Three-dimensional methane distribution simulated with FLEXPART 8-CTM-1.1 constrained with observation data
Authors: Groot Zwaaftink Christine D. , Henne Stephan , Thompson Rona L. , Dlugokencky Edward J. , Machida Toshinobu , Paris Jean-Daniel , Sasakawa Motoki , Segers Arjo , Sweeney Colm , Stohl Andreas 
Published: November 2018

Abstract. A Lagrangian particle dispersion
model, the FLEXible PARTicle dispersion chemical transport model (FLEXPART
CTM), is used to simulate global three-dimensional fields of trace gas
abundance. These fields are constrained with surface observation data through
nudging, a data assimilation method, which relaxes model fields to observed
values. Such fields are of interest to a variety of applications, such as
inverse modelling, satellite retrievals, radiative forcing models and
estimating global growth rates of greenhouse gases. Here, we apply this
method to methane using 6 million model particles filling the global model
domain. For each particle, methane mass tendencies due to emissions (based on
several inventories) and loss by reaction with OH, Cl and O(1D), as
well as observation data nudging were calculated. Model particles were
transported by mean, turbulent and convective transport driven by 1∘×1∘ ERA-Interim meteorology. Nudging is applied at 79 surface
stations, which are mostly included in the World Data Centre for Greenhouse
Gases (WDCGG) database or the Japan–Russia Siberian Tall Tower Inland
Observation Network (JR-STATION) in Siberia. For simulations of 1 year
(2013), we perform a sensitivity analysis to show how nudging settings affect
modelled concentration fields. These are evaluated with a set of independent
surface observations and with vertical profiles in North America from the
National Oceanic and Atmospheric Administration (NOAA) Earth System Research
Laboratory (ESRL), and in Siberia from the Airborne Extensive Regional
Observations in SIBeria (YAK-AEROSIB) and the National Institute for
Environmental Studies (NIES). FLEXPART CTM results are also compared to
simulations from the global Eulerian chemistry Transport Model version 5
(TM5) based on optimized fluxes. Results show that nudging strongly improves
modelled methane near the surface, not only at the nudging locations but also
at independent stations. Mean bias at all surface locations could be reduced
from over 20 to less than 5 ppb through nudging. Near the surface, FLEXPART
CTM, including nudging, appears better able to capture methane molar mixing
ratios than TM5 with optimized fluxes, based on a larger bias of over 13 ppb
in TM5 simulations. The vertical profiles indicate that nudging affects model
methane at high altitudes, yet leads to little improvement in the model
results there. Averaged from 19 aircraft profile locations in North America
and Siberia, root mean square error (RMSE) changes only from 16.3 to
15.7 ppb through nudging, while the mean absolute bias increases from 5.3 to
8.2 ppb. The performance for vertical profiles is thereby similar to TM5
simulations based on TM5 optimized fluxes where we found a bias of 5 ppb and
RMSE of 15.9 ppb. With this rather simple model setup, we thus provide
three-dimensional methane fields suitable for use as boundary conditions in
regional inverse modelling as a priori information for satellite retrievals
and for more accurate estimation of mean mixing ratios and growth rates. The
method is also applicable to other long-lived trace gases.