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BT simulations in the context of SST retrieval P. Brunel, H. Roquet MF/CMS (with some slides from J. Hocking, Met Office). Contents. Which RTM ? (RTTOV overview) Which “reference” atmospheric profiles for SST algorithms derivation ? (from radio-soundings to NWP outputs)

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  1. BT simulations in the context of SST retrievalP. Brunel, H. Roquet MF/CMS(with some slides from J. Hocking, Met Office)

  2. Contents • Which RTM ? (RTTOV overview) • Which “reference” atmospheric profiles for SST algorithms derivation ? (from radio-soundings to NWP outputs) • Which approach for BT simulations in operational conditions ? • Input profiles from NWP outputs • First guess SST • (BT simulations adjustment, see Igor’s presentation) • Other issues : IR surface emissivity, solar contribution to IR BTs etc… OSI SAF workshop on SST

  3. RTTOV overview (1) • Radiative Transfer for TOVS • Fast radiative transfer model • First version developed at ECMWF in early 90s by John Eyre • Since v6 in 2000, RTTOV has been developed within the NWP SAF by the Met Office, Météo-France, and ECMWF • Simulates TOA radiances for a wide range of IR and MW instruments • Includes direct, tangent linear, adjoint and jacobians models • Used in data assimilation, 1-D VAR, re-analysis, Autosat, … *TOVS = TIROS** Operational Vertical Sounder **TIROS = Television Infrared Observation Satellite OSI SAF workshop on SST

  4. RTTOV overview (2) • Input consists of p, T, q profiles and surface variables • Other trace gas profiles may optionally be supplied (O3, CO2, CO, CH4, N2O) • For IR instruments: • cloud-affected radiances calculated by a parameterized scattering scheme • profiles of one or more of 11 pre-defined aerosol types may be supplied • For MW instruments: • an alternative interface is provided which calculates scattering by cloud and precipitation OSI SAF workshop on SST

  5. RTTOV overview Most important outputs: • Clear-sky TOA radiances and BTs • Cloudy TOA radiances and BTs • Surface to space transmittance • Level to space transmittances OSI SAF workshop on SST

  6. RTTOV v10 scientific improvements (2012) • New IR and MW land surface emissivity atlases • Improved MW sea surface emissivity model (FASTEM-4) • Principal Components calculations of AIRS and IASI spectra • Explicit treatment of Zeeman splitting for SSMI/S and AMSU-A • Updates to RTTOV_SCATT for cloudy MW simulations • Up to two cloud types per level for cloudy IR simulations • Include top layer (0.005hPa) in input profile • Updated coefficient files on new set of 51 levels OSI SAF workshop on SST

  7. RTTOV v11 scientific improvements • Capability to simulate visible and near-IR channels • Inclusion of non-local thermodynamic equilibrium effects for shortwave infrared sounding channels • Support for new instruments (MTG-IRS, ATMS, ...) • Updates to the land surface emissivity atlases • Improvements to simulation of PCs for advanced IR sounders (including HT-FRTC) • Due for release in 2013... OSI SAF workshop on SST

  8. Reference profiles : the SAFREE data base (François, 2002) - 402 profiles OSI SAF workshop on SST

  9. Ref. profiles : the Chevallier data base (Chevallier 2006) • http://research.metoffice.gov.uk/research/interproj/nwpsaf/rtm/profile_datasets.html • Extracted from ECMWF operational global forecasts between July 2005 and June 2006 (T799, 91 levels) • For each profile : T, q, O3, CC, CLW, CIW, rain, snow and surface variables (Ps, Tskin, T2m, Td2m, U10m, V10m…) • Initial data base : 121 462 560 profiles • Selection algorithm : • Criterion based on the distance between the new profile and the ones which have already been selected • Criterion separately applied for each variable (T, q, O3, CLW+CIW, rain+snow) • Final data base : 5 files containing 5 000 profiles each • For our purpose (IR SST algorithm derivation), we start from the 2 files based on T and q selection criterion (10 000 profiles), keeping profiles representative of clear-sky conditions : • Threshold on maximum cloud cover (CC) or relative humidity (RH) in the profile OSI SAF workshop on SST

  10. Ref. profiles : the Chevallier data base (Chevallier 2006) : 4245 profiles over open ocean OSI SAF workshop on SST

  11. Ref. profiles : the Chevallier data base with 0.4 CC threshold : 1499 profiles over open ocean OSI SAF workshop on SST

  12. Ref. profiles : the Chevallier data base with 95 % RH threshold : 1183 profiles over open ocean OSI SAF workshop on SST

  13. Ref. profiles : direct selection from ECMWF outputs • Different from Chevallier’s approach : • We don’t want to maximise T or q variability within the selected profiles data base • We want to respect the “natural” T and q distribution over global oceans • ECMWF operational short term forecasts (9, 12, 15 and 18 h) from analyses on 15 March, June, September and December 2012 at 00 and 12 UTC (global, 0.5° x 0.5° grid, 91 vertical levels) • One grid point every 10 is kept in latitude and longitude • Resulting profile is selected only if Land Fraction = 0 and Total Cloud Cover (TCC) = 0 and Tskin > 0 • Final number of profiles : 2108 OSI SAF workshop on SST

  14. Ref. profiles : ECMWF outputs with TCC = 0 and Tskin > 0 : 2108 profiles over open ocean OSI SAF workshop on SST

  15. Operational BT simulations in OSI SAF SST processing chains (GOES-E, MSG) • RTTOV v9.3 • (T,q) profiles : 3-hourly operational ECMWF short term forecasts from analyses at 00 and 12 UTC received in NRT at M-F centre in Toulouse, global 0.5° x 0.5° grid, 15 pressure levels : 1 000 to 100 hPa (recently upgraded to 20 pressure levels : 1 000 to 10 hPa) • Vertical interpolation to RTTOV levels internally performed by RTTOV • Tskin : most recent OSTIA SST analysis minus 0.2 K • 3-hourly BT simulations (no time interpolation) • BT simulations performed directly in satellite spaceview projection (at full IR resolution, in all ocean pixels) : for all IR window channels, Tb, Tb / Tskin, Tb / TWVC OSI SAF workshop on SST

  16. NRT BT simulations in OSI SAF SST prototype chain (METOP-A) • Similar but : • RTTOV v10.2 • BT simulations performed on global 0.05° x 0.05° grid, based on 12-hourly global METOP-A/AVHRR workfiles from the operational OSI SAF SST processing chain (in all grid cells where a METOP-A/AVHRR SST value is present) : for all IR window channels, Tb, Tb / Tskin, Tb / TWVC OSI SAF workshop on SST

  17. Future OSI SAF operational BT simulations for polar orbiters : open issues • RTTOV v10.2 or RTTOV v11 ? (contribution of solar reflection in IR channels) • IR emissivity model ? (current RTTOV IR emissivity model is ISEM, with no wind speed dependency) • Use of ECMWF ozone profiles ? (negligible impact at 11 and 12 microns) • Which NRT computation strategy ? (computational cost) • Should we do something when ECMWF profiles are cloudy ? • Next slides (“quick and dirty” experiment) : thresholding ECMWF relative humidity at 95 % OSI SAF workshop on SST

  18. T12obs – T12simu 1st of March 2012 OSI SAF workshop on SST

  19. TWVC (g / cm2) 1st of March 2012 OSI SAF workshop on SST

  20.  TWVC (g / cm2) 1st of March 2012 OSI SAF workshop on SST

  21.  T12simu 1st of March 2012 OSI SAF workshop on SST

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