Rutherford Appleton Laboratory Remote Sensing Group Ozone Profile Retrieval from MetOp R. Siddans,...

Rutherford Appleton Laboratory

Remote Sensing Group

Ozone Profile Retrieval from MetOp

R. Siddans, G. Miles, B. Latter A. Waterfall, B. Kerridge

Acknowledgements:NERC/NCEO for funding this workEUMETSAT & ECMWF for provision of dataDr N Richards (U. Leeds) TOMCAT data

NCEO Atmospheric Composition Theme

Remote Sensing GroupDevelopmentsOzone ECV Project• Delivered sonde-matched pixels for 2008 test year to round

robin exercise• Additionally 3 days per month (all orbits) for more detailed

spatial comparison• Begun comparisons with models and official Ozone SAF

product (KNMI)• Identified opportunities to improve aspects of our retrieval

scheme

Additionally• Contribution to NCEO-theme partner N. Richards (U. Leeds)• Through visiting scientist project delivered prototype modules

from RAL scheme for experimental use at KNMI• Ongoing comparisons to models (TOMCAT, MACC)

Remote Sensing GroupPerformance against

sondes in troposphere and stratosphere

Remote Sensing GroupSonde Comparisons with time

© 2010 RalSpace


GO

ME

GO

ME

TOMCAT

TOMCAT +G1AK

GOME-1 (1995-2011) aboard ERS-2 platform as compared to TOMCAT

24-26th February 1997 average

TOMCAT + G1AK

TOMCAT

GOME-1

MACC + G2AK

TOMCAT + G2AK

GOME-2

24-26th July 2008 average

Model data sampled as GOME-2 would see it, with averaging kernels applied

GO

ME-

2G

OM

E-2

TOMCAT + G2AK

MACC +G2AK

TOMCAT

TOMCAT G2AK

MACC G2AK

MACC

A Priori

Orbit model cross-sections

GOME-2

August 24th 2008


In Development• Comparisons to both sondes and models indicate some aspects

of the scheme can be improved upon• Some spurious high tropospheric ozone values in NH spring • This might be improved by implementing a polarisation

correction in the radiative transfer

0-6km

G2-sonde mean biasG2-sonde(AK) mean bias 6-12km


Broad absorption bands 420-650 nm – large continuum

overlapped with diffuse vibrational structure

– First steps to analyse the information content for GOME-2 in this spectral region

© 2010 RalSpace

Next steps – Chappuis Bands


© 2010 RalSpace

Next steps – Chappuis Bands

- Potentially more information about ozone closer to the surface over land as higher reflectance in the visible

- Requires accurately characterised surface properties to fit the measured reflectance due to broad nature of Chappuis bands

Remote Sensing GroupNext steps – Polarisation CorrectionNeglecting polarisation in radiative transfer calculation can:– Cause inaccuracies in surface albedo and scattering

parameter estimates– This can lead to overestimate in ozone absorption• Implement polarisation correction in Huggins Bands

© 2010 RalSpace

0-6k

m O

zone

Albedo

Remote Sensing GroupNext steps – Joint Scheme

• Further develop Joint IASI-GOME-2 retrieval scheme• Compare to existing individual schemes

GOME-2 Only GOME-2 + IASI

AK 6km

AK 12km

AK 6km

AK 12km


PREVIOUS TALK/SUPPLEMENTARY SLIDES

© 2010 RalSpace


Jan Feb Mar

Apr May Jun

Jul Aug Sept

Oct Nov Dec

Remote Sensing GroupIn Development• Comparisons to both sondes and models indicate some

aspects of the scheme can be improved upon• High tropospheric ozone values in NH spring suggesting need

to implement a polarisation correction in the radiative transfer

24th April 2008

With extended QC and filtering:

Scheme with basic QC:

0-6k

m O

zone


1st June 0-6km Sub-column

Why do we retrieve the slit function?Not retrieving the slit function:

With slit function retrieval:2007 2008 2009

Remote Sensing GroupGlobal monthly mean sonde bias

G2-sondeG2-sonde+G2AK

• Bias changing with time due to instrument degradation

• Small positive bias• larger in 2010 after last decontamination test

Without retrieving instrument slit function width

Retrieving instrument slit function width

Remote Sensing GroupThrough-put degradation

Retrieved slit function width Mean absolute Wavelength shift (band 2)

• Dashed lines indicate instrument decontamination tests. • Final test in September 2009.


© 2010 RalSpace

Fitting of the Eigenvectors

Mean residual fitting only

Band 2 fit cost


© 2010 RAL Space

RAL GOME-2 Ozone Scheme Overview

• 3-step retrieval: band 1a, surface albedo, band 2b.• Use sun-normalised radiance in Hartley and Huggins bands to measure ozone

in Earth’s atmosphere• Forward model inc. Rayleigh + cloud scattering, surface • Huggins band reveals information on tropospheric ozone, requires precision of

fit >0.1%.• For band 1, absolute calibration is important, especially for stratospheric

ozone.• For band 2, a good estimate of noise is important for precision of the fitting for

tropospheric ozone

Fit residuals < 0.1%

cloud-free

cloudy

Measured spectra in Huggins bandsOzone absorption

Remote Sensing GroupRAL MetOp Ozone

GOME-2 Retrieval Scheme

• UV/Vis spectrometer• Optimal estimation retrieval

with sun-normalised radiance

• Uses Huggins band to add information for tropospheric ozone

• Requires fit precision < 0.1% for tropospheric ozone

IASI Retrieval Scheme

• Optimal estimation scheme

• Uses RTTOV as forward model (with recomputed coefficients)

• State vector: Ozone, Surface Temperature, H2O and Surface Emissivity (using MODIS/Wisconsin data as prior)

© 2010 RalSpace


Ozone SAF OOP 0-6km

Co-adding 4 pixels along track, cloud screened

RAL GOME-2 Ozone 0-6 km

We have experimented with co-adding pixels along and across track, to improve the quality of GOME-2 tropospheric ozone.

Remote Sensing GroupAVHRR/3 Cloud Products for GOME-2 and IASI

• Oxford RAL Aerosol Cloud (ORAC) Scheme• Retrieve cloud properties for high resolution AVHRR imager pixels,

combine for GOME-2 or IASI pixels– optical depth– effective radius– cloud top pressure– cloud fraction +

• Products can be used for screening data for quality control or used directly in radiative transfer model Effective RadiusOptical DepthFalse colour (measured)

Remote Sensing GroupUse of AVHRR/3 imager data for GOME and IASI ozone

Relative sensitivity of GOME to lower

tropospheric ozone

Across-track pixel

Relative sensitivity of GOME to ozone profile compared to cloud-free conditions (from RTM)

ORAC

Cloud height / kmOptical Depth/ km


• AVHRR and GOME-2 derived O3 factors are comparable

• AVHRR O3SF more sensitive than GOME-2 cloud flag (contributing sub-pixel information)

• Used for screening for the affects of cloud

• Putting factors directly into RTM

GOME-2 vs AVHRR Derrived Ozone Sensitivity Factor (O3SF)

IASI GOME-2 Pixel matching

IASI pixel within GOME-2 pixel selected for lowest cloud factor,

derived with AVHRR

TES 0-6 km Ozone:1 month of observations gridded 2x2 degrees

22-24th August 2008

RAL GOME-2 Ozone:Co-added 4 pixels along track

GOME-2 and TES 0-6km

Statistical comparison of GOME-2 vs Sondes

(2008, global)Prior vs Sondes

Retrieval vs Sondes

Retrievals vs Sondes x Averaging

kernels

IASI + GOME dashed

Linear Averaging kernelsLog averaging kernels

Remote Sensing GroupAVHRR Cloud products for GOME-2 and IASI• Oxford RAL Aerosol Cloud (ORAC)

Scheme• Retrieve cloud properties for high

resolution AVHRR imager pixels, combine for GOME-2 or IASI pixels

• Retrieve optical depth, effective radius, cloud top pressure, cloud fraction

Z* / km

Effective RadiusOptical Depth

Remote Sensing GroupAVHRR Cloud and Ozone Sensitivity Factor (O3SF)• Ozone factor derived from

AVHRR or GOME-2 cloud properties

• Quantifies relative sensitivity to ozone in the troposphere

• AVHRR has sub-pixel cloud sensitivity

• AVHRR O3SF more sensitive to high cloud

• GOME-2 O3SF potentially better over multilayer cloud


Single orbit: GOME only retrieval + Avg. Kernels on 23 Aug 2008

10

12 M

ole

c/cm

3


Single orbit: GOME+IASI retrieval + Avg. Kernels on 23 Aug 2008

Remote Sensing GroupRAL IASI Ozone Retrieval Scheme

• Optimal estimation scheme

• Uses RTTOV as forward model (with recomputed coefficients)

• State vector = Ozone, Surface Temperature, H2O and Surface Emissivity


GOME-2 only Ozone 0-6km

IASI only Ozone 0-6km

NIGHT

DAYMetOp Joint Ozone Scheme:

- Directly fits GOME-2 and IASI spectra simultaneously in non-linear retrieval- IASI FM based on RTTOV- Uses AVHRR ORAC scheme to identify IASI pixel least affected by cloud- Fits down to noise in most scenes

Rutherford Appleton Laboratory Remote Sensing Group Ozone Profile Retrieval from MetOp R. Siddans,...

Documents

Transcript of Rutherford Appleton Laboratory Remote Sensing Group Ozone Profile Retrieval from MetOp R. Siddans,...