Evaluation of vertical transport in the Asian monsoon 2017 from CO2 reconstruction in the ERA5 and ERA-Interim reanalysis
Authors: Vogel Bärbel , Volk Michael , Wintel Johannes , Lauther Valentin , Clemens Jan , Grooß Jens-Uwe , Günther Gebhard , Hoffmann Lars , Laube Johannes C. , Müller Rolf , Ploeger Felix , Stroh Fred 
Published: June 2023

Abstract. Atmospheric concentrations of many greenhouse gases especially CO2 are increasing globally. In particular the rapid increase of anthropogenic CO2 emissions in Asia contributes strongly to the acceleration of the CO2 growth rate in the atmosphere. During the Asian monsoon season, greenhouse gases as well as pollution emitted near the ground rapidly propagate up to an altitude of 13 km (~360 K potential temperature) with slower ascent and mixing with the stratospheric background above. However, CO2 sources in South Asia are poorly quantified. Here, differences in transport of air in the regions of the Asian summer monsoon 2017 were inferred using the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by three data sets, namely two ECMWF reanalyses in different resolutions (ERA-Interim, ERA5 and ERA5 1° x 1°). These model results are assessed using unique airborne measurements up to altitudes of ~20 km (~475 K) during the Asian summer monsoon 2017 conducted with the Geophysica aircraft during the StratoClim campaign in Nepal. Trajectory-based transport times, air mass source regions at the Earth's surface, mean effective ascent rates and age spectra as well as mean age of air from 3-dimensional CLaMS simulations are compared using the three data sets and evaluated by observation-based ascent rates. Our findings confirm that because of a better spatial and temporal resolution, ERA5 reanalysis yields a better representation of convection than ERA-Interim. Further, our findings show that transport times from the surface to the Asian monsoon anticyclone as well as the origin of air at the Earth's surface are both very sensitive to the used reanalysis. Above 430 K, the mean effective ascent rates derived from ERA5 back-trajectories and ERA5 1° x 1° (~0.2–0.3 K/day) are in good agreement with the observation-based mean ascent rates inferred from long-lived trace gases such as C2F6 and HFC-125 derived from air samples collected by the whole air sampler aboard Geophysica. Mean effective ascent rates derived from ERA-Interim back-trajectories are much faster ~0.5 K/day at these altitudes. In the Asian monsoon region at 470 K, mean age of air is larger than 3 years for ERA5 1° x 1° and about 2 years for ERA-Interim, whereas an observation-based age of air is up to 2.5 years. A reliable reconstruction (simulation) of vertical CO2 profiles during the Asian monsoon is a challenge for model simulations because the seasonal variability of CO2 at the ground, mixing with aged stratospheric air and the vertical velocities (including convection as well as vertical ascent caused by diabatic heating in the UTLS) have to be represented accurately in the simulations. Up to 410 K, the presented CO2 reconstruction agrees best with high-resolution in situ aircraft CO2 measurements using ERA5 compared to ERA5 1° x 1° and ERA-Interim, indicating a better representation of Asian monsoon transport for the newer ECMWF reanalysis product ERA5. Above 410 K the uncertainties of the CO2 reconstruction are increasing because of mixing with aged air.