A model study of the response of mesospheric ozone to short‐term solar ultraviolet flux variations

Abstract

A one‐dimensional photochemical model and a time‐dependent heat equation which incorporates non‐local thermodynamic equilibrium (non‐LTE) IR radiative transfer are used to study the response of mesospheric ozone concentration to short‐term solar UV flux variations. We compare our model results with the observed ozone response obtained from a statistical analysis of Solar Mesosphre Explorer (SME) data (Keating et al., 1987). Our model with sinusoidal 27‐day‐period forcing of mesospheric chemistry by solar ultraviolet flux, when combined with temperature‐chemistry feed‐back, time‐dependent atmospheric temperature effects, and low eddy mixing rates, reproduces the major characteristics of the observed ozone response but not the observed temperature response. Below 60 km the calculated response shows a strong dependence on the magnitude of the assumed flux modulation in the Hartley region of the spectrum. A comparison of the model O3 response with the SME observations suggests that there is negligible 27‐day variation of solar flux longward of 2400 Å, in agreement with Lean (1987). The magnitude of the computed ozone response to increased solar UV flux in the upper mesosphere (above 70 km) is strongly coupled to the water vapor abundance through the HOx catalytic cycle that removes ozone. Our model predicts the essential aspects of the observed ozone response only when the water vapor mixing ratio and vertical eddy diffusion coefficient are a factor of 5 below observed and inferred mid‐latitude values. But then the model results are in almost complete disagreement with the observed temperature amplitude and phase lag. Calculations performed with imposition of the temperature response observed by Keating et al. (1987) produces an ozone response in the middle mesosphere in severe conflict with observations, suggesting a need for better mesospheric and solar data and further modeling efforts to determine the role of dynamics in the mesospheric ozone and temperature response to solar UV variability.