Kathrin Höppner

Comparison of a Satellite based Alpine Cloud Climatology with Observations of Synoptic Stations

A five-year cloud climatology (1992 to 1996) of the Alpine region in a 15-km resolution has been evaluated by means of the APOLLO cloud detection algorithm applied to daytime AVHRR data of several NOAA satellites. The study area comprises three different climatic regions, the moderate climate north of the Alps, the Alpine climate and the Mediterranean climate in the Po-valley. Synoptic observations of the total cloud cover at 40 stations have been compared to the satellite based monthly mean data. Hourly ground observations allowed to estimate the variance in the monthly mean diurnal cycle of total cloud cover due to the fact that the satellite overpass time shifts from noon to afternoon for the NOAA-11 platform and for different NOAA satellites as well. This time shift of satellite observation effects the cloud climatology only slightly, because the changes of the cloud cover between 11 and 16 UTC are in most cases considerably smaller than the year-to-year variability. Furthermore, these cloud cover variations due to the time of the day are in monthly means below the validation accuracy. The comparison of monthly means reveals an overestimation of the satellite cloud cover of about 10% mainly due to additional detection of thin cirrus. A good agreement is found in the Alpine and rural moderate climates (corr. coeff. r > 0.75), whereas the cloud detection in the satellite data is too high in the Mediterranean zone due to urban and aerosol haze effects. In both data sets a rather small amplitude of the annual cycle of cloud cover results in the mountains compared to the lowlands. The high spatial variability of cloud cover in mountainous terrain is obvious with the satellite data and is substantiated by the sparse synoptic stations within the Alps.

Earth and space observation at the German Antarctic Receiving Station O’Higgins

The German Antarctic Receiving Station (GARS) O’Higgins at the northern tip of the Antarctic Peninsula is a dual purpose facility for earth observation and has existed for more than 20 years. It serves as a satellite ground station for payload data downlink and telecommanding of remote sensing satellites as well as a geodetic observatory for global reference systems and global change. Both applications use the same 9 m diameter radio antenna. Major outcomes of this usage are summarised in this paper. The satellite ground station O’Higgins (OHG) is part of the global ground station network of the German Remote Sensing Data Centre (DFD) operated by the German Aerospace Centre (DLR). It was established in 1991 to provide remote sensing data downlink support within the missions of the European Remote Sensing Satellites ERS-1 and ERS-2. These missions provided valuable insights into the changes of the Antarctic ice shield. Especially after the failure of the on-board data recorder, OHG became an essential downlink station for ERS-2 real-time data transmission. Since 2010, OHG is manned during the entire year, specifically to support the TanDEM-X mission. OHG is a main dump station for payload data, monitoring and telecommanding of the German TerraSAR-X and TanDEM-X satellites. For space geodesy and astrometry the radio antenna O’Higgins significantly improves coverage over the southern hemisphere and plays an essential role within the global Very Long Baseline Interferometry (VLBI) network. In particular the determination of the Earth Orientation Parameters (EOP) and the sky coverage of the International Celestial Reference Frame (ICRF) benefit from the location at a high southern latitude. Further, the resolution of VLBI images of active galactic nuclei (AGN), cosmic radio sources defining the ICRF, improves significantly when O’Higgins is included in the network. The various geodetic instrumentation and the long time series at O’Higgins allow a reliable determination of crustal motions. VLBI station velocities, continuous GNSS measurements and campaign-wise absolute gravity measurements consistently document a vertical rate of about 5 mm/a. This crustal uplift is interpreted as an elastic rebound due to ice loss as a consequence of the ice shelf disintegration in the Prince Gustav Channel in the late 1990s. The outstanding location on the Antarctic continent and its year-around operation make GARS O’Higgins in future increasingly attractive for polar orbiting satellite missions and a vitally important station for the global VLBI network. Future plans call for the development of an observatory for environmentally relevant research. That means that the portfolio of the station will be expanded including the expansion of the infrastructure and the construction and operation of new scientific instruments suitable for long-term measurements and satellite ground truthing.

Bathymetric Controls on Calving Processes at Pine Island Glacier

Pine Island Glacier is the largest current Antarctic contributor to sea-level rise. Its ice loss has substantially increased over the last 25 years through thinning, acceleration and grounding line retreat. However, the calving line positions of the stabilising ice shelf did not show any trend within the observational record (last 70 years) until calving in 2015 led to unprecedented retreat and changed the alignment of the calving front. Bathymetric surveying revealed a ridge below the former ice shelf and two shallower highs to the north. Satellite imagery shows that ice contact on the ridge was likely lost in 2006 but was followed by intermittent contact resulting in back stress fluctuations on the ice shelf. Continuing ice-shelf flow also led to occasional ice-shelf contact with the northern bathymetric highs, which initiated rift formation that led to calving. The observations show that bathymetry is an important factor in initiating calving events.