| The aim of this work is to study the local impact on the upper
troposphere/lower stratosphere air composition of an extreme deep convective
system. For this purpose, we performed a simulation of a convective cluster
composed of many individual deep convective cells that occurred near Bauru
(Brazil). The simulation is performed using the 3-D mesoscale model RAMS
coupled on-line with a chemistry model. The comparisons with meteorological
measurements show that the model produces meteorological fields generally
consistent with the observations.
The present paper (part I) is devoted to the analysis of the ozone
precursors (CO, NOx and non-methane volatile organic compounds) and
HOx in the UTLS. The simulation results show that the distribution of
CO with altitude is closely related to the upward convective motions and
consecutive outflow at the top of the convective cells leading to a bulge of
CO between 7 km altitude and the tropopause (around 17 km altitude). The
model results for CO are consistent with satellite-borne measurements at 700 hPa. The simulation also indicates enhanced
amounts of NOx up to 2 ppbv in the 7–17 km altitude layer mainly produced by the lightning associated
with the intense convective activity. For insoluble non-methane volatile
organic compounds, the convective activity tends to significantly increase
their amount in the 7–17 km layer by dynamical effects. During daytime in
the presence of lightning NOx, this bulge is largely reduced in the
upper part of the layer for reactive species (e.g. isoprene, ethene) because
of their reactions with OH that is increased on average during daytime.
Lightning NOx also impacts on the oxydizing capacity of the upper
troposphere by reducing on average HOx, HO2, H2O2 and
organic hydroperoxides. During the simulation time, the impact of convection
on the air composition of the lower stratosphere is negligible for all ozone
precursors although several of the simulated convective cells nearly reach
the tropopause. There is no significant transport from the upper troposphere
to the lower stratosphere, the isentropic barrier not being crossed by
convection.
The impact of the increase of ozone precursors and HOx in the upper
troposphere on the ozone budget in the LS is discussed in part II of this
series of papers. |