Martine De Mazière, Tobias Kerzenmacher, Nicolas Kumps, Bart

IASI: Space­based observations of the atmospheric composition using high spectral resolution Fourier transform infrared spectroscopy. Martine De Mazière, Tobias Kerzenmacher, Nicolas Kumps, Bart Dils, Christian Hermans, Alexis Merlaud, Eddy Neefs, Francis Scolas, Cindy Senten, Ann Carine Vandaele, Gauthier Vanhaelewyn, Corinne Vigouroux, Michel Kruglanski
The Infrared Atmospheric Sounding Interferometer (IASI) is a Fourier transform spectrometer onboard METOP­1 (which will have a time coverage up to 2020 with the follow­up satellites METOP­2 and ­3), observing at nadir the radiance from the Earth surface and atmosphere.
 Satellite/orbit characteristics: Launched on 19 October 2006 by a Soyouz into a sun­synchronous polar orbit, at 817 km altitude, with a 9h30 local equator crossing time
 Instrument characteristics:
 Spectral range : 645­2760 cm­1 (15.5 – 3.62 µm) ; spectral resolution: 0.35 to 0.5 cm­1; radiometric resolution: 0.25 to 0.5 K
 viewing geometry: nadir, up to ± 48.3° off­axis; swath width: ±1200 km
 spatial resolution at nadir: 3.3° x 3.3°, 50 x 50 km2 at nadir. Each cell is analyzed simultaneously by a 2 x 2 array of detectors. With a instantaneous FOV diameter of 14.65 mrad, each pixel diameter is 12 km at nadir.
Global coverage twice daily ; 1.3 106 measurements per day (day and night)
Products:
 Meteo products: high accuracy temperature (1 K) and humidity (10%) profiles; cloud cover
 Atmospheric chemistry products (trace gases):  Operational products : O3 partial columns, CO, CH4, N2O, CO2 total columns
 Scientific products: SO2, HNO3, CH4 partial columns, aerosol/dust, … in progress Important dates:
 30 November 2006: first IASI measurements available
 May 2007 : L1c data operationally distributed → reception at BIRA via EUMETCAST
 22 February 2008: operational distribution of L2 trace gas products
Ground view from space. Swath lines and pixels are indicated in the figure above.
Applications: meteorology; climate; stratospheric an tropospheric chemistry
Work at BIRA­IASB
Development of the ASIMUT line-by-line radiative transfer and inversion
code
 Possibility to retrieve columns and/or profiles of atmospheric constituents simultaneously from different spectra, which may have been recorded by different instruments or obtained under different geometries. This allows the possibility to perform combined retrieval, e.g., of a ground based measurement and a satellite­based one probing the same air mass, or from spectra recorded by different instruments on the same platform;
Analytical derivation of the Jacobians;
Use of the Optimal Estimation method (OEM), using diagonal or full covariance matrices; Portable
Modular → now being re­written in C++
Verified against LBLRTM as to the forward model part; verified against SFIT2 as to the inversion of ground­based spectra
Development of an aerosol scattering module to be
coupled to ASIMUT
 thin aerosol layer in the troposphere at altitude haer, characterised by 2 aerosol components with densities n1 and n2 and Henvey­
Greenstein phase function  RT: discrete stream approximation (doubling method) including o surface and atmospheric path emission o scattering by surface and aerosol
 inversion using OEM
 analytically calculated Jacobians with respect to Tsurf, n1 and n2
Objective: retrieve aerosol optical densities from IASI spectra
Retrieval of the scientific CH4 product, total and partial column abundances, above Ile de La Réunion and selected areas
Retrievals are performed in a two stage process: Window 1.1: 1084.0—1134.0 cm­1
mainly CO2, H2O and O3, noise = 0.5 10­7 W/cm2/Sr/cm­1
Window 1.2: 1392.0—1442.0 cm­1 mainly H2O, noise = 0.2 10­7 W/cm2/Sr/cm­1
Fitted parameters: surface temperature Ts, H2O in seven layers and the O3 column Window 2.1: 1175.5—1185.5 cm­1
mainly H2O and N2O, noise = 0.4 10­7 W/cm2/Sr/cm­1
Window 2.2: 1228.0—1238.0 cm­1
mainly CH4 and H2O, noise = 0.2 10­7 W/cm2/Sr/cm­1
Window 2.3: 1299.0—1309.0 cm­1
mainly CH4, N2O and H2O, noise = 0.2 10­7 W/cm2/Sr/cm­1
Fitted parameters: CH4 in 13 layers and the N2O column
Radiances retrieved separately for CH4, H2O, N2O and all of them compared to the IASI radiance. Selection criteria for good retrievals and cloud reduction: CH4 total columns at St Denis in 2007 showing comparisons of two ground based retrievals and IASI retrievals
χ ²y < # of spectra/3 for first pass, χ²y < # of spectra/3 for second pass
Ground
All the pixels for the 2 September 2007 within 100 km from St. Denis. Blue: sea pixels; red: mixed sea/ground pixels; green: ground pixels. Only sea pixels are used in the analyses. Validation of IASI atmospheric
chemistry products using ground-based
Ny Ålesund
FTIR network
Kiruna
Bremen
cruise ship
Jungfraujoch
Merida
Izaña
RMS < median RMS + 1σ for both passes
Ts > 283.15 K and other restrictions on Ts in order to minimise cloud contamination using T from ECMWF and solar intensity observations over St. Denis.
However: preliminary comparisons with ground­based measurements show high variability of IASI retrievals. Cloud information of IASI AVHRR has to be taken into account to exclude all the cloudy pixels.
Supporting organisms : national PRODEX contracts, BIRA/IASB funding, EUMETSAT satellite validation programme
Wollongong
Reunion Island
Arrival Heights
Lauder
 Focus on O3, CO, N2O, CH4 and HNO3