Heinrich Hoeber

Boundary-layer observations over water and arctic sea-ice during on-ice air flow

Observations made on 8 and 9 May 1988 by aircraft and two ships in and around the marginal ice zone of the Fram Strait during on-ice air flow under cloudy and cloud-free conditions are presented.The thermodynamic modification of the air mass moving from the open water to the ice over horizontal distances of 100–300 km is only a few tenth of a degree for temperature and a few tenth of a gram per kilogram for specific humidity. This is due to the small temperature differences between sea and ice surfaces. During the day, the ice surface is even warmer than the sea surface. The stably stratified 200–400 m deep boundary layer is often topped by a moisture inversion leading to downward fluxes of sensible as well as latent heat.The radiation and energy balance at the surface are measured as functions of ice cover, cloud cover and sun elevation angle. The net radiationRNis the dominating term of the energy budget. During the day, the difference ofRNbetween clear and overcast sky is only a few W/m2 over ice, but 100–200 W/m2 over water. During the night,RNover ice is more sensitive to cloud cover.The kinematic structure is characterized by strong shears of the longitudinal and the transversal wind component. The profile of the latter one shows an inflection point near the top of the boundary layer. Dynamically-driven roll circulations are numerically separated from the mean flow. The secondary flow patterns have wavelengths of about 1 km and contribute substantially to the total variances and covariances.

A mesoscale cyclone over the Fram Strait and its effects on sea ice

The ice export from the Arctic Ocean through the Fram Strait into the Atlantic Ocean is steered by atmospheric processes on the large scale as well as on the local scale. A case study of a mesoscale cyclone over the Fram Strait and its effects on the sea ice drift is presented. The cyclone developed on March 13, 1993, at the ice edge of the Greenland Sea and moved northward over the ice and over the position of the research icebreaker Polarstern and an array of six meteorological ice buoys. The cyclone had a radius of about 150 km and was essentially restricted to the lowest 2 km of the atmosphere. Near the surface the temperature difference between the air in the warm sector and the surrounding cold air was 25 K resulting in a pressure difference of about 15 hPa between center and the surrounding with low-level wind speeds of up to 20 m/s. The passage of the cyclone caused a full loop of the ice drift otherwise oriented from northeast to southwest. The ratio of ice drift speed to wind speed, increased from 5% before to 9% after the passage. Extreme values of ice deformation and large variations of the ice strain rate tensor occurred during the cyclone passage. It is concluded that passing cyclones can affect the sea-ice drift through the bottleneck of the Fram Strait and may have integral effects on larger time scales and space scales.

One-year temperature records in the atmospheric surface layer above sea ice and open water

Temperature observations of three buoys drifting in the Weddell Sea for one year and covering the ice-water-ice cycle from July 1986 to July 1987 are presented. Significant differences between winter and summer are shown to be a consequence of the air-sea heat exchange being drastically modified by the sea ice cover. Over ice, prevailing variance is in the synoptic scale (periods 3 to 5 days) with amplitudes of 25 °C, whereas over water, the diurnal wave dominates with amplitudes of less than 1 °C.