PCp where F(z)gW is the net downwelling solar radiation flux at level z, p is the atmospheric density and cp the atmospheric specific heat capacity at constant pressure. In Fig. 11.4 is given the mean global vertical heating rate due to solar radiation absorption for an RC model with mean cloud cover of 0.5, surface albedo of 0.1, with the relative humidity falling with altitude according to (Man-abe and Whetherald 1967)
with a surface relative humidity of 0.8, planetary albedo of 0.33, a tropospheric moist lapse rate determined according to eqn (11.10) with a resulting water column of 2.09 g cm~2 and ozone vertical distribution as given in Fig. 7.10, and surface CO2 of 365 ppmv. The heating rate follows the ozone profile in the upper atmosphere, while in the troposphere the heating rate shown is due to the single cloud layer of the model and absorption of water vapour in the near-infra-red. Note, however, that within the convective equilibrium zone the radiative heating is not used for the determination of the atmospheric temperature, which is determined by the lapse rate. The surface temperature Ts and the temperature structure, T(z), within the radiative equilibrium zone are determined by balancing the net outgoing terrestrial longwave flux and the net incoming solar shortwave flux at all levels in the radiative equilibrium zone, which includes the top of the atmosphere,
Thus, the radiative equilibrium temperature structure at any altitude depends on the radiative processes at all levels from which radiation can reach the specific level. The vertical temperature structure, T(z), within the convective equilibrium zone is determined by the surface temperature and the moist lapse rate at each altitude (Vardavas and Carver 1984b).
In Fig. 11.5 are shown mean global vertical temperature profiles obtained by an RC-photochemical model (described in Vardavas and Carver 1984a, and Lavvas et al. 2007), without climatic feedbacks associated with cloud cover and relative humidity, for increasing CO2 levels from 1 PAL = 365 ppmv, the present atmospheric level, to 8 PAL. As the CO2 level rises the lapse rate decreases and the convection zone extends to higher altitudes. The decrease in the lapse rate acts as a negative feedback that limits the rise in the surface temperature due
flG. 11.4. Variation of the global mean vertical heating rate due to solar radiation absorption primarily by ozone in the upper atmosphere. The heating rate follows the variation in the ozone mixing ratio with altitude. Heating due to water vapour and heating within the cloud layer occurs in the lower troposphere.
to the greenhouse effect. As the lapse rate decreases the troposphere is overall warmer and so contributes more towards the outgoing longwave flux that is needed to balance the net incoming solar flux. In the other extreme, a surface temperature that is determined by radiative equilibrium would need to be significantly higher to compensate the greenhouse reduction in outgoing longwave flux, as the troposphere would be relatively cooler.
In the stratosphere, the atmosphere is optically thin with regard to the thermal infrared absorption by CO2 and so any photons emitted upwards in this spectral range will escape to space. The cooling to space by the CO2 increases as the mixing ratio of CO2 increases with the result that the stratosphere becomes colder.
In Table 11.3 are shown the effects of increasing CO2, without cloud and relative humidity feedbacks, from its present atmospheric level on various climatic parameters of the atmosphere. The model gives a surface temperature increase of 1.3 K for a doubling of the present level of CO2. As the CO2 content of the atmosphere increases the water-vapour content increases significantly, and this introduces an important feedback in the climate model. The
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