The Airbus Perlan II Mission: Forecasting and Modeling for the Argentinian Flights into the Stratosphere in a Piloted, Engineless Glider

The Perlan Project is an international scientific endeavor to soar a manned, engineless glider into the stratosphere. The Perlan Project uses the coupling between mountain waves and stratospheric waves associated with the polar vortex in order to provide the lift for a pressurized sailplane navigated by two pilots. In September 2017, the Airbus Perlan II Mission set a new world altitude record for gliders when it achieved an altitude of more than 15.9 kilometers (52,221 feet), beating the Perlan Mission I world record set back in 2006. Modeling and observations, however, suggest that it may be possible to achieve much greater altitudes, perhaps as high as 30 kilometers.

While such a project is interesting in its own right, there are multiple ways in which it provides valuable scientific knowledge:

• It provides a unique in situ platform for high altitude research. The Perlan aircraft is designed to accept self-contained scientific experiments designed to fit the NASA Cubesat form factor. Additionally Perlan routinely monitors temperature, wind speed, UVA/UVB, atmospheric radiation and ozone concentration. Since Perlan is usually flown in a near-polar environment, such measurements can provide valuable information on ozone layer health.
• Since the aircraft depends on lift from mountain waves and coupling to the polar vortex (i.e., polar night jet) it provides valuable insight on the existence of these high-altitude waves and the turbulence associated with them, which will become increasingly relevant to commercial flying as carriers increase their cruising altitude. It is no coincidence that Airbus is the primary sponsor of Perlan Mission II. The coupling between mountain waves and the polar vortex also provides an important mass exchange mechanism between the troposphere and stratosphere that the aircraft should provide valuable data to help study.
• At the highest levels of flight, the atmospheric environment approximates the density and temperature of the Martian atmosphere and can give insight into the issues that may be faced by using aircraft on Mars.

While that is some of the motivation behind the project, in this particular paper/presentation we are primarily concerned with the atmospheric science for the Perlan Project, from the climatology used in the choice of site to the operational forecasting necessary for the day-to-day flight tests. The project currently operates out of two bases, Minden, Nevada and El Calafate, Argentina. By having bases in both northern and southern hemisphere, Perlan can always be flying in the cool season, when favorable wave flying conditions occur most often. Due to its proximity to the Antarctic circumpolar vortex, the attempt at setting new world records will take place primarily in Argentina, while Minden will be used for testing and proof-of-concept.

Forecasting for Perlan involves both short and long term planning. The Perlan team consists of team of paid professionals and unpaid volunteers, all with a common enthusiasm for the project. It is important to know as far in advance as possible whether the aircraft will be flying on particular day so that the necessary personnel will be available. In the three to five day forecasting window the primary forecasting tools are the global models (GFS and ECMWF), with graphical output customized to look at whether strong mountain waves will occur in the area, and whether conditions will permit flying.

For the nearer time window, a custom WRF model is run out to 72 hours, several times each day. The WRF model analyses have become an invaluable tool for the Perlan project as the Perlan 2 aircraft cannot fly in instrument meteorological conditions (due to visibility and icing), must avoid breaking waves and their associated severe/extreme turbulence and also has wind limits for its takeoff and landing. Specialized WRF graphical output is provided for the pilots and ground crew, such as vertical cross sections with shaded vertical velocities up to 600 meters/minute, as well as horizontal plots at standard heights, also with vertical velocity shading and flight waypoints plotted on them. Rawinsonde balloons are also launched early in the morning each flight day and for days leading up to potential flight days. There are future plans to assimilate the balloon soundings into the WRF model. Near real-time GOES-East imagery also improves the nowcasting capability and allows the flight planners on the ground to give updates during the flights. It is hoped that this suite of forecasting tools will allow Perlan to reach the aerodynamic limit for winged flight.