A simulated observation database to assess the impact of the IASI-NG hyperspectral infrared sounder

Abstract

Abstract. The highly accurate measurements of the hyperspectral Infrared Atmospheric Sounding Interferometer (IASI) are used in numerical weather prediction (NWP), atmospheric chemistry and climate monitoring. As the second generation of the European Polar System (EPS-SG) is being developed, a new generation of IASI instruments has been designed to fly on board the MetOp-SG constellation: IASI New Generation (IASI-NG). In order to prepare the arrival of this new instrument, and to evaluate its impact on NWP and atmospheric chemistry applications, a set of IASI and IASI-NG simulated data was built and made available to the public to set a common framework for future impact studies. This paper describes the information available in this database and the procedure followed to run the IASI and IASI-NG simulations. These simulated data were evaluated by comparing IASI-NG to IASI observations. The result is also presented here. Additionally, preliminary impact studies of the benefit of IASI-NG compared to IASI on the retrieval of temperature and humidity in a NWP framework are also shown in the present work. With a channel dataset located in the same wave numbers for both instruments, we showed an improvement of the temperature retrievals throughout the atmosphere, with a maximum in the troposphere with IASI-NG and a lower benefit for the tropospheric humidity.

An update on the RTTOV fast radiative transfer model (currently at version 12)

“…Concerning simulating cloudy radiances, the original version of RTTOV could only compute the radiance from a fractional cover of uniform grey cloud layer at a defined level. The inclusion of cloud and hydrometeor scattering and absorption at each level and at all wavelengths within RTTOV has allowed cloudy radiances, often referred to as all-sky, to be computed for more complicated cloud regimes which match the observed radiances well (Aumann et al, 2018). This has proven useful for producing simulated satellite imagery from model fields to compare with the real imagery (Blackmore et al, 2014) and for developing an all-sky radiance assimilation system (Geer et al, 2017).…”

“…Saunders et al: An update on the RTTOV fast radiative transfer model There have been several other fast models developed over the years, notably the Community Radiative Transfer Model (CRTM) (Chen et al, 2008;Ding et al, 2011), which is also used in several NWP models, the Optimal Spectral Sampling (OSS) model (Moncet et al, 2015) and many others which have taken part in several inter-comparisons with RTTOV (e.g. Garand et al, 2001;Saunders et al, 2007;Aumann et al, 2018). These fast models are not only forward models (i.e.…”

A new gas absorption optical depth parameterisation for RTTOV version 13

“…Instead a fast parameterisation of Rayleigh extinction is applied within RTTOV at run time. This follows Bucholtz (1995), who provides a parameterisation of the Rayleigh volume scattering coefficient β s at a standard temperature T s and pressure p s as a function of wavelength and gives the scattering coefficient β at arbitrary temperature T and pressure p as…”

“…RTTOV was originally developed at the European Centre for Medium-Range Weather Forecasts (ECMWF) in the 1990s to enable the direct assimilation of radiances in their operational numerical weather prediction system and since 1998 has undergone much development within the EUMETSAT-funded Numerical Weather Prediction Satellite Applications Facility (NWP SAF). Today RT-TOV is widely used around the world in a range of applications including operational data assimilation in NWP (Lupu and Geer, 2015), physical retrievals using satellite data (Ghent et al, 2017), generating simulated satellite imagery from NWP models (Lupu and Wilhelmsson, 2016), and studies assessing future satellite instruments (Andrey-Andrés et al, 2018). In order to support variational assimilation and retrieval applications, RTTOV comprises not only a direct (or forward) model, but also tangent linear, adjoint, and full Jacobian models.…”

Response to reviewer 2

“…Concerning simulating cloudy radiances, the original version of RTTOV could only compute the radiance from a fractional cover of uniform grey cloud layer at a defined level. The inclusion of cloud and hydrometeor scattering and absorption at each level and at all wavelengths within RTTOV has allowed cloudy radiances, often referred to as all-sky, to be computed for more complicated cloud regimes which match the observed radiances well (Aumann et al, 2018). This has proven useful for producing simulated satellite imagery from model fields to compare with the real imagery (Blackmore et al, 2014) and for developing an all-sky radiance assimilation system .…”