New opportunities from HEO satellites

Erik Andersson, Niels Bormann, Katie Lean


The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and the European Space Agency (ESA) are considering the prospects for observations from satellites in a highly elliptical orbit (HEO). This is a new opportunity that could bring weather imaging capabilities to the Arctic and high latitudes of similar quality to what is currently available in the tropics and mid-latitudes from geostationary satellites. Two such imagers in HEO would provide full and frequent coverage at high latitudes of clear benefit to numerical weather prediction (NWP). ECMWF is therefore supportive of the new mission concept.

Highly elliptical orbit
Highly elliptical orbit. Satellites in highly elliptical orbits move more slowly in high-altitude parts of the orbit than in low-altitude parts, thus maximising viewing times over high latitudes. (Earth image: EUMETSAT)

At a recent EUMETSAT HEO meeting, the participants agreed to support an HEO mission. However, further investigations are needed to establish the user requirements for such a mission in terms of instrument capabilities, and to substantiate the benefits as a complement to a baseline polar-orbiting constellation. Furthermore, the performance and value of a dedicated HEO mission versus a hosted payload on a telecommunications satellite would have to be analysed in detail.

There is a recognised gap in the global observing system at latitudes between 50–70°N regarding the provision of tropospheric winds in the form of Atmospheric Motion Vectors (AMVs) derived from cloud and water vapour image sequences. This gap corresponds to a gap in coverage between winds provided by geostationary satellites (GEO) on the one hand and polar wind products from Low Earth Orbit (LEO) satellites on the other.

Whilst this gap can be mitigated by other satellite observation techniques combining images from multiple satellites, the data quality is lower than that from GEO and the quality expected from HEO. Furthermore, the temporal and spatial coverage is limited, leading to areas still sparse in data for each data assimilation cycle.

Satellite coverage
Satellite coverage. Two imagers in highly elliptical orbits would provide good coverage over the North polar region. The illustration shows one possible scenario, with the actual coverage dependent on the final choice of orbit.

The value of high-latitude winds is well established through the recent addition of further polar AMV data at ECMWF and data denial experiments at other NWP centres. The inclusion of such winds is also believed to have been effective in reducing forecast busts. These investigations also confirm that operational NWP systems are not yet saturated with high-latitude good-quality wind observations. Forecasts of high-impact weather events such as cold-air outbreaks and polar lows are expected to benefit from improved observations in the Arctic.

Whilst the provision of good-quality wind observations is seen as the primary benefit HEO missions would bring, other benefits are anticipated from much-improved snow and ice monitoring capabilities and radiance assimilation.

With respect to regional NWP and limited-area modelling, there is an increased need for high spatial and temporal resolution observations as the model domains expand further north, in response to the increased economic exploitation of the Arctic.