Hanschmann, Timo (2009) Wolken-Strahlungs-Effekte beobachtet mit SEVIRI auf Meteosat Second Generation. (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 147 pp.

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Abstract

The earth receives energy from the sun’s inhomogeneous diabatic heating. At the same time the earth looses energy via thermal emission to space. The resulting energy distribution creates gradients in the ocean and the atmosphere, leading to global general circulation. However it is too simple to characterize the energy budget of the earth as simply incoming radiation minus outgoing radiation. Ice covered areas causes a cooling of the earth due to the Ice-Albedo-Feedback. Water vapour, carbone dioxide, and other greenhouse gases lead to a heating of the earth due to absorption and reemission of thermal radiation. A larger effect of water vapour is the formation of clouds through condensation. One effect of clouds is to reduce Earth’s surface temperature through the reflection of solar radiation. A second effect of clouds is to increase the surface temperature by absorbing thermal radiation, which prevents the loss of this energy to space. The clouds then reemit the energy in proportion to their skin temperature in all directions, provining an additional heat source to the earth’s surface. It is difficult to determine which effect is dominant. This will be examined in this study using top of the atmosphere observations of convective clouds. Measuring at the top of atmosphere allows warming effects to be clearly distinguished from cooling effects. The background information for radiative transfer and cloud radiative effects are describes in chapter 2 and 4. These chapters outline the impact of clouds on the energy budget. Also, the instrument utilized on board Meteosat Second Generation is described. In chapter 5, two methods will be introduced for cloud tracking. One method is to mark the clouds manually at each timestep. The second method employs automatic tracking. Cloud tracking is important for both measuring the current state of the cloud and examining its complete life cycle and evolution. Results are presented in chapter 6, 7, and 8. Five clouds in May 2008, five clouds in August 2008, and 5 clouds in January 2009 were tracked and analysed. The last five clouds in January were used to compare the manual and automatic tracking method to determine the viability of atomatic cloud tracking. The computed cloud radiative effect found in this study will be compared to results from previous studies. Differences between single clouds and temporal and spatial mean values will be determined. Also, the influence of the cloud lifecycle on the cloud radiative effect is shown. For example, clouds with a lifetime shorter than one day are found to have a dominant thermal effect. The diverse errors resulting from the use of the automatic tracking method are illustrated in chapter 8.

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