IASI CH4 Operational Retrieval Feasibility -

Optimal Estimation Method

Task 1+3: Updates

Richard Siddans, Jane Hurley

PM2, webex 10 October 2014

Overview• Quick description of the OEM retrieval

– This is based on the RAL scheme developed for IASI, with funding from the UK National Centre for Earth Observation (NCEO)

– ATBD level description of the scheme• Updates to scheme since PM1

– Introduction of trend in modelled N2O

– See validation presentation.– Comparison over longer time series to GEOSCHEM model– Interface to L1C Principal Component– Analysis of mean residual patterns (MetA+B, before/after 16 May)

RAL IASI CH4 retrieval scheme

Retrieval Technique: Optimal estimation

Radiative Transfer RTTOV (with customised coefficients to improve accuracy and vary HDO)

Spectral range: 1240-1290 cm-1 (cf. Razavi et al, ACP 2009)

Retrieval species: CH4

N2OH2OHDO scaling factorSurface temperature

Measurement errors: Noise derived from spectral fits(dependent on scene radiance)

A priori + covariance matrix

Fixed a priori profileCovariance: MACC global covariance or 10% (whichever larger) with 6km correlation length

Background profiles Temperature, and a priori surface temperature, H2O from ECMWF

Line strengths scaled by 1.04

1232-1290 cm-1 Better transmission to ground

N2O retrieval replaced by (i) Fixing N2O based on ECMWF PV

+ zonal climatology(ii) Retrieve effective cloud-height

and fraction

Also scan dependent systematic residuals derived (accommodate systematic errors in RT or IASI)

Stratosphere: TOMCAT zonal mean (no time dependence)Troposphere: Fixed value

Current scheme

Measurement vector• 1232.25 - 1290 cm-1 (some gaps)

– Based on Razavi et al assessment of trade-off between information • Measurement errors combine diagnosed IASI noise

and RTTOV errors

State vector / priors• Surface temperature (K).

– a priori value: from ECMWF analysis (see section 3.8 below). – a priori error: 5K

• Methane mixing ratio (ppmv) defined on 12 fixed pressure levels corresponding to z* = 0, 6, 12, 16, 20, 24, 28, 32, 36, 40, 50, 60 km, – z* = 16(3-log10(p))

• Natural logarithm of H2O mixing ratio (ppmv) defined on z*= 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 16, 20, 30, 40, 50, 60 km. – A priori profiles from ECMWF analysis

• HDO scale factor – a priori: 1 1

• Log cloud fraction (RTTOV black body model)– a priori: 0.01 factor 10

• Cloud pressure• a priori: 500 hPa 500 hPa

• Scale factors for two systematic residual patterns• a priori: [0,1] [1,1]

Modelling N2O• Tropospheric N2O is even more well-mixed than CH4, with much more

consistent (easier to model) growth-rate of 0.75 ppb/yr (0.23% per year). – Variations are < 0.5% with latitude, seasonally etc

• So far we assumed N2O has a fixed tropospheric mixing ratio of 322 ppbv, and model the (significant) variation in the stratosphere using ECMWF PV:

1. ECMWF Vorticity convert to PV on potential temperature levels

2. Equivalent latitude derived

3. ACE-FTS N2O zonal, seasonal climatology interpolated in equivalent latitude / pot. Temp.

4. Resulting profiles interpolated back to original ECMWF grid

New: N2O profile scaled by

f = 1 + 0.0023*(year-2009)

• Existing retrievals corrected by applying this factor to CH4

• Better to scale N2O used in retrieval• Only TCCON co-locations re-run so far

Systematic residual patterns• Early retrievals exhibit systematic residuals + scan dependent biases.• These currently mitigated by

– Scaling CH4 input to FM by 1.04 (like line-strength change)– Fitting scale factor for two residual patterns (mean nadir residual and

across-track variation of residual)• The mean patterns are determined from a run in which CH4 is constrained

– Only latitudes between 75-15oS considered– CH4 profile shape defined in same way as N2O – Only H2O, cloud, surface temperature retrieved

– Resulting mean residuals determined– CH4 profiles scaled to minimise correlation of mean residual with

weighting function for CH4 column perturbation (process iterated to achieve this)

• Mean nadir residual determined• Mean extreme swath residual determined• Patterns from mean nadir + difference between (Mean extreme-swath and

nadir) scaled during standard retrievals (2 fit parameters)

Systematic residual patterns

Comparison of residuals

IASI vs models two-year time-series (2009-2010)West - East

North - S

outh

Updated GEOSCHEM comparisons for 4 years (new data from AMF)

• Scheme should have trend in N2O modelled

• Not yet included in large scale data processor (or delivered code)

• Can approximate effect by scaling retrieved CH4 by linear time factor (= expected N2O trend)

• Analysis of fit residuals shows• Residual patterns quite stable – all differences small cf basic pattern• Update on 16 May stable “IPSF tuning” – introduces small residual

pattern which correlates with spectral shift.• Code to generate PC scores and use reconstructed radiances in retrieval

implemented• Systematic residuals similar to other sets• Initial retrievals seem to “work” (results soon…)

• New GEOSCHEM comparisons show excellent agreement, except for bias (mainly in new GEOSCHEM run)

Summary / plans (beyond study)