By means of complex interaction-processes sea ice not only modifies the regional climate in the ocean-atmosphere-sea-ice system but also the general circulation of the atmosphere and the ocean's circulation. Besides a strong interannual variability sea-ice extent shows an arcticwide significant negative trend during the last two decades with maximum rates in spring and summer. These are often linked to (small-scale) processes in the Siberian Arctic and the Laptev Sea, respectively. The objective of this thesis is the expansion of the understanding of the processes concerning atmosphere-sea-ice interactions on the regional scale during the summer from 1979 to 2002 in the Arctic with a special emphasis on the Laptev Sea. To achieve this, numerical simulations of the regional climate model HIRHAM4 are used in conjunction with ground- and satellite-based observational data. A precondition for the numerical experiments and the realistic reproduction of atmospheric processes is an improved lower boundary forcing dataset for HIRHAM4 based on observational datasets, which is developed, validated and described. To investigate the effects of the sea-ice distribution, its properties and small-scale features on the atmosphere, HIRHAM4 is used in sensitivity studies systematically with different model settings, each of which incorporates the lower boundary forcing data in a different manner. Even little changes in the lower boundary forcing fields, while retaining the lateral boundary forcing, are sufficient to cause the model to produce significantly different atmospheric circulation patterns relative to the control simulations which use standard forcings and settings. Cyclone activity, which is a special focus of this study, is also altered. The mean atmospheric circulation patterns and the near-surface air temperature distribution can be reproduced more realistically with the new forcing dataset, which is shown by validation experiments with observational data. The biggest relative impact, besides an altered sea-ice coverage and distribution, can be reached by using sea-ice concentrations instead of a binary sea-ice mask. By utilizing sea-ice drift data, dynamic and thermodynamic processes can be partially separated from each other to investigate the development of sea-ice anomalies in the Laptev Sea. They depend on a time-critical succession of atmospheric conditions and the properties of sea ice during May and August. Positive air temperature anomalies are identified to be the key driving factors for the development of negative sea-ice anomalies. They are found to be a result of enhanced short-wave radiation balances, which are coupled to high pressure areas and intermediate anticyclones. The polynyas during early summer seem to have an important influence too. Because of lower process rates, the wind-induced sea-ice drift is enhancing and damping the development of the sea-ice area anomalies, but it cannot cause an anomaly all by itself. A precise separation of the effectiveness of the sea-ice transport and the melting rates is not possible due to the available data.