Evaluation of column-averaged methane in models and TCCON with a focus on the stratosphere

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Abstract

<p><strong>Abstract.</strong> The distribution of methane (CH<sub>4</sub>) in the stratosphere can be a major driver of spatial variability in the dry-air column-averaged CH<sub>4</sub> mixing ratio (XCH<sub>4</sub>), which is being measured increasingly for the assessment of CH<sub>4</sub> surface emissions. Chemistry-transport models (CTMs) therefore need to simulate the tropospheric and stratospheric fractional columns of XCH<sub>4</sub> accurately for estimating surface emissions from XCH<sub>4</sub>. Simulations from three CTMs are tested against XCH<sub>4</sub> observations from the Total Carbon Column Network (TCCON). We analyze how the model–TCCON agreement in XCH<sub>4</sub> depends on the model representation of stratospheric CH<sub>4</sub> distributions. Model equivalents of TCCON XCH<sub>4</sub> are computed with stratospheric CH<sub>4</sub> fields from both the model simulations and from satellite-based CH<sub>4</sub> distributions from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) and MIPAS CH<sub>4</sub> fields adjusted to ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) observations. Using MIPAS-based stratospheric CH<sub>4</sub> fields in place of model simulations improves the model–TCCON XCH<sub>4</sub> agreement for all models. For the Atmospheric Chemistry Transport Model (ACTM) the average XCH<sub>4</sub> bias is significantly reduced from 38.1 to 13.7<span class="thinspace"></span>ppb, whereas small improvements are found for the models TM5 (Transport Model, version 5; from 8.7 to 4.3<span class="thinspace"></span>ppb) and LMDz (Laboratoire de Météorologie Dynamique model with zooming capability; from 6.8 to 4.3<span class="thinspace"></span>ppb). Replacing model simulations with MIPAS stratospheric CH<sub>4</sub> fields adjusted to ACE-FTS reduces the average XCH<sub>4</sub> bias for ACTM (3.3<span class="thinspace"></span>ppb), but increases the average XCH<sub>4</sub> bias for TM5 (10.8<span class="thinspace"></span>ppb) and LMDz (20.0<span class="thinspace"></span>ppb). These findings imply that model errors in simulating stratospheric CH<sub>4</sub> contribute to model biases. Current satellite instruments cannot definitively measure stratospheric CH<sub>4</sub> to sufficient accuracy to eliminate these biases. Applying transport diagnostics to the models indicates that model-to-model differences in the simulation of stratospheric transport, notably the age of stratospheric air, can largely explain the inter-model spread in stratospheric CH<sub>4</sub> and, hence, its contribution to XCH<sub>4</sub>. Therefore, it would be worthwhile to analyze how individual model components (e.g., physical parameterization, meteorological data sets, model horizontal/vertical resolution) impact the simulation of stratospheric CH<sub>4</sub> and XCH<sub>4</sub>.</p>

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APA

Ostler, A., Sussmann, R., Patra, P. K., Houweling, S., De Bruine, M., Stiller, G. P., … Robinson, J. (2016, September 28). Evaluation of column-averaged methane in models and TCCON with a focus on the stratosphere. Atmospheric Measurement Techniques. Copernicus GmbH. https://doi.org/10.5194/amt-9-4843-2016

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