Optical properties

1. From dust microphysical properties to dust hyperspectral infrared remote sensing Optical depth Layer altitude BT 165 (re) calculated BT 165 observed aspect ratio Volz Volz dust dust OPAC OPAC mineral mineral SHADE SHADE aspect ratio ? Microphysical properties Reid et al., 2003 T,H2O… 1. effective radius 2. shape • Aerosol microphysical properties • Size distribution • Refractive index at each wavelength • Shape • Aerosol optical properties • Extinction cross-section Cext • / efficiency Qext • Single-scattering albedo  • Phase function or asymetry parameter g g Norm. Qext  -Mie code -T Matrix code Optical properties Cext g  3.75 m 3.75 m 3.75 m Norm. Qext Qext 9.5 m 9.5 m 9.5 m Wavelength (µm) Wavelength (µm) 3. Refractive indices Phase function  g Qext Imaginary part Real part 3.75 m 9.5 m 3. Size and shape 2. Optical depth and altitude 1. Atmospheric situation size shape BT (3µm-1µm) BT (spheroids-spheres) 1. Dust altitude and 10 µm AOD (LUT retrieval) AIRS 10 µm AOD MODIS 0.55 µm AOD AIRS dust altitude July 2003 July 2003 AIRS night WVC AIRS night SST April 2003 SSMI WVC night+day MODIS night SST Input AOD Output AOD May 2003 Size 0 60 285 SST (K) 303 WVC (mm) 3. Dust effective radius June 2003 Effective radius (µm) Effective radius (µm) RetrievalAtmosphere, dust AOT, dust altitude Shape July 2003 Validation with simulations Results: April-May 2003 August 2003 Ref. indices September 2003 Clémence Pierangelo(1), Michael Mishchenko(2), Alain Chédin(1) (1) Laboratoire de Météorologie Dynamique - Institut Pierre Simon Laplace, Ecole Polytechnique (2) Goddard Institute for Space Sciences – NASA clemence.pierangelo@lmd.polytechnique.fr Why studying dust in the infrared? Advantages: • night and day detection • sensitivity to dust vertical distribution : altitude retrieval • sensitivity to dust mineralogical composition : might be retrieved • over deserts Limitations: • Spatial resolution (20km) • Aerosol refractive indices poorly known + high spectral dependency • High sensitivity to temperature and gas profiles • No direct validation IPCC 2001: dust radiative forcing poorly known. Since then, most studies focus on the visible wavelength, whereas the closure of the Earth radiative balance also needs knowledge of the dust effect on terrestrial and atmospheric infrared radiation (3.5 to 15 µm), and computations of IR forcing from visible or near-IR measurements are not reliable enough. Satellite observation Dust optical properties at infrared wavelengths Purpose: modelling of the maximum effect of shape: spheroids with aspect ratio=2 (effect stronger than a mixture of spheroids with several aspect ratios) For realistic values of re (1 to 3µm), moderate impact on , g and Qext normalized (max 30%). The impact of size is greater in the visible than in the IR. The ratio of IR to visible extinction increases with dust size. Optical properties are greater for oblate or prolate spheroids than spheres. The maximum impact is about 10%. • Weak impact of the aspect ratio on the phase function because the size parameter is relatively small. • Note that the impact of the phase function on the radiance at satellite level is not as crucial as in the visible (no reflected solar radiation). … and big variability of optical properties with data set too! (“SHADE” model probably not realistic) Huge variability of refractive indices with wavelength + with data set…  Refractive index is a more problematic issue than size or shape Radiative transfer computations with aerosols at infrared wavelengths Simulations of AIRS (Advanced Infrared Sounder-AQUA) brightness temperatures for 324 channels with a code coupling the line-by-line « 4A » and DISORT BT 177 (8.14µm)– BT 165 (9.33µm)(K.) Simulations for 100 tropical atmospheric situations (Same aerosol properties) The aerosol impact itself depends on the atmospheric situation First component to the signal: the temperature and the water vapor profiles. Aerosol optical depth (AOD) and altitude impact: a few K. Aerosol size and shape impact: a few tenth of K. Remote sensing of dust with hyperspectral infrared sounders : Application to AIRS 2. Validation of the LUT approach Validation of the LUT retrieved atmospheric situation is performed comparing its surface temperature (SST) and water vapor content (WVC) to MODIS / SSMI observations. Method: Look-Up-Tables (LUT) built for 8 AIRS channels, several dust altitudes and 10 µm AOD, one aerosol model (OPAC) and almost 600 atmospheric situations (Pierangelo et al., ACP, in press) Robustness of the retrieval to dust microphysical properties The error in the retrieval caused by the use of one fixed size distribution is below 10% for the AOD, and still lower for the altitude Method: Channel 165 (1072cm-1) sensitive to size, not to shape The error in the retrieval caused by the use of spherical particles is below 10% for the AOD, and still lower for the altitude Very strong effect of the refractive index on the AOD!!! But, with the data sets “dust”, “mineral” or “SHADE”, the retrieved AOD could not be greater than 0.5… way to exclude these models The effective radius of Saharan dust decreases from 2.8 µm to 1.2 µm with transport.