The group PHAROS (Physical Analysis of Remote Sensing Images) analyses satellite images in order to derive information about the surface of the Earth (e.g., the ice cover of the oceans) or the atmosphere (e.g., the water vapour content in polar regions). The satellite data mainly come from
- AMSR-E (Advanced Microwave Scanning Radiometer - Earth Observing System) on the NASA satellite Aqua AMSU (Advanced Microwave Sounding Unit) on various weather satellites
- SMOS (Soil Moisture and Ocean Salinity)
- MODIS (Moderate Resolution Imaging Spectroradiometer) on the European ESA satellite Envisat.
AMSR-E, AMSU, and SMOS measure microwave radiation that is emitted by the Earth surface and by the atmosphere. Therefore, these satellites are independent of sunlight. MODIS measures visible and infrared light, which is mainly sunlight scattered and reflected by the Earth surface and by clouds.
As the sea ice plays a major role in the climate system and as it is also an indicator for changes in the climate, this topic is mainly relevant for climate research. On the one hand, sea ice insulates the ocean from the atmosphere and vice versa, and on the other hand, it reacts quite fast to a warming atmosphere by melting. As the thinning ice insulates less, the ocean can also be warmed and the ice can melt from below as well. Thus, sea ice reacts sufficiently fast for being used as a climate change indicator.
We daily generate global maps of the sea ice cover from AMSR-E data; the spatial resolution of these maps is 6.25 km. The sea ice cover is given as ice concentration, i.e. the percentage of one resolution cell (6.25 by 6.25 km) that is covered by sea ice. In addition to the global maps, about 20 different regional sea ice maps are generated (e.g., Baltic Sea, Bering Strait, Ross Sea). Sea ice maps are, of course, interesting not only for climate research, but also for navigation.
The current and archived sea ice maps can be accessed here
Besides the sea ice cover, the thickness of the sea ice is important: The thicker the sea ice, the more it insulates the sea from the atmosphere; also, thicker sea ice takes longer to melt. The new satellite instrument SMOS (Soil Moisture and Ocean Salinity) operates at a much lower microwave frequency (1 GHz) than all other space-borne microwave radiometers. Thus its signal does not only contains information about the surface, but also about the upper decimeters of the sea ice. We are working on a method for determining the thickness of thin sea ice (up to 50 cm) from SMOS data.
Snow grain size
There is a new method for determining the grain size of snow on sea ice and on land ice (Greenland and Antarctica), based on the satellite instrumend MODIS (visible and near infrared spectral range) The grain size of snow greatly determines it ability to backscatter light (i.e., its albedo), and thus it is relevant for the radiation budget of the Earth. Since little is known about the snow grain size on sea and land ice, progress in this field is of particular interest for climate modelling and prediction.
Topography of Tidal Flats
We have developed a method for mapping the vast tidal flats at the German North Sea coast. This method relies on delineating the water line on number of radar images of the same area at differet stages of the tidal cycle. Linking the differet water lines with sea level and gauge data yields height contours that can be converted into a topographic map. This method can produce suchs maps faster than the traditional method of measuring the depth at high tide by echo-sounding from ships (which can take up to 6 years to cover the German Bight). However, the method using radar images there are some drawbacks as the weather can influence the quality of the water lines, and the geolocation of the images might not be accurate enough. This field of research is relevant for coastal management and conservation.
Water vapour and clouds in polar regions
Water vapour and the cloud cover are significant parameters for meteorology and climate research. In polar regions, however, there is a lack of water vapour and cloud measurements as meteorological stations are sparse there, and as standard measurement methods with satellites do often not work very well in polar regions. We have developed a method to retrieve the total water vapour (vertically integrated water vapour) over polar regions from microwave data of AMSU-B.
In addition, we are working on a method for deriving the cloud liquid water path over the Arctic from AMSR-E data; there is no established algorithm for this yet.