Interpol-IAGOS: a new method for assessing long-term chemistry–climate simulations in the UTLS based on IAGOS data, and its application to the MOCAGE CCMI REF-C1SD simulation

Abstract

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 these modelling exercises 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. Even though a few model assessments have been made using the 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. As a first application, the REF-C1SD reference simulation generated by the MOCAGE (MOdèle de Chimie Atmosphérique à Grande Echelle) CTM in the framework of Chemistry-Climate Model Initiative (CCMI) phase I has been evaluated during the 1994–2013 period for ozone (O3) and the 2002–2013 period for carbon monoxide (CO). The concept of the new comparison software proposed here (so-called Interpol-IAGOS) is to project all IAGOS data onto the 3-D grid of the model with a monthly resolution, since generally the 3-D outputs provided by chemistry–climate models for multi-model comparisons on multi-decadal timescales are archived as monthly means. This provides a new IAGOS data set (IAGOS-DM) mapped onto the model's grid and time resolution. To get a model data set consistent with IAGOS-DM for the comparison, a subset of the model's outputs is created (MOCAGE-M) by applying a mask that retains only the model data at the available IAGOS-DM grid points. Climatologies are derived from the IAGOS-DM product, and good correlations are reported between with the 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 midlatitudes. They are remarkably well reproduced by the model for lower-stratospheric O3 and also good for upper-tropospheric CO. Along this model evaluation, we also assess the differences caused by the use of a weighting function in the method when projecting the IAGOS data onto the model grid compared to the scores derived in a simplified way. We conclude that the data projection onto the model's grid allows us to filter out biases arising from either spatial or temporal resolution, and the use of a weighting function yields different results, here by enhancing the assessment scores. 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 the IAGOS data set.

Impact of present and future aircraft NOxand aerosol emissions on atmospheric composition and associated direct radiative forcing of climate

“…Information on the instruments is available in Thouret et al (1998) and Marenco et al (1998) for ozone and in Nédélec et al (2003Nédélec et al ( , 2015 for CO. The model assessment against the aircraft-based measurements is performed using the Interpol-IAGOS software (Cohen et al, 2021) that entails projecting the IAGOS data onto the model grid and deriving monthly means for each sampled grid cell. The model monthly averages are then derived from the daily output by applying a mask with respect to the IAGOS sampling.…”

Impact of present and future aircraft NO<sub>x </sub>and aerosol emissions on atmospheric composition and associated direct radiative forcing of climate

“…Information on the instruments is available in Thouret et al (1998) and Marenco et al (1998) for ozone and in Nédélec et al (2003Nédélec et al ( , 2015 for CO. The model assessment against the aircraft-based measurements is performed using the Interpol-IAGOS software (Cohen et al, 2021) that consists in projecting the IAGOS data onto the model grid and to derive monthly means for each sampled grid cell. The model monthly averages are then derived from the daily output by applying a mask with respect to the IAGOS sampling.…”

“…Last, for each data set, these monthly outputs are used to derive seasonal and annual climatologies. For this comparison we apply the methodology for comparison with the global model described in Cohen et al (2021). Since all the four cruise-altitude levels are regularly crossed by the tropopause, each of them will contain both high-ozone (low-CO) and low-ozone (high-CO) grid cells.…”

The Climate Impact of Hypersonic Transport

“…Seasonal and annual climatologies are then calculated on the 3D models grids. As in Cohen et al (2021a), a grid point is filtered out, if the total amount of IAGOS data is below a minimum threshold, the latter decreasing with latitudes in order to account for the grid cell area. The validated grid points are then averaged together as partial columns, with a 400 hPa lower bound in order to exclude the IAGOS data recorded during ascent and descent phases near airports.…”

“…Discussion started: 19 May 2022 c Author(s) 2022. CC BY 4.0 License.In order to allow a direct comparison between simulation outputs and the IAGOS data, the Interpol-IAGOS software used here(Cohen et al, 2021a) first projects observations onto model grids, then derives monthly means. The subsequent products are called IAGOS-DM-INCA and IAGOS-DM-EMAC.…”