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The present work investigates the quality of the shortwave and and longwave downward radiation (DSR, DLR) at the sea surface over the Atlantic Ocean as retrieved from Meteosat Second Generation (MSG) measurements and EUMETSAT's Climate Monitoring - Satellite Application Facility (CM-SAF) algorithms. The observations taken at two transatlantic research cruises have been an ideal basis to be compared with the MSG products for DSR and DLR derived from Meteosat-8 and Meteosat-9. Onboard the research vessels "Akademik Ioffe" and "Polarstern" high quality in situ measurements of both radiation fluxes have been performed. Continuous full sky imagery and standard meteorological observations enable a comprehensive evaluation of the skills of MSG DSR- and DLR-retrievals in different climate zones and under various cloud and weather conditions. The DSR was retrieved by MSG with a positive bias of 2.77 Wm−2 during the Ioffe cruise, and 22.23 Wm−2 during the Polarstern cruise. The bias for the DLR was −1.73 Wm−2 and 2.76 Wm−2, respectively. The differences between the two cruises mainly arise from the different weather conditions. No significant differences between the satellite products from Meteosat-8 and Meteosat-9 were found. In general DSR and DLR for clear sky conditions are captured with a high accuracy. Largest retrieval errors occur for fast fluctuating broken cloud conditions, though on average the MSG algorithm match the in-situ observations well. Semitransparent cirrus was found to cause a negative bias for the retrieved DSR. In tropics and subtropics the errors for DLR are smaller compared to higher latitudes. Most importantly, no significant dependencies of the satellite retrieval errors for both the DSR and the DLR on the solar elevation, near-surface humidity, cloud cover, SST and the shift of day and night were found, indicating that the CM-SAF radiation products are not subject to significant systematic errors.

The influence of the external and internal structure of clouds on the incoming solar radiation cannot yet be included in parameterizations used in numerical models. Based on numerical simulations, Schewski and Macke (2003) (Schewski-parameterization) have shown that a robust link exists between the domain averaged cloud and the domain averaged solar broadband radiation fluxes, despite the 3d nature of the cloud fields involved. The present work revisits this approach with observed cloud (cloud cover and liquid water path) and radiation (downwelling shortwave radiative flux) properties obtained from the Richard Assmann Observatory (RAO) of the German Weather Service in Lindenberg. Applying the original (model based) cloud-radiation parameterization by Schewski and Macke (2001) to observed domain averaged cloud fields yields an overall good correlation between observed and parameterized downwelling solar radiation fluxes. However, the parameterized fluxes strongly underestimate the observations. The Schewski parameterization has been modified by removing the bias and re-adjusting the parameterization coefficients to match the observed cloud and radiation correlation. Furthermore, the empirical parameterization by Zillman (1972) has been implemented for describing the clear conditions. Applying the new parameterization to an independent data set provides significant improvements. However, the accuracy remains in the order of previously used one- or two-parameter empirical cloud-radiation parameterizations. We conclude that cloud cover and liquid water path, i.e. those data that are available from large scale climate models, cannot be regarded as sufficient to describe the cloud radiative effect at the surface.

Sky observations by means of digital full sky imagers enable a continuous monitoring and archiving of the state of the atmosphere. A direct application is the estimation of the total amount of clouds with a very high temporal resolution. From simple methods of image analysis, we used the full sky imager additionally as a sunshine recorder to replace complex pyrheliometric and pyranometric measurements for detecting direct solar irradiation. The Meridional Ocean Radiation Experiment MORE has realized four transatlantic research cruises with a special focus on the surface radiation budget, remote sensing of clouds and automated sky monitoring. This dataset was used to study different parametrizations of solar downward radiation at the sea surface based on standard meteorological measurements. It was found that the parameterization by Zillman (1972) provided the best performance for the use with operational meteorological data. Furthermore, this parametrization was modified to create a fast responding parametrization to reproduce short-term fluctuations of the insolation. This new parametrization has been applied to independent datasets. It is shown that based on measurements of a few weeks it was not possible to reduce the overall bias of parameterized insolation significantly due to the fact that the parametrization does not consider the optical thickness of clouds. The standard deviation between calculated and measured flux has been reduced by 4%. The performance for separated broken cloud conditions was improved significantly.

The impact of cloudiness on the shortwave downwelling radiation (SDR) at the surface is investigated by means of collocated pyranometer radiation measurements and all-sky imager observations. The measurements have been performed in Westerland, a seaside resort on the North Sea island of Sylt, Germany, during summer 2004 and 2005. A main improvement to previous studies on this subject resulted from the very high temporal resolution of cloud images and radiation measurements and, therefore, a more robust statistical analysis of the occurrence of this effect. It was possible to observe an excess of solar irradiation compared to clear sky irradiation by more than 500 W/m2, the largest observed excess irradiation to our knowledge so far. Camera images reveal that largest excess radiation is reached close to overcast situations with altocumulus clouds partly obscuring the solar disk, and preferably with cumulus clouds in lower levels. The maximum duration of the enhancements depends on its strength and ranges from 20 seconds (enhancements > 400 W/m2) up to 140 seconds (enhancements > 200 W/m2).

In this modelling study the effects of cloud inhomogeneities on the observations of the passive microwave radiometer SSM/I (Special Sensor Microwave/Imager) are investigated. Brightness temperatures in the radiometer's field-of-view are calculated by using the results of a numerical cloud model as input for a 3-dimensional radiative transfer model. These brightness temperatures are compared with those obtained by the assumption of a plane parallel homogeneous cloud with the equivalent mean liquid water path (LWP). Differences between the two temperatures (the so-called 'beam-filling effect') are related to the mean LWP and to cloud inhomogeneity. It was found that not only the strength of the beam-filling effect but also its dependencies on mean LWP and its inhomogeneity are different for the different microwave frequencies. For 19 and 22 GHz the beam-filling effect increases monotonically with the mean LWP and inhomogeneity, while the beam-filling effect decreases with increased LWP at 37 and 85 GHz at large LWP. In general the beam-filling effect is more pronounced in the horizontal polarisation. Its maximum is found for 37 GHz with differences up to -60 Kelvin compared to the homogeneous case. The effect of the lowered brightness temperatures of inhomogeneous clouds in a LWP algorithm (WENG and GRODY, 1994) is analysed. The largest effect arises from decreased 37 GHz brightness temperature. The mean underestimation of LWP is 0.0179 kg/m2 and the maximum amounts to 0.7 kg/m2. In einer Modellstudie werden die Effekte von Wolkeninhomogenitäten auf die Helligkeitstemperaturen, die das passive Mikrowellenradiometer SSM/I (Special Sensor Microwave/Imager) misst, untersucht. Die Helligkeitstemperaturen im Radiometerblickwinkel werden dazu mit einem dreidimensionalen Strahlungstransportmodell für die Wolken eines numerischen Wolkenmodells berechnet. Sie werden verglichen mit Helligkeitstemperaturen plan-parallel homogener Wolken mit äquivalentem mittlerem Flüssigwasserpfad (LWP). Die Differenz dieser beiden Helligkeitstemperaturen, der sogenannte ,,Beam-Filling“ Effekt, wird untersucht bezüglich seiner Abhängigkeit vom mittleren LWP und der Wolkeninhomogenität. Dabei ergibt sich, dass nicht nur die Größe des Beam-Filling Effektes frequenzabhängig ist, sondern auch dessen Abhängigkeit von mittlerem LWP und Wolkeninhomogenität: Für 19 und 22 GHz steigt der Beam-Filling Effekt monoton mit zunehmendem LWP und Inhomogenität, wohingegen bei 37 und 85 GHz der Beam-Filling Effekt bei hohen mittleren Flüssigwassergehalten abnimmt. Insgesamt ist der Beam-Filling Effekt in der horizontalen Polarisationsebene ausgeprägter und ist maximal für 37 GHz mit einer Differenz von -60 Kelvin verglichen mit einer homogenen Wolke. Anhand eines vielfach angewendeten Algorithmus (WENG und GRODY, 1994) werden Auswirkungen von Wolkeninhomogenität auf den abgeleiteten LWP gezeigt. Dabei spiegeln sich die erniedrigten Helligkeitstemperaturen im 37 GHz Kanal besonders deutlich wider und führen zu einer mittleren Unterschätzung des LWPs von 0,0179 kg/m2 und einer maximalen Unterschätzung von 0,7 kg/m2.