The mean ozone profile and its temperature sensitivity in the upper stratosphere and lower mesosphere: An analysis of LIMS observations

Abstract

A detailed analysis of one week (May 1–7, 1979) of data from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) is presented, with emphasis on the ozone abundance and its temperature sensitivity between 0.1 and 6 mbar, covering the upper stratosphere and lower mesosphere. The time period was chosen to minimize possible ozone transport from large‐scale dynamical disturbances. The zonally averaged ozone profile (30°–35°N latitude) is compared with results from a simplified photochemical model that assumes ozone to be in photochemical steady state. The model is constrained by the simultaneous LIMS observations of temperature, H2O, and NO2. Such constraints for both the stratosphere and mesosphere have not been available prior to the LIMS data set. The model ozone profile is systematically lower than the observed profile, which is in good agreement with the observations of other experiments below ∼0.2 mbar. However, the calculated uncertainties in the model values do overlap the range defined by the observational uncertainties. Certain key parameters are identified, changes in which can systematically increase the ozone profile in both the stratosphere and mesosphere. In particular, changes in O2 photolysis and O3 formation could eliminate most of the differences in the whole altitude range. Thus one does not have to invoke one or more missing key reactions in current photochemical models in order to explain this systematic discrepancy. The LIMS‐derived values for the sensitivity of ozone to changes in temperature, ΘL, are compared with equilibrium model calculations, ΘE, which include the temperature‐driven opacity feedback effect on photodissociation rate constants. Given the noise of the data, there is fair agreement in the mesosphere, but below 1 mbar, ΘL/ΘE decreases with increasing pressure. Uncertainties in the photochemical model (or in the data) cannot account for this discrepancy. The theoretical ozone response to temperature perturbations is investigated. We show how, in general, the ozone‐temperature sensitivity coefficient, Θ, is affected by zonal and vertical advection terms as well as by the photochemical coupling between O3 and T. Zonal advection produces a Doppler shift of the wave frequency, thereby affecting the ozone‐temperature sensitivity at a fixed point. Vertical wave advection can be strong enough (for local gravity waves) to lead to an “advective equilibrium” situation, whereby ozone and temperature changes (reflected in Θ) merely follow the vertical gradients of mean ozone and potential temperature. Long (greater than a week) period planetary‐scale waves, on the other hand, do not act fast enough to perturb ozone away from photochemical equilibrium (at least above about 5 mbar). The theoretical Θ values for the lower mesosphere are close to the photochemical values, for waves other than short (less than a day) period gravity waves. In the upper stratosphere the LIMS‐derived Θ values can be explained by a combination/superposition of waves with 1‐ to 5‐day periods. Simultaneous observations of ozone and temperature in the middle atmosphere can provide indirect information about wave activity (frequency). The parameter ΘE is not very sensitive to changes in model photochemistry. Similarly, “observed Θ” is not a very good indicator of potential future changes in the photochemistry caused by chlorine increases.