Overflights of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere‐Troposphere Exchange Project (STEP) show the presence of two mesoscale phenomena: a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclone's entire cloud shield (wavelength of 250 km or greater). The larger feature is fairly steady, though its physical interpretation is ambiguous. The 110‐km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is comparable to that of 100‐to 150‐km‐diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield; these cells have lifetimes of 4.5 to 6 hours. Aircraft flights through the anvil show that these cells correspond to regions of enhanced convection, higher cloud altitude, and upwardly displaced potential temperature surfaces. A three‐dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variables associated with the 110‐km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400–600 m, a lifetime of 4.5–6 hours and a size comparable to the cells of low brightness temperature. The forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity waves is definitely important to the westerly phase of the stratopause semiannual oscillation and possibly important to the easterly phase of the quasi‐biennial oscillation. There is strong evidence that some of the mesoscale gravity waves break below 20 km as well. The effect of this wave breaking on the diabatic circulation below 20 km may be comparable to that of above‐cloud diabatic cooling.

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Instrumentation on the NASA ER‐2 high‐altitude aircraft has been used to observe engine exhaust from the same aircraft while operating in the lower stratosphere. Encounters with the exhaust plume occurred approximately 10 min after emission with spatial scales near 2 km and durations of up to 10 s. Measurements include total reactive nitrogen, NOy, the component species NO and NO2, CO2, H2O, CO, N2O, condensation nuclei, and meteorological parameters. The integrated amounts of CO2 and H2O during the encounters are consistent with the stoichiometry of fuel combustion (1:1 molar). Emission indices (EI) for NOx (=NO + NO2), CO, and N2O are calculated using simultaneous measurements of CO2. EI values for NOx near 4 g (kg fuel)−1 are in good agreement with values scaled from limited ground‐based tests of the ER‐2 engine. Non‐NOx species comprise less than about 20% of emitted reactive nitrogen, consistent with model evaluations. In addition to demonstrating the feasibility of aircraft plume detection, these results increase confidence in the projection of emissions from current and proposed supersonic aircraft fleets and hence in the assessment of potential long‐term changes in the atmosphere.

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N205 agree well with measured concentrations of NO and C10, but concentrations of HO2 and OH are underestimated by 10 to 25%, concentrations of NO 2 are overestimated by 10 to 30%, and concentrations of HC1 are overestimated by a factor of 2. Discrepancies for [OH] and [HO2] are reduced if we allow for higher yields of O( 1 D) from O3 photolysis and for heterogeneousproduction of HNO:. The data suggest more efficient catalytic removal of O3 by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [03] in the lower stratosphere may be larger in response to inputs of NO• from supersonic aircraft than estimated by current assessment models.