The EUFAR workshop on stratospheric airborne research was held in Rome, at the CNR headquarters, from 4 to 5 May 2017. Its aims were to discuss the future research topics that may be tackled by a stratospheric research aircraft, and to provide suggestions on how to secure the availability of such an aircraft in Europe in the coming decade.
The organizer F. Cairo (CNR), leader of the Networking Activity Future of the Fleet, welcomed and thanked those attending the meeting: C. Voigt (DLR), F. Stroh (FZ Juelich), P. Smith (U. Cambridge), B. Legras (CNRS), M. Höpfner (KIT), N. Harris (U. Cranfield), U. Cortesi (CNR), F. Fierli (CNR), G. Gratton (FAAM), R. G. Gallardo (SJ Berwin); then introduced the first topic under discussion: for which scientific subjects the role of high-altitude aircraft observations is privileged or irreplaceable. In the debate, various themes were analyzed; for what concerns high altitude clouds, their radiative impact can only be assessed when both microphysical in-situ and radiation measurements are simultaneously available, thus allowing a full characterization of the radiation field and how it changes with respect to different clouds and their altitude. A particular mention deserve contrail cirrus, which, although lingering at the level of the commercial air traffic, should be remotely sensed from altitudes well above them.
The role high altitude clouds play in fixing the amount of stratospheric water vapor and trace species is still poorly known, and it is difficult to understand its possible change, in response to alteration of the flux of water vapor, Cloud Condensation Nuclei, Ice Nuclei, and other chemical compounds, (among which an ever increasing interest is e.g. in ammonia). As their role impacts both the radiative budget and the composition of the stratosphere, both climate and weather prediction models would benefit from a better understanding of microphysical and chemical processes for, and in, these clouds.
A climatologically crucial, and heavily under sampled region by vertically/horizontally high resolved observations (which is particularly vulnerable to climatic changes) is the tropical tropopause layer, where clouds can dwell as high as 18 km, the air is chemically and physically processed before entering the stratosphere, and steep vertical gradients of water and other trace species (e.g. ozone) make it difficult to estimate its chemical composition by remote sensing. The lack of proper assessment of processes in this region severely hampers our ability to predict the evolution of the stratosphere and its impact on the global climate, and airborne measurements are there of paramount importance.
The recent debate on geoengineering strategies to modify the Earth Radiation Budget, by increasing the stratospheric aerosol albedo, has renowned the interest in stratospheric chemistry, in the sulphur budget and their climate sensitivity. Observational studies are needed in combination with regional climate modelling both to predict and possibly to verify the effect of such strategies. It has also been stressed the need to keep the capability to in situ survey of the stratosphere, in response to climatic disruptive effects of possible major volcanic eruptions, or “nuclear winter” induced by even limited use of nuclear weapons.
Although the use of high altitude aircraft is not best suited for a continuous monitoring of the upper atmosphere, which has indeed to rely on the satellites, airborne stratospheric observations are beneficial for satellite validation, with particular emphasis on products from limb viewing satellites, and for fast deployment of proof of concept instruments and retrieval and data fusion exercises.
Moreover, aircraft observations may help to fill the future potential gap in limb sounding satellite instruments. The lively discussion well traced and summarized the science questions present in the current WCRP Stratosphere-troposphere Processes And their Role in Climate (SPARC) implementation plan, currently being worked out; these were presented and summarized by Harris, again underlining the importance of airborne observations to assess the climatic effects of volcanic eruptions, to predict the evolution of tropopause layer cirrus, water vapor content and chemical composition of the stratosphere and their feedback on global climate, and to possibly study the regional impacts of stratospheric geoengineering activities.
The meeting continued with surveys on current activity and future developments in stratospheric air research at national level. Gratton presented results from a UK national poll showing that about a quarter of the UK atmospheric scientific community is interested in improving the stratospheric airborne research capabilities, and this could be done either by acquiring a high altitude jet (as Falcon 7X), High Altitude Pseudo Satellites (HAPS like the solar powered Qinetiq-Airbus Zephyr, that fills a capability gap between satellites and UAV, bringing few kilos payload into the stratosphere, or adapting former reconnaissance aircraft (Canberra P9) Moreover, it is envisaged to foster and encourage the use of foreign platforms, either American (NASA Global Hawk, ER2, WB57) or European (DLR Halo).
Legras presented a panoramic view of the French activities in airborne research, and informed about the intention to replace the current Falcon-20 with a more performing jet. This is considered a high priority for the French community although funding for such replacement are not yet secured.
Cortesi reported on High Altitude Pseudo Satellites, stratospheric platforms as airships or solar powered lightweight aircraft - that fly at about or above 20 km, can stay over a fixed point on Earth from weeks to months, complementing or extending the capabilities of satellites in the domains of Earth Observation, (and Telecommunications). Current prototypes can carry payloads of tens of Kg, powered with tens of Watts.
Voigt presented the past and ongoing activities and scientific perspectives of the DLR HALO, the new aircraft for atmospheric research and earth observation of the German science community. HALO is based on a production G550 business jet. The operational standby costs are shared by some German research centers and German universities, together forming the HALO consortium; Its partners are the main scientific users of the aircraft, but it is in principle also open to other users, opportunity that will be more pursued in the future.
Cairo has informed about the possible access to the Nasa WB57 for European instrumentation. The WB57 is also offered as a freelance carrier for European driven missions.
In the follow-up discussion, existing and possible near-realization stratospheric platforms were analyzed.
Apart from the in service heavy payload high altitude stratospheric research aircraft, like NASA WB57 and ER2, or the Russian M55 Geophysica, a new generation of stratospheric platforms is arising. White Knight One and Two, from the well proven Scaled Composites Model 281 Proteus, are jet-powered cargo aircraft that are used to lift heavy payloads, like SpaceShipOne spacecraft to release altitude, with service ceiling at 21km. Other initiatives to develop small to ultra-small stratospheric platforms, either manned or remotely piloted, are ongoing in Germany (DLR) and Italy (ASI).
There was a general consensus that, though at national level there is an intent to acquire high altitude research aircraft, as their ceiling is limited to the high troposphere (about 15 km), and though a new generation of lightweight platforms is emerging, in the medium term the sole chance to maintain airborne stratospheric research is either supporting the well-established European ones, namely the M55 Geophysica with already well proven EU payload, and fostering forms of collaborations with the NASA carriers .
Cairo presented the future evolution of the EUFAR consortium into a legal structure beyond the limit of the EU funded projects. This structure should support the core activities of the network, implement the scheme of Open Access to the existing national facilities, facilitate negotiations with EC and other funding agencies. It was stressed that this coordinated approach may facilitate the replacement of existing aircraft or the acquisition of new facilities (including stratospheric).
Garcia Gallardo explained how the legal structure EUFAR consortium will look like; details can be found in his presentation that is, like the others presented during the workshop, on the EUFAR website.
The workshop saw the need to maintain the community's observational capacity for stratospheric research, given the multitude of scientific issues that can predominantly or only be studied through airborne observations. Looking ahead to the availability of new, lightweight and manageable stratosphere platforms, it is essential, in the medium term, to maintain the present observation capabilities, supporting the existing European stratosphere platform Geophysica and promoting collaborations with non-European countries for the joint exploitation of their infrastructures.
In this respect, the creation of a legal structure of the EUFAR consortium can provide a coordinating framework and a common interface for dealing with funding agencies or aircraft operators.
For more information, please contact Francesco Cairo.