E3SIM potential role in Cal/Val activities
D.P. Donovan (KNMI) with contributions from H. Baars•

2nd ESA EarthCARE Validation Workshop•

ESA UNCLASSIFIED – For ESA Official Use Only

25-28 May 2021 (online)Outline• The general problem with direct comparisons of ATBs.
• E.G. The viewing direction does matter (even in the simplest conditions).

• A solution!
• Terrestrial L1èL2=è”Simulation” èSpace-based L1• Simulation strategies• Simple direct approaches (advantages and limitations)• Heavy (e.g. ECSIM) approaches (advantages and disadvantages)• Conclusions

Why direct comparison of ATBs is not enough forquantitative validation*.

(Even in the case of where space/time co-location is perfect, and/or the atmosphericis horizontally homogeneous.)
Geometry is important !
(because with lidars, attenuation is important)

ATB(r ) = ( b (r ) M + b (r ) R ) exp[ -2T (0, r )]From Ground

From space*Note: using R and Depol info from ground-based could be somewhat directly compared…but these are ratios so e.g. absolutecalibration factors, radiometric assessment can not be checked/validated.

Another issue: 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

The role of simulation in general¹

ATBs from ground-bases systems can
NOT be directly compared to spacebased ATBs, even in the best ofcircumstances.

So, something (at least somewhat) more involved is needed !

Retrieval ofbeta,alpha,depol“Simulation”
of ATLID
ATBs

CompareValidation +Radiometric assessment

“Simulation”
of ATLID
ATBsThis step, in particular, can be done at various levels of detail and sophistication..

Simple: e.g. Use single-scattering lidar equations for each ATLID channel.
• Big advantage…simplicity !!
• Limitations: Limited ability to traceback potential problems to ATLID in case of problems.

Complex: e.g. Use a more detailed approach using a lidar RT model that calculates
spectral/polarization characteristics (and included multiple scattering effects) coupled with adetailed ATLID instrument model.
• Ability to investigate issues in depth (e.g. track down specific cross-talk correction issues byadjusting virtual FP parameters).
• Ability to pull in other sources of information (e.g. in-situ data) .
• Ability to use data not at 355nm in a manner more accurate than simple approaches.
• Disadvantage: NOT SIMPLE!!

Example at 355 nm (using a “simple” approach).
PollyXT_TROPOS; Leipzig, Germany,
27 March 2020. 1830-1930 UTC

Height [km]16Pl ( z ) =particle
molecular

128400480 50 100 150 200 250-1-1
Backscatter coeff. [Mm sr ] Extinction coeff. [Mm -1]lC O ( z)z2ìïüïlll
éë b paùé lù
r ( z ) +b mol ( z ) û exp í -2 ò ëa par ( r ) + a mol ( r ) û dr ý
îï z1
þï
z2

16
from ground: z1=0 km
from space: z1= 20 km

Height [km]

2020l
sys

With measured backscatter andextinction profiles from groundone can calculate theattenuated backscatter fromspace!

from groundfrom space

128
40

0
48
Attenuated Backscatter coeff.
[Mm-1 sr-1]

An example of a non-simple approach is theuse of E3SIM
ModelAtmosphereForwardRT model(s)Forward
Instrument model(s)Synthetic
observations

CompareInversions

The simulator can ingest varied data
steams in a consistent manner

Modular unified approach

Mission Performance
EarthCARE Simulator - Example atmospheric scene
Extinction

MSI 8.6 um

stratuscirruscirrusstratusaerosol

Lidar+RadarReff

cirrusZeaerosolstratusSo the E3SIM route would look like..

Aux data(A concrete example will be given in G-J’s talk)

Retrievalof
beta,alpha,depol(3D) field of
Scatteringtype andSize
Distribution
parameters !

L1 Validation +Radiometric assessment
Possible iterations
with different
instrument model
settings/adjustments

E3SIM UFF file

Check ?

Apply ECSIM
RT code(s) in
satellite mode
Apply ATLID
instrument
model

Apply ECSIM
RT code(s) interrestrial
mode
Apply
Ground-based
lidar
instrument
model

How Could this work in practice ?
• Simple simulation approach:• Each group could do its own thing (and maybe this is well enough !).

• E3SIM(-like) approach.
• KNMI (maybe in cooperation with established partners) will most-likely use
ECSIM in a few selected cases but we would not be able to offer a ``generalservice’’ to the community.
• Maybe resources can be found to assembling an e.g. virtual machine with an
example that people could then follow ?
• Making things ``plug and play’’ would be very difficult !
• There would have to be enough demand to go this route…and...(arguably underresourced) earlier efforts by ESA to do similar things with ECSIM met with little success.

Conclusions• Simulation (loosely defined) will play an important role in ATLID L1validations !
• Hopefully, simple approaches will be sufficient !
• If not, more sophisticated approaches are possible..
• The E3SIM(and like) approaches are potentially powerful !
• At the same time E3SIM is far from a Plug-and-Play tool (and likely, practicallyspeaking, could never be made into one without significant resources andcommitment) !