMWF andtrajectory data from HYSPLIT.

Insights:• The growing presence of atmospheric lidars in space is
expected to enhance synergies among different lidars
Ø EarthCARE (scheduled for launch in 2024)
Ø AEOLUS-2 (expected by the end of the decade)ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy

Conclusions• Due to the absence of depolarization measurements, ALADIN faces challenges in effectively
differentiating aerosols and clouds. To address this, this study uses collocated SEVIRI measurements tocreate a dust mask specifically for ALADIN observations;• During the the 2020 Sahara dust event (14th – 24th June), averaged retrievals reveal a good agreement
in both backscatter and extinction coefficients;• Within the bottom layer of the dust, ALADIN’s extinction retrievals are strongly affected by diminished
SNRs as a result of signal attenuation;• A large discrepancy in total AOD is observed when comparing collocated A-Train data from CALIOP andMODIS;• Our experimental results underscore that using the lidar ratio derived from ALADIN observations cannotably enhance CALIOP’s extinction retrievals.
• Overall, this research demonstrates the potential of spaceborne HSRL ALADIN in aerosol retrievals andoffers invaluable insights for forthcoming HSRL missions such as EarthCARE and AEOLUS-2.

ESA-JAXA Pre-Launch EarthCARE Science and Validation Workshop | 13 – 17 November 2023 | ESA-ESRIN, Frascati (Rome), Italy