Tessendorf, Alrun (2010) Strahlungsbilanz arktischer Bewölkung aus Modell und Beobachtung. (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 79 pp.

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Abstract

The effects of clouds and sea-ice on the surface radiative balance in the Arctic are studied. Clouds during arctic summer are characterized by a low optical thickness and high cloud amounts. To determine the effect of these clouds in combination with sea-ice, measurements onboard the research vessel FS Polarstern were obtained in August and September 2009 during the expedition ARK24-3 in the Greenland Sea.
Data evaluation was accompanied by model calculations with the radiative transfer model GRIMALDI. In particular, the effect of multiple reflexions between sea-ice surface and clouds on the shortwave radiative balance was determined. The GRIMALDI model from R. Scheirer is based on the Monte-Carlo method and was adjusted within this work to radiative transfer calculations in the broadband solar spectrum.
The average downward shortwave and longwave irradiance of 80W/m2 and 296W/m2 measured during ARK24-3, as well as the average sea-ice albedo of 0.45 at this time of the year are within the scope of expectations from previous studies (Intrieri et al., 2002b; Shupe and Intrieri, 2004; Wang and Key, 2005; Persson et al., 2002). This applies also for the shortwave and longwave cloud radiative forcing (-70W/m2 and 46W/m2) which depends not only on sun zenith angle but also from surface albedo and temperature. For zenith angles less than 82°, surface cloud radiative forcing had a cooling effect during ARK24-3 and for zenith angles greater than 82° a warming effect.
Sea-ice increases the downward shortwave irradiance at the surface by multiple reflexions between sea-ice and clouds. In case of a broken cloud cover the effect is amplified in places where the irradiance is already increased due to cloud distribution.
In the local mean, the effect depends on the cloud cover and the optical thickness of clouds. The absolute magnitude of the effect increases with cloud fraction. With an increasing optical thickness, the relative magnitude of the effect increases, while the absolute irradiance decreases. This results in an optical thickness tau with a maximal increase of the absolute sea-ice effect. This optical thickness is near tau = 5 which equals the average optical thickness of clouds during ARK24-3. With a zenith angle of theta = 60°, sea-ice albedo of 0.5 and 100% cloud cover the total increase is 45W/m2, representing a relative increase of the downward irradiance of 15%.
With the effect of increased downward shortwave irradiance over sea-ice compared to water, a difference in the cloud radiative forcing is to be expected. This difference was not observed during ARK24-3. Because of the small dataset it remains uncertain if this is due to additional low-level clouds over sea-ice or to increased cloudiness in relation with frontal systems.

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