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Abstract:

The Reference Forward Model (RFM) is a general purpose line-by-line radiative transfer model, currently 91̽»¨ed by the UK National Centre for Earth Observation. This paper outlines the algorithms used by the RFM, focusing on standard calculations of terrestrial atmospheric infrared spectra followed by a brief summary of some additional capabilities and extensions to microwave wavelengths and extraterrestrial atmospheres. At its most basic level — the ‘line-by-line’ component — it calculates molecular absorption cross-sections by applying the Voigt lineshape to all transitions up to ±25 cm−1 from line-centre. Alternatively, absorptions can be directly interpolated from various forms of tabulated data. These cross-sections are then used to construct infrared radiance or transmittance spectra for ray paths through homogeneous cells, plane-parallel or circular atmospheres. At a higher level, the RFM can apply instrumental convolutions to simulate measurements from Fourier transform spectrometers. It can also calculate Jacobian spectra and so act as a stand-alone forward model within a retrieval scheme. The RFM is designed for robustness, flexibility and ease-of-use (particularly by the non-expert), and no claims are made for superior accuracy, or indeed novelty, compared to other line-by-line codes. Its main limitations at present are a lack of scattering and simplified modelling of surface reflectance and line-mixing.

Abstract:

Sulfur dioxide (SO2) is an important atmospheric constituent that plays a crucial role in many atmospheric processes. Volcanic eruptions are a significant source of atmospheric SO2 and its effects and lifetime depend on the SO2 injection altitude. The Infrared Atmospheric Sounding Interferometer (IASI) on the METOP satellite can be used to study volcanic emission of SO2 using high-spectral resolution measurements from 1000 to 1200 and from 1300 to 1410 cm−1 (the 7.3 and 8.7 µm SO2 bands) returning both SO2 amount and altitude data. The scheme described in Carboni et al. (2012) has been applied to measure volcanic SO2 amount and altitude for 14 explosive eruptions from 2008 to 2012. The work includes a comparison with the following independent measurements: (i) the SO2 column amounts from the 2010 Eyjafjallajökull plumes have been compared with Brewer ground measurements over Europe; (ii) the SO2 plumes heights, for the 2010 Eyjafjallajökull and 2011 Grimsvötn eruptions, have been compared with CALIPSO backscatter profiles. The results of the comparisons show that IASI SO2 measurements are not affected by underlying cloud and are consistent (within the retrieved errors) with the other measurements. The series of analysed eruptions (2008 to 2012) show that the biggest emitter of volcanic SO2 was Nabro, followed by Kasatochi and Grímsvötn. Our observations also show a tendency for volcanic SO2 to reach the level of the tropopause during many of the moderately explosive eruptions observed. For the eruptions observed, this tendency was independent of the maximum amount of SO2 (e.g. 0.2 Tg for Dalafilla compared with 1.6 Tg for Nabro) and of the volcanic explosive index (between 3 and 5).

Abstract:

Line-by-line (LBL) methods of numerically solving the equations of radiative transfer can be inhibitingly slow. Operational trace gas retrieval schemes generally require much faster output than current LBL radiative transfer models can achieve. One option to speed up computation is to precalculate absorption cross sections for each absorbing gas on a fixed grid and interpolate. This work presents a general method for creating, compressing, and validating a set of individual look-up tables (LUTs) for the 11 most abundant trace gases to use the Reference Forward Model (RFM) to simulate radiances observed by the Infrared Atmospheric Sounding Interferometer (IASI) at a more operational pace. These LUTs allow the RFM to generate radiances more than 20 times faster than LBL mode and were rigorously validated for 80 different atmospheric scenarios chosen to represent variability indicative of Earth’s atmosphere. More than 99 % of all IASI simulated spectral channels had LUT interpolation errors of brightness temperature less than 0.02 K, several factors below the IASI noise level. Including a reduced spectral grid for radiative transfer sped up the computation by another factor of six at the expense of approximately doubling interpolation errors, still factors below IASI noise. Furthermore, a simple spectral compression scheme based upon linear interpolation is presented, which reduced the total LUT file size from 120 Gbytes to 5.6 Gbytes; a compression to just 4.4 % of the original. These LUTs are openly available for use by the scientific community, whether using the RFM or to be incorporated into any forward model.