Analysis of CO in the tropical troposphere using Aura satellite data and the GEOS-Chem model: insights into transport characteristics of the GEOS meteorological products

摘要

We use the GEOS-Chem chemistry-transport model (CTM) to interpret thespatial and temporal variations of tropical tropospheric CO observed by theMicrowave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer(TES). In so doing, we diagnose and evaluate transport in the GEOS-4 andGEOS-5 assimilated meteorological fields that drive the model, with aparticular focus on vertical mixing at the end of the dry season whenconvection moves over the source regions. The results indicate that overSouth America, deep convection in both GEOS-4 and GEOS-5 decays at too lowan altitude early in the wet season, and the source of CO from isoprene inthe model (MEGAN v2.1) is too large, causing a lag in the model's seasonalmaximum of CO compared to MLS CO in the upper troposphere (UT). TES and MLSdata reveal problems with excessive transport of CO to the easternequatorial Pacific and lofting in the ITCZ in August and September,particularly in GEOS-4. Over southern Africa, GEOS-4 and GEOS-5 simulationsmatch the phase of the observed CO variation from the lower troposphere (LT)to the UT fairly well, although the magnitude of the seasonal maximum isunderestimated considerably due to low emissions in the model. A sensitivityrun with increased emissions leads to improved agreement with observed CO inthe LT and middle troposphere (MT), but the amplitude of the seasonalvariation is too high in the UT in GEOS-4. Difficulty in matching CO in theLT and UT implies there may be overly vigorous vertical mixing in GEOS-4early in the wet season. Both simulations and observations show a time lagbetween the peak in fire emissions (July and August) and in CO (Septemberand October). We argue that it is caused by the prevailing subsidence in theLT until convection moves south in September, as well as the low sensitivityof TES data in the LT over the African Plateau. The MLS data suggest thattoo much CO has been transported from fires in northern Africa to the UT inthe model during the burning season, as does MOZAIC aircraft data, perhapsas a result of the combined influence of too strong Harmattan winds in theLT and too strong vertical mixing over the Gulf of Guinea in the model.