Submitted as: methods for assessment of models 13 Oct 2020

Submitted as: methods for assessment of models | 13 Oct 2020

Review status: this preprint is currently under review for the journal GMD.

On a new assessment method for long-term chemistry-climate simulations in the UTLS based on IAGOS data: application to MOCAGE CCMI-REFC1SD simulation

Yann Cohen1,2,a, Virginie Marécal1, Béatrice Josse1, and Valérie Thouret2 Yann Cohen et al.
  • 1Centre National de Recherches Météorologiques, CNRS-Météo-France, UMR 3589, Toulouse, France
  • 2Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, France
  • anow at: Laboratoire des Sciences du Climat et de l’Environnement, LSCE-IPSL (CEA-CNRS-UVSQ), Université Paris-Saclay, 91191 Gif-sur-Yvette, France

Abstract. A wide variety of observation data sets are used to assess long-term simulations provided by chemistry-climate models (CCMs) and chemistry-transport models (CTMs). However, the upper troposphere – lower stratosphere (UTLS) has hardly been assessed in the models yet. Observations performed in the framework of IAGOS (In-service Aircraft for a Global Observing System) combine the advantages of in situ airborne measurements in the UTLS with an almost global-scale sampling, a ~ 20-year monitoring period and a high frequency. If a few model assessments have been made using IAGOS database, none of them took advantage of the dense and high-resolution cruise data in their whole ensemble yet. The present study proposes a method to compare this large IAGOS data set to long-term simulations used for chemistry-climate studies. For this purpose, a new software (named Interpol-IAGOS) projects all IAGOS data on the 3D grid of the chosen model with a monthly resolution, since generally chemistry-climate models provide 3D outputs as monthly means. This provides a new IAGOS data set (IAGOS-DM) mapped at the model's grid and time resolution. As a first application, the REF-C1SD simulation generated by MOCAGE CTM in the framework of CCMI phase-I has been evaluated during the 1994–2013 period for ozone and the 2002–2013 period for carbon monoxide. This comparison is exclusively based on the grid cells sampled by IAGOS, thus the assessed model output (MOCAGE-M) is obtained by applying a corresponding mask onto the grid. First, climatologies are derived from the IAGOS-DM product. Good correlations are reported between IAGOS-DM and MOCAGE-M spatial distributions. As an attempt to analyse MOCAGE-M behaviour in the upper troposphere (UT) and the lower stratosphere (LS) separately, UT and LS data in IAGOS-DM were sorted according to potential vorticity. From this, we derived O3 and CO seasonal cycles in eight regions well sampled by IAGOS flights in the northern mid-latitudes. They are remarkably well-reproduced by the model for lower-stratospheric O3 and also good for upper-tropospheric CO. The data projection onto the model's grid is a necessary step for a more accurate assessment, as it allows to filter out biases only due to either spatial or temporal resolution. Beyond the MOCAGE REF-C1SD evaluation presented in this paper, the method could be used by CCMI models for individual assessments in the UTLS and for model intercomparisons with respect to IAGOS data set.

Yann Cohen et al.

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Yann Cohen et al.

Data sets

IAGOS-DM-MOCAGE Yann Cohen, Virginie Marécal, Béatrice Josse, and Valérie Thouret

Model code and software

Interpol-IAGOS Yann Cohen, Virginie Marécal, Béatrice Josse, and Valérie Thouret

Yann Cohen et al.


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Short summary
Assessing long-term chemistry-climate simulations with in situ and frequent observations near the tropopause is possible with the IAGOS commercial aircraft data set. This study presents a method that distributes the IAGOS data (ozone and CO) on a model monthly grid, limiting the impact of resolution for the evaluation of the modelled chemical fields. We applied it to the CCMI-REFC1SD simulation from the MOCAGE CTM, and notably highlighted a well-reproduced O3 behaviour in the lower stratosphere.