Preparing the assimilation of the future MTG‐IRS sounder into the mesoscale numerical weather prediction AROME model

Abstract

The IRS (InfraRed Sounder) instrument is an infrared Fourier transform spectrometer that will be on board the Meteosat Third Generation series of the future EUMETSAT geostationary satellites. It will measure the radiance emitted by the Earth at the top of the atmosphere using 1960 channels. IRS will provide high spatial and temporal frequency 4D information on atmospheric temperature and humidity, winds, clouds, surfaces, as well as on the chemical composition of the atmosphere. The assimilation of these new observations represents a great challenge and opportunity for the improvement of Numerical Weather Prediction (NWP) forecast skill, especially for mesoscale models such as AROME at Météo‐France (Brousseau et al. 2016). The objectives of this study are to prepare for the assimilation of IRS in this system and to evaluate its impact on the forecasts when added to the currently assimilated observations.By using an Observing System Simulation Experiment (OSSE) constructed for a mesoscale NWP model. This OSSE framework makes use of synthetic observations of both IRS and the currently assimilated observing systems in AROME, constructed from a known and realistic state of the atmosphere. The latter, called the Nature Run, is derived from a long and uninterrupted forecast of the mesoscale model. These observations were assimilated and evaluated using a 1 h update cycle 3D‐Var data assimilation system over two‐month periods, one in the summer and one in the winter.This study demonstrates the benefits that can be expected from the assimilation of IRS observations into AROME NWP system. The assimilation of only 75 channels over oceans increases the total amount of observations used in the AROME 3D‐Var by about 50 %. The IRS impact in terms of forecast scores was evaluated and compared for the summer and winter periods. The main findings are that (i) over both periods the assimilation of these observations lead to statistically improved forecasts over the whole atmospheric column, (ii) for the summer season experiment, the forecast ranges up to +48h are improved, (iii) for the winter season experiment, the impact on the forecasts is globally positive but is smaller compared to the summer period and extends only to 24 h. Based on these results, it is foreseen that the addition of future IRS observations in the AROME NWP systems will significantly improve mesoscale weather forecasts.This article is protected by copyright. All rights reserved.

A global observing‐system simulation experiment for the EPS–Sterna microwave constellation

“…A calibration of the noise added to the simulated observations is performed in order to have a weight in the assimilation similar to that of real observations. The calibration procedure applied is similar to the one adopted for the previous OSSEs conducted at Météo-France (Coopmann et al, 2023;Duruisseau et al, 2017;Guedj et al, 2014). The calibration is performed by tuning the magnitude of perturbations added to the simulated observation.…”

“…Different methodologies exist to measure these benefits, such as ensemble data assimilation (EDA) or observing-system simulation experiments (OSSEs: Bormann et al, 2023). In this article, we assess the impact of the EPS-Sterna constellation using the OSSE methodology, building on Météo-France's past experience conducting OSSEs for other observing systems (Coopmann et al, 2023;Duruisseau et al, 2017;Guedj et al, 2014). An OSSE mimics a real NWP system with simulated data by providing forecasts with realistic errors compared with a known truth.…”

Enhancing Clear Radiance Generation for Geostationary Hyperspectral Infrared Sounder Using High Temporal Resolution Information

“…However, GeoHIS provides high temporal resolution observations, which are critical for tracking rapidly changing parameters such as atmospheric water vapor. The temporal resolution for GeoHIS can be as high as 5 min (Coopmann et al., 2023) over a designated region, this offers a unique opportunity to enhance clear radiance generation using temporal information. For example, within a relatively short period (e.g., less than 15 min), it is usually reasonable to assume that the atmospheric profile remains unchanged or changes minimally within that time window.…”