Estimation of the total cloud cover with high temporal resolution and parametrization of short-term fluctuations of sea surface insolation

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.