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The article presents a study which assesses the retrieval of MetOp-A/IASI carbon monoxide (CO) profiles. Two retrieval algorithms are examined which include the Software for a Fast Retrieval of IASI Data (SOFRID) and Fast Optimal Retrievals on Layers for IASI (FORLI). It also outlines CO profiles from the measurements of ozone, water vapour, carbon monoxide and nitrogen oxides by airbus in-service aircraft (MOZAIC) program.

Within the African Monsoon Multidisciplinary Analysis (AMMA), we investigate the impact of nitrogen oxides produced by lightning (LiNOx) and convective transport during the West African Monsoon (WAM) upon the composition of the upper troposphere (UT) in the tropics. For this purpose, we have performed simulations with 4 state-of-the-art chemistry transport models involved within AMMA, namely MOCAGE, TM4, LMDz-INCA and p-TOMCAT. The model intercomparison is complemented with an evaluation of the simulations based on both spaceborne and airborne observations. The baseline simulations show important differences between the UT CO and O3 distributions simulated by each of the 4 models when compared to measurements of the African latitudinal transect from the MOZAIC program and to distributions measured by the Aura/MLS spaceborne sensor. We show that such model discrepancies can be explained by differences in the convective transport parameterizations and, more particularly, the altitude reached by convective updrafts (ranging between ∼200-125 hPa). Concerning UT O3, the majority of models exhibit low concentrations compared to both MOZAIC and MLS observations south of the equator, with good agreement in the Northern Hemisphere. Sensitivity studies are performed to quantify the effect of deep convective transport and the influence of LiNOx production on the UT composition. These clearly indicate that the CO maxima and the elevated O3 concentrations south of the equator are due to convective uplift of air masses impacted by Southern African biomass burning, in agreement with previous studies. Moreover, during the WAM, LiNOx from Africa are responsible for the highest UT O3 enhancements (10-20 ppbv) over the tropical Atlantic between 10° S-20° N. Differences between models are primarily due to the performance of the parameterizations used to simulate lightning activity which are eval uated using spaceborne observations of flash frequency. Combined with comparisons of in-situ NO measurements we show that the models producing the highest amounts of LiNOx over Africa during the WAM (INCA and p-TOMCAT) capture observed NO profiles with the best accuracy, although they both overestimate lightning activity over the Sahel. [ABSTRACT FROM AUTHOR]

The mechanisms of transport on annual, semi-annual and quasi-biennial time scales in the equatorial (10° S-10° N) stratosphere are investigated using the nitrous oxide (N2O) measurements of the space-borne ODIN Sub-Millimetre Radiometer instrument from November 2001 to June 2005, and the simulations of the three-dimensional Chemistry Transport Models MOCAGE and SLIMCAT. Both models are forced with analyses from the European Centre for Medium-range Weather Forecasts, but the vertical transport is derived either from the forcing analyses by solving the continuity equation (MOCAGE), or from diabatic heating rates using a radiation scheme (SLIMCAT). The N2O variations in the mid-to-upper stratosphere at levels above 32 hPa are shown to be generally captured by the models though significant differences appear with the observations as well as between the models, attributed to the difficulty of capturing correctly the slow vertical velocities of the Brewer-Dobson circulation. In the lower stratosphere (LS), below 32 hPa, the variations are shown to be principally seasonal with peak amplitude at 400K (∼19 km), and are totally missed by the models. The decrease in diabatic radiative heating in the LS during the Northern Hemisphere summer is found to be out of phase by one month and far too small to explain the observed N2O seasonal cycle. The proposed explanation for this annual variation is a combination of i) the annual cycle of tropopause height of 1 km amplitude, ii) the convective overshooting above 400 K peaking in May and absent in the models, and iii) an annual cycle of 15 ppbv amplitude of the N2O concentration at the tropopause, but for which no confirmation exists in the upper troposphere in the absence of global-scale measurements. The present study indicates i) a significant contribution of deep convective overshooting on the chemical composition of the LS at global scale up to 500 K, ii) a preferred region for that over the African continent, and iii) a maximum impact in May when the overshoot intensity is the largest and horizontal winds are the slowest. [ABSTRACT FROM AUTHOR]

The transport pathways of carbon monoxide (CO) in the African Upper Troposphere (UT) during the West African Monsoon (WAM) is investigated through the assimilation of CO observations by the Aura Microwave Limb Sounder (MLS) in the MOCAGE Chemistry Transport Model (CTM). The assimilation setup, based on a 3-D First Guess at Assimilation Time (3-D-FGAT) variational method is described. Comparisons between the assimilated CO fields and in situ airborne observations from the MOZAIC program between Europe and both Southern Africa and Southeast Asia show an overall good agreement around the lowermost pressure level sampled by MLS (∼215 hPa). The 4-D assimilated fields averaged over the month of July 2006 have been used to determine the main dynamical processes responsible for the transport of CO in the African UT. The studied period corresponds to the second AMMA (African Monsoon Multidisciplinary Analyses) aircraft campaign. At 220 hPa, the CO distribution is characterized by a latitudinal maximum around 5° N mostly driven by convective uplift of air masses impacted by biomass burning from Southern Africa, uplifted within theWAMregion and vented predominantly southward by the upper branch of the winter hemisphere Hadley cell. Above 150 hPa, the African CO distribution is characterized by a broad maximum over northern Africa. This maximum is mostly controlled by the large scale UT circulation driven by the Asian Summer Monsoon (ASM) and characterized by the Asian Monsoon Anticyclone (AMA) centered at 30° N and the Tropical Easterly Jet (TEJ) on the southern flank of the anticyclone. Asian pollution uplifted to the UT over large region of Southeast Asia is trapped within the AMA and transported by the anticyclonic circulation over Northeast Africa. South of the AMA, the TEJ is responsible for the tranport of CO-enriched air masses from India and Southeast Asia over Africa. Using the high time resolution provided by the 4-D assimilated fields, we give evidence that the variability of the African CO distribution above 150 hPa and north of the WAM region is mainly driven by the synoptic dynamical variability of both the AMA and the TEJ. [ABSTRACT FROM AUTHOR]