Modelling study of the impact of deep convection on the utls air composition - Part I: Analysis of ozone precursors
[摘要] The aim of this work is to study the local impact on the uppertroposphere/lower stratosphere air composition of an extreme deep convectivesystem. For this purpose, we performed a simulation of a convective clustercomposed of many individual deep convective cells that occurred near Bauru(Brazil). The simulation is performed using the 3-D mesoscale model RAMScoupled on-line with a chemistry model. The comparisons with meteorologicalmeasurements show that the model produces meteorological fields generallyconsistent with the observations.
The present paper (part I) is devoted to the analysis of the ozoneprecursors (CO, NOx and non-methane volatile organic compounds) andHOx in the UTLS. The simulation results show that the distribution ofCO with altitude is closely related to the upward convective motions andconsecutive outflow at the top of the convective cells leading to a bulge ofCO between 7 km altitude and the tropopause (around 17 km altitude). Themodel results for CO are consistent with satellite-borne measurements at 700 hPa. The simulation also indicates enhancedamounts of NOx up to 2 ppbv in the 7–17 km altitude layer mainly produced by the lightning associatedwith the intense convective activity. For insoluble non-methane volatileorganic compounds, the convective activity tends to significantly increasetheir amount in the 7–17 km layer by dynamical effects. During daytime inthe presence of lightning NOx, this bulge is largely reduced in theupper part of the layer for reactive species (e.g. isoprene, ethene) becauseof their reactions with OH that is increased on average during daytime.Lightning NOx also impacts on the oxydizing capacity of the uppertroposphere by reducing on average HOx, HO2, H2O2 andorganic hydroperoxides. During the simulation time, the impact of convectionon the air composition of the lower stratosphere is negligible for all ozoneprecursors although several of the simulated convective cells nearly reachthe tropopause. There is no significant transport from the upper troposphereto the lower stratosphere, the isentropic barrier not being crossed byconvection.The impact of the increase of ozone precursors and HOx in the uppertroposphere on the ozone budget in the LS is discussed in part II of thisseries of papers.
