Middle Atmosphere Ozone Response to Solar UV Variations Over Solar Rotation and Solar Cycle Periods

A one-dimensional photochemical-radiative time -dependent model with vertical diffusion is developed to investigate the response of ozone in the middle atmosphere to solar UV irradiance variations over 27-day solar rotation periods. The results of the 1D model show that ozone decreases at altitudes of 60-80 km with increasing solar irradiance at Lyman alpha. The amplitude of the ozone decrease is about 1% of its equilibrium value at about 70-75 km. Below 50 km, the ozone increase is due to the increased photolysis of molecular oxygen by solar irradiance over the Herzberg continuum. The peak of the ozone increase is located at about 35 km and is about 0.3% of its equilibrium value at that level. The peaks of the ozone oscillation in the upper stratosphere precede the peaks of the solar oscillation. The phase lag of the ozone response decreases from about 3 days at 30 km to 0 at 35 km and -9 days at 50 km as a result of the strong influence of the temperature feedback on the ozone response through the temperature -dependent chemical reaction rates. The temperature feedback has a greater impact on the stratospheric ozone response than on the mesospheric ozone response. The modified NCAR two-dimensional model originally developed by Guy Brasseur's group is also used to study the short-term ozone response. The comparison of the 2D results with the 1D results shows that horizontal transport increases the phase lag of the temperature response. This large phase lag is essential to simulate the observed ozone response given by Nimbus 7 SBUV observations. The sensitivity of the mesospheric ozone response to water vapor, vertical diffusion and the spectral structure of solar variations is tested by the 1D and 2D models and the results are discussed. The seasonal and latitudinal (up to 45^circ ) variations in the ozone response are evaluated by using the 2D model. The 2D model is also used to study the ozone response to solar UV irradiance variations over solar cycle periods. The amplitudes of the solar UV variations during solar cycle 21, as derived from SME observations, is employed in the 2D model. A negative ozone response at altitudes of 60 -78 km, except in the region over high latitudes in winter, results from the enhanced solar Lyman alpha at solar maximum. The largest negative ozone response, revealed by the 2D model, about -10%, is found at 72-78 km at middle to high latitudes in summer. The latitudinal dependence of the ozone response above 60 km except at high winter latitudes is very much determined by the latitudinal dependence of water vapor. The positive ozone response around 80 km is caused by the dramatic decrease in water vapor and the enhanced solar irradiance over the SRB. The increased solar irradiance over the Herzberg continuum is responsible for the positive ozone response below about 45 km. The calculated ozone response at the equator is compared with the limited available observations, and the differences between them are discussed. The sensitivity of the ozone response to solar variations over different spectral regions, to different photochemical families and to increased methane are evaluated. (Abstract shortened by UMI.).