Contributions of the troposphere and stratosphere to CH4 model biases

Zhiting Wang,Voltaire A Velazco,Thorsten Warneke,Michel Ramonet,Justus Notholt,Marielle Saunois,Ralf Sussmann,Dave F Pollard,Ute Karstens,Rigel Kivi,Matthias Schneider,Christof Petri,Harjinder Sembhi,Nicholas M Deutscher,David W T Griffith,Huilin Chen

doi:10.5194/acp-17-13283-2017

Abstract

Abstract. Inverse modelling is a useful tool for retrieving CH4 fluxes; however, evaluation of the applied chemical transport model is an important step before using the inverted emissions. For inversions using column data one concern is how well the model represents stratospheric and tropospheric CH4 when assimilating total column measurements. In this study atmospheric CH4 from three inverse models is compared to FTS (Fourier transform spectrometry), satellite and in situ measurements. Using the FTS measurements the model biases are separated into stratospheric and tropospheric contributions. When averaged over all FTS sites the model bias amplitudes (absolute model to FTS differences) are 7.4 ± 5.1, 6.7 ± 4.8, and 8.1 ± 5.5 ppb in the tropospheric partial column (the column from the surface to the tropopause) for the models TM3, TM5-4DVAR, and LMDz-PYVAR, respectively, and 4.3 ± 9.9, 4.7 ± 9.9, and 6.2 ± 11.2 ppb in the stratospheric partial column (the column from the tropopause to the top of the atmosphere). The model biases in the tropospheric partial column show a latitudinal gradient for all models; however there are no clear latitudinal dependencies for the model biases in the stratospheric partial column visible except with the LMDz-PYVAR model. Comparing modelled and FTS-measured tropospheric column-averaged mole fractions reveals a similar latitudinal gradient in the model biases but comparison with in situ measured mole fractions in the troposphere does not show a latitudinal gradient, which is attributed to the different longitudinal coverage of FTS and in situ measurements. Similarly, a latitudinal pattern exists in model biases in vertical CH4 gradients in the troposphere, which indicates that vertical transport of tropospheric CH4 is not represented correctly in the models.

Highlights

Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas
Compared to total column data the surface measurements characterise the boundary layer only and CH4 concentrations in the boundary layer are sensitive to boundary layer height, which is difficult to accurately simulate in a global transport model
Turner et al (2015) compared GOSAT CH4 with GEOS-Chem simulations, and found large differences at high latitudes. They proposed that the model bias in total column CH4 at high latitudes comes from the stratosphere since the validation with TCCON (Total Carbon Column Observing Network), NOAA surface and aircraft measurements, and HIPPO shows good performances of the model in the troposphere

Summary

Introduction

Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas. Atmospheric CH4 concentrations began to rise again in 2007 after a decade of nearzero growth (Rigby et al, 2008). Turner et al (2015) compared GOSAT CH4 with GEOS-Chem simulations, and found large differences at high latitudes They proposed that the model bias in total column CH4 at high latitudes comes from the stratosphere since the validation with TCCON (Total Carbon Column Observing Network), NOAA surface and aircraft measurements, and HIPPO shows good performances of the model in the troposphere.

Measurements and models

Comparison between measurements and models

Conclusions