1. IASI CH 4 Operational Retrieval Feasibility - Optimal Estimation Method Task 1+3: Updates Richard Siddans, Jane Hurley PM2, webex 10 October 2014

2. 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 N 2 O –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)

3. RAL IASI CH 4 retrieval scheme Current scheme

4. 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

5. State vector / priors

6. Modelling N 2 O Tropospheric N 2 O is even more well-mixed than CH 4, 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 N 2 O used in retrieval Only TCCON co-locations re-run so far

7. 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 75-15 o S considered –CH4 profile shape defined in same way as N2O –Only H 2 O, cloud, surface temperature retrieved –Resulting mean residuals determined –CH 4 profiles scaled to minimise correlation of mean residual with weighting function for CH 4 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)

8. Systematic residual patterns

9. Comparison of residuals

10. IASI vs models two-year time-series (2009-2010) West - East North - South

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

12. Scheme should have trend in N 2 O modelled Not yet included in large scale data processor (or delivered code) Can approximate effect by scaling retrieved CH 4 by linear time factor (= expected N 2 O 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)