Ke. Pickering et al., UPPER TROPOSPHERIC OZONE PRODUCTION FOLLOWING MESOSCALE CONVECTION DURING STEP EMEX, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 98(D5), 1993, pp. 8737-8749
Aircraft data from the Stratosphere-Troposphere Exchange Project (STEP
) and the Equatorial Mesoscale Experiment (EMEX) flights conducted on
February 2, 1987, off northern Australia are used in cumulus cloud and
photochemical models to determine the effects of convection on upper
tropospheric O3 production. Ozone production is calculated as the amou
nt integrated over cloud outflow layers for the first 24 hours after c
onvection. Ozone production with convection is compared to ozone forma
tion in undisturbed conditions. Model simulations of the EMEX 9 convec
tive system indicate lower tropospheric air relatively rich in CO and
low in NO(x) exiting in cloud outflow, slightly depressing the rate of
O3 formation in the middle and upper troposphere. Other convective co
mplexes, 800-900 km upstream, caused even greater perturbations to mea
sured profiles of CO, NO(x), O3, and H2O and implied a 15-20% reductio
n in the rate of O3 production from 14.5 to 17 km. The greatest factor
affecting O3 formation in the upper troposphere in the STEP/EMEX flig
ht might have been lightning-produced NO(x). We estimate that O3 produ
ction from 12 to 17 km is 2-3 times more rapid than it would be with n
o lightning. This STEP/EMEX event adds to a climatology of half a doze
n cases we have analyzed to determine the effects of convection on fre
e tropospheric O3 production. The study region represents the ''mariti
me continent'' in contrast to continental regions studied previously.
Relatively small quantities of species from the lower troposphere were
transported to the upper troposphere because of the relatively weak v
ertical velocities in the storm and because chemical species gradients
had been minimized by frequent convection prior to the February 2 eve
nt. Earlier in the convective season, the chemical consequences of a s
ingle episode might have been more substantial.