Experiments with a two-dimensional model of the general circulation
Experiments have been conducted with a two-dimensional timedependent, numerical model of the general circulation of the atmosphere up to the mesopause. A scheme for the calculation of cooling rates due to the 15(mu)m band of carbon dioxide is developed. It uses the Curtis matrix approach which incorporates cooling-to-space, transfer of radiation between atmospheric layers and non-equilibrium effects in the upper mesosphere. The sensitivity of the cooling rate calculations to the choice of collisional relaxation time is investigated. An 'almost exact' scheme to calculate heating rates due to the absorption of solar radiation by ozone and molecular oxygen is presented. Use of both new radiation schemes enables the diabatic heating rate to be calculated to the upper boundary of the model. Other heat sources in the region of the mesopause are discussed. Incorporation of the new schemes considerably improves the modelled temperature structure of:the stratosphere and lower mesosphere. The upper mesosphere is not well reproduced with no indication of the observed cold summer mesopause. The heat and momentum budgets of the mesosphere are studied. Eddy momentum fluxes derived from satellite observations of planetary waves are found to be significant for the circulation and transport properties of the stratosphere but incapable of producing the required distribution of angular momentum in the mesosphere. A Rayleigh friction parameterisation is included in the mesosphere to reproduce the observed zonal wind and temperature structure. Momentum deposition by tides and gravity waves is discussed. Curtis matrices are calculated with higher mixing ratios of carbon dioxide and the effects of increased atmospheric CO2 on stratospheric temperatures and ozone is investigated. Temperature decreases of up to about 10K are predicted with increases in ozone concentration in the upper stratosphere. In the lower stratosphere the ozone increases are restricted to high latitudes and a decrease shown in equatorial regions. The latitudinal variations are reflected in the ozone column density. An experiment is conducted in which chlorofluorocarbons are released into the model atmosphere and the effects on stratospheric ozone are in exactly the opposite sense to those predicted for the CO2 case. A run in which CO2 and CFCs are introduced simultaneously shows that the two effects are not linearly additive. A simple photochemical theory is used to investigate the temperature dependence of ozone and to explain the non-linearity of the coupled experiment.