For the planet around the F-type star the structure of the stratospheric meridional cir-culation does not change as dramatically as for the planet around the K-type star. The typical characteristic pattern of the Brewer-Dobson circulation can be identied spanning over the entire upper atmosphere. The mass transport is however stronger for the forb scenario than for the sunorb scenario, consistent with the strong temperature inversion in the winter stratopause shown in Fig. 9.4.
9.4.1 Water vapor
Water vapor plays an important role for the climate of a planet since it is an eective greenhouse gas. It absorbs radiation predominantly in the infrared, thereby blocking a part of the outgoing radiation at long wavelengths. This eect leads to an increase in surface and lower atmospheric temperatures. Higher surface temperatures cause a higher evaporation of water from the surface, hence lead to an additional increase in atmospheric water vapor. This is the water vapor feedback.
Figure 9.12: Global annual mean specic humidity (kg/kg) proles for planets around dierent stars. Sunorb scenario in black, korb in red and forb in blue. Left panel shows specic humidity on a linear scale, right panel on a logarithmic scale.
To investigate the general behavior of water vapor the global annual mean prole of specic humidity is shown in Fig. 9.12. Specic humidity is the atmospheric mass concentration of water vapor, hence the ratio of mass of water vapor to the mass of dry air in the atmo-sphere, see section 3.3.4.
For the korb scenario (red) water vapor mass concentrations are higher than for the sunorb
9.4. HYDROLOGY 141 scenario (black) reaching values of over 0.09 kg/kg (9 %). Values corresponding to the near surface specic humidityof the planet around the Sun (0.009 kg/kg) are reached at much higher altitudes at pressures of about 5000 Pa (50 hPa). Generallythe upper atmospheric specic humidityis much larger for the korb scenario than for the planet around the Sun, about two orders of magnitude, see right panel of Fig. 9.12.
For the planet around the F-type star the specic humidity decreases to a global mean value near the surface of 0.003 kg/kg (0.3 %). The specic humidityis lower throughout the troposphere than for the planet around the Sun. In the upper atmosphere however, water vapor mass concentrations are comparable.
Water vapor distribution
Figure 9.13: Zonal mean distribution of specic humidity (% kg/kg) for planets around dierent stars in NHW. Results for the sunorb scenario in the upper panels, for the korb scenario in the middle panels and for the forb scenario in the lower panels. In the left panels the pressure scale is linear in the right panels logarithmic.
Figure 9.13 shows the zonal mean specic humidityfor northern hemispheric winter, for
the concentrations are nearly constant over all latitudes. These high water vapor concen-trations are caused by the coupling of the the water vapor feedback cycle and the NIR radiation of the star. Increased surface temperatures lead to an increased evaporation of water from the surface, increasing the amount of water in the atmosphere. This leads to a higher optical depth due to absorption by water. Since strong water absorption takes place in the infrared, near infrared stellar radiation and thermal infrared radiation originating at the surface and within the atmosphere are absorbed eciently. This has been shown on the basis of the shortwave and thermal infrared uxes in section 9.2. For the planet around the K-type star the water vapor feedback is enhanced due to the shift of the stellar spectrum towards longer wavelengths. Therefore, a small increase in water vapor leads to a larger climate eect for a planet around a K-type star than for a planet around the Sun, or hotter stars. The cycling of water vapor at high altitudes also contributes to these high concentrations (see sec. 9.4.3).
For the forb scenario specic humidity is lower than for the planet around the Sun. Maxi-mum values of specic humidity are found in the summer equatorial regions, which have a maximum of about 0.5 %. As for the sunorb scenario water concentrations steeply decrease towards the poles. Due to the lower water vapor amount in the atmosphere the greenhouse eect becomes less ecient as has been shown in section 9.2.
9.4.2 Clouds
Clouds inuence the climate of a planet by scattering and absorbing incident stellar ra-diation and absorbing outgoing thermal infrared rara-diation, as well as by releasing latent heat during their formation. The impact of clouds thereby depends on the cloud cover and the cloud properties, such as the composition (water droplets or ice crystals), on the particle sizes and crystal shapes, the thickness and on the temperature of the cloud.
These properties are a strong function of various microphysical processes which act on very small temporal and spatial scales. It is usually impossible to resolve these small scales in a global climate model. Thus, they have to be included by using highly parametrized schemes, making the impact of clouds one of the major uncertainties in climate modeling (Houghton et al., 2001). Therefore, the results presented in this section strongly depend on the parametrization of the cloud microphysical processes, described in section 5.3.4 of the model description.
Table 9.3 summarizes the global annual mean cloud properties. For the korb scenario cloud cover is lower than for the planet around the Sun (sunorb) and it is higher for the
9.4. HYDROLOGY 143