In this section the results of the 3D and 1D model, described in Chapter 5, for an Earth-like planet irradiated with a normalized K-type stellar spectrum and a solar spectrum are compared for xed sea surface conditions in the 3D model, in contrast to the model re-sults shown in the previous Chapters, which included interactively calculated sea surface temperatures and sea ice. This has been done prior to the model calculations presented in the last Chapter to evaluate, whether the dynamical response is already large for xed surface conditions. The Earth-like planet around the K-type star has been chosen, since for K-type stellar radiation 1D model studies revealed the largest change in the stratospheric temperature structure, such as a very weak stratospheric temperature inversion.
In this study the sea surface temperatures, sea ice concentrations and sea ice depth were prescribed from the AMIP II climatology (Taylor et al., 2000). The concentrations of the radiative gases are the same in both models, except for ozone and water. The 1D model uses a global annual mean ozone prole, while for the 3D model the climatology of Fortuin and Kelder (1998) is used, see also sec. 5.5. The concentrations of water vapor are
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calculated by both models. For the 1D model it is calculated from the temperature prole using a relative humidity prole of Manabe and Wetherald (1967), and in the 3D model by solving the continuity equation for water compounds. In the 1D model the surface albedo isxed to 0.21, in order to obtain the global annual mean temperature of the Earth (288.1 K) under solar irradiation.
In addition, the impact of the resolution of the shortwave radiative transfer scheme in the 3D model was evaluated by using the two dierent shortwave radiation schemes available in EMAC (see sec. 5.3.2.1). For the 3D model scenario k4sst (see also Table 11.1) the stan-dard radiation scheme with four bands covering a wavelength regime from 250 nm to 4μm was utilized. For the other 3D model scenarios, with K-type stellar irradiation (k52sst) and solar irradiation (sunsst) the FUBRAD radiation scheme was applied, which calcu-lated the shortwave heating rates for pressures smaller than 70 hPa in 52 bands ranging from 121.4 nm to 4μm.
11.1.1 Comparison of the global annual mean temperature proles Fig. 11.1 shows the global annual mean temperature proles for the 1D and 3D model calculations.
The model results obtained with the solar spectrum are shown in black (solid: 3D model, dash-dotted: 1D model), whereas the proles for the K-type stellar spectrum are displayed in green. The 1D model result is indicted by the dashed line, the solid line shows 3D proles for the K52sst, scenario and the dotted line the 3D model result for the K4sst scenario, respectively.
For the K-type stellar radiation the temperature prole changes drastically (compared to the result for solar irradiation) without showing any signicant stratospheric temperature inversion for the 1D model result (green dash-dotted) and 3D model scenario with high spectral resolution in the shortwave radiative transfer (k52sst, solid green line). This is caused by the reduced stellar UV ux and has been found in previous 1D model calcula-tions, e.g. by Segura et al. (2003).
In the case of solar radiation the dierences between the 1D and 3D model results are smaller, yet discernible, such as dierent tropospheric lapse rates resulting from dier-ent treatmdier-ents of convection, and therefore also dierdier-ent inversion heights. Nevertheless, stratospheric proles agree well for pressures larger than 1 hPa, where the 1D model also
re-11.1. RESULTS FOR FIXED SEA SURFACE CONDITIONS 163 sembles the US-Standard-Atmosphere from 1976. The deviations for pressures smaller than 1 hPa originate from dierent vertical domains of the models and corresponding boundary conditions, such as the neglected downward infrared radiation at the top of the atmosphere, as well as from the missing absorption of far UV radiation by O2 in the 1D model.
For both stellar ux distributions the stratospherictemperature proles computed with the 1D model (dash-dotted) and the 3D model with the high resolution radiative scheme (solid) agree well up to about 1 hPa. The comparison of the standard radiation scheme in EMAC and FUBRAD for solar irradiation was presented in Nissen et al. (2007).
For the K-type stellar radiation, signicant dierences become obvious in the stratospheric temperature proles for the 3D model calculations using dierent spectral resolutions.
Since in the low resolution scheme (dotted), the UV and visible wavelength regime is rep-resented by one band only, the stellar radiation is integrated over the whole band (see Fig. 7.2 in the scenario description). Therefore, spectral dierences at wavelengths where ozone heating is important are not resolved and the low resolution scheme fails to produce realisticheating rates.
This clearly demonstrates that a radiation scheme with a suciently high spectral reso-lution is needed for investigations of the inuence of the stellar spectral type upon atmo-sphericproperties.
Figure 11.1: Global annual mean temperature proles for an Earth-like exoplanet around a K-type star (green) in comparison to a planet around the Sun (black).
noteworthy response to the changed stellar spectrum. However, the stratospheric temper-ature structure is strongly inuenced. Comparing the summer tempertemper-ature structures for the solar (upper left panel of Fig. 11.2) and the K-type radiation (upper right panel of Fig. 11.2) suggests that the stratopause is located at lower altitudes for the K-type radia-tion (at 10 hPa instead of 1 hPa). This region is about 50 K colder than in the solar case, which leads to an overall sinking of the atmosphere in the vertical.
The stratospheric temperature inversion in the winter hemisphere, which is a consequence of the Brewer-Dobson circulation is weak for the k52sst scenario, suggesting a weakening in meridional circulation.
Temperatures typical of Earth's lower stratosphere now appear nearly in the entire winter stratosphere for the K-type stellar irradiation.
The temperature in the lower winter stratosphere appears to be mostly similar for both scenarios, a result which has been found also in the 3D model results with interactive ocean (Chapter 9). It is a consequence of the missing stellar radiation during polar night.
In addition to overall decreasing stratospheric temperatures, the stratospheric meridional temperature gradient weakens for the K-type radiation, e.g. the temperature gradient be-tween the stratospheric temperature maximum in summer and the equator is about 10 K smaller. This leads to reduced zonal wind speeds, as shown in the middle panels of Fig. 11.2.
For solar radiation (see left middle panel of Fig. 11.2) stratospheric jet streams in the winter hemisphere are usually westerly (denoted by positive values) and easterly in the summer hemisphere (denoted by negative wind speeds). Furthermore, the southern hemispheric jet stream is stronger compared to the northern hemisphere due to less deceleration by plan-etary waves, since the southern hemisphere has orography than the northern hemisphere.
For K-type radiation (Fig. 11.2 right middle panel) the stratospheric jet streams in the southern hemisphere decrease by approximately 30ms and about 20ms in the northern hemisphere compared to the sunsst scenario.
The maximum wind speeds in the summer hemispheres appear at nearly the same pressures around 0.1 hPa as for solar radiation, whereas in the winter hemisphere the wind maxi-mum occurs at larger pressures for the K-type radiation. Additionally, due to the lower stratospheric temperatures for the K-type radiation, the atmosphere sinks in the vertical and zonal wind features similar to Earth's thermosphere appear at the upper model lid.
For more reliable results in this region a model including processes relevant in the thermo-sphere would be required.