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Assimilation of AIRS & IASI at ECMWF

This article provides an overview of the assimilation configuration, channel selection, cloud detection, and forecast impacts of AIRS and IASI instruments at ECMWF. It discusses the operational configurations, noise assumptions, and the selection of channels for assimilation. The article also covers the first guess departures, water vapor, data compression, and concludes with the forecast impacts of AIRS and IASI.

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Assimilation of AIRS & IASI at ECMWF

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  1. Assimilation of AIRS & IASI at ECMWF Andrew Collard Acknowledgement to Tony McNally

  2. Overview • Introduction • Assimilation Configuration • Channel Selection • Cloud Detection • IASI First Guess Departures • AIRS & IASI Forecast Impacts • Water Vapour • Data Compression • Conclusions

  3. Introduction

  4. Current and Future High-Spectral Resolution InfraRed Sounders on Polar Orbiters

  5. Shortwave window (with solar contribution) Longwave window H2O CO2 Brightness Temperature (K) O3 Q-branch CO2 Wavelength (μm) An IASI Spectrum

  6. IASI vs HIRS: The Thermal InfraRed

  7. AIRS vs HIRS Jacobiansin the 15μm CO2 band 100 hPa HIRS-4 HIRS-5 Selected AIRS Channels: 82(blue)-914(yellow) HIRS-6 HIRS-7 HIRS-8 1000 hPa

  8. HIRS vs IASI:TemperatureRetrieval Accuracy

  9. HIRS vs IASI: Response to Important Atmospheric Structure IASI HIRS Response to a structure the observation of which would have improved the forecast of the reintensification of Hurricane Floyd over SW France and SW England on 12th September 1993. (Rabier et al., 1996)

  10. Assimilation Configuration

  11. Current Operational Configurations AIRS • Operational at ECMWF since October 2003 • 324 Channels Received in NRT • One FOV in Nine • Up to 155 channels may be assimilated (CO2 and H2O bands) IASI • Operational at ECMWF since 12th June 2007 • 8461 Channels Received in NRT • All FOVS received; Only 1-in-4 used • 366 Channels Routinely Monitored • Up to 168 channels may be assimilated (CO2 band only)

  12. Assumed Noise for AIRS & IASI Assimilation AIRS Instrument Noise Assumed Observational Error Clear Sky Background Departures Noise or First Guess Departure (K) Wavelength (μm)

  13. Assimilation Configuration:Channel Selection

  14. Why Select Channels? • The volume of IASI data available is such that we do not have the computational resources to simulate and assimilate all these data in an operational timeframe • Not all channels are of equal use when assimilated into an NWP system • We choose channels that we wish to monitor (often with a view to future use) • We choose a subset of these channels which we actively assimilate

  15. Pre-screened channels

  16. Selected Channels (2)

  17. AIRS 324 vs IASI 366 IASI 366 AIRS 324

  18. Comparison of Actively Assimilated Channels (1)

  19. Comparison of Actively Assimilated Channels (2)

  20. Jacobians of 15μm CO2 Band Pressure (hPa) Temperature Jacobian (K/K)

  21. Assimilation Configuration:Cloud Detection

  22. Cloud detection scheme for Advanced Sounders A non-linear pattern recognition algorithm is applied to departures of the observed radiance spectra from a computed clear-sky background spectra. This identifies the characteristic signal of cloud in the data and allows contaminated channels to be rejected AIRS channel 226 at 13.5micron (peak about 600hPa) The large number of AIRS or IASI channels allows improved measurement of the cloud-top height compared to HIRS obs-calc (K) Vertically ranked channel index AIRS channel 787 at 11.0 micron (surface sensing window channel) unaffected channels assimilated CLOUD pressure (hPa) contaminated channels rejected temperature jacobian (K)

  23. Number of Clear Channels High Peaking Channels Window Channels

  24. IASI First Guess Departures

  25. First –Guess Departure Standard Deviations in 15μm CO2 Band Calculated Std. Dev. Observed Std. Dev.

  26. First Guess Departure Biases in 15μm CO2 Band Ordered by Wavenumber Ordered by Jacobian Peak Pressure

  27. First Guess Departure Standard Deviations and Biases in the Longwave Window Bias Standard Deviation

  28. First-Guess Departure Biases in Water Band

  29. First –Guess Departure Standard Deviations in Water Band Calculated Std. Dev. Observed Std. Dev.

  30. First –Guess Departure Standard Deviations in Shortwave Band Observed Std. Dev. Calculated Std. Dev.

  31. AIRS & IASI Forecast Impacts

  32. AIRS Impact at ECMWF 500hPa Geopotential Anomaly Correlation Northern Hemisphere Southern Hemisphere

  33. IASI Impact on SH Geopot. AC No IASI With IASI

  34. IASI Forecast Scores Again: 500hPa Geopot. AC 8th March-5th May 2007 NH IASI Better IASI Worse SH

  35. IASI Forecast Scores Again: 500hPa Geopot. AC 11th July-26th August 2007 NH IASI Better IASI Worse SH

  36. AIRS/IASI Impact at ECMWF in Context: N. Hemis. No satellite experiments

  37. Single instrument experiments Anomaly correlation of 500hPa height for the Southern Hemisphere (average of 50 cases summer and winter 2003 verified with OPS analyses)

  38. 168 channels assimilated … 8293 to go

  39. Using the IASI SpectrumLongwave CO2 Band 156 of 1st 500 channels are active Adjacent channels not used because of oversampling Other channels omitted because of ozone and humidity contamination

  40. Using the IASI SpectrumShortwave CO2 Band Short wave temperature bands: 350 channels Solar contamination, High Noise, Interfering species

  41. Using the IASI SpectrumChannels Primarily Sensitive to the Surface 2900 Window Channels

  42. Using the IASI SpectrumTrace Gases and RT Challenges 700 channels affected by non-LTE during the day 350 channels in the main ozone band Many other channels (not shown) affected by O3, CH4, NO2, CO etc.

  43. Using the IASI SpectrumThe 6.3μm Water Band Water band 3800 channels

  44. Water Vapour

  45. Use of Water Vapour Channels at ECMWF (For AIRS) • We get a small positive impact from using the water vapour channels • We use a cloud detection scheme that uses the first guess departures in the water band itself • The signal from water vapour can mimic cloud • The resulting clear channels also tend to be those where water vapour departures are smallest • We also assume 2K observation errors (c.f. instrument noise is ~0.2K).

  46. In-band vs Cross-Band Cloud Detection AIRS 1583 1402cm-1 Cross-Band In-Band

  47. Choosing 10 IASI Water Vapour Channels Grey channels are the 120 H2O channels distributed via the GTS

  48. Fit to other observations:10 IASI Water Channels Best value at ~1.5K Normalised to unity here

  49. Fit to other observations: Other Platforms Black is expt. with IASI humidity channels Red is Control NOAA-17 HIRS S.Hemis. EOS-Aqua AIRS S.Hemis. The addition of the IASI water band improves the analysis fit to HIRS on NOAA-17 and increases usage of AIRS data.

  50. Fit to other observations: Sondes The addition of the IASI water band improves the analysis fit to radiosondes Spec. Hum. N.Hemis. Sondes Spec. Hum. S.Hemis. Sondes Rel. Hum. S.Hemis. Sondes Black is expt. with IASI humidity channels Red is Control

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