High above the King Air altitude, NASA is also flying one the agency's two U-2 derivative Lockheed ER-2 aircraft on a mission to test three different polarimeters to measure aerosol and cloud properties. A polarimeter gauges the intensity and polarization of reflected light, and is therefore crucial to analyzing the composition of droplets or ice crystals in clouds that impact Earth's radiation budget and climate.
The Polarimeter Definition Experiment will test the three instruments mounted in pods under the wings of the ER-2 as a preliminary evaluation before selecting instruments for the upcoming Aerosol-Cloud-Ecosystem (ACE) satellite. “Each have different engineering realizations on how you might do this,” says ACE science lead David Starr of Goddard. “All three have different ways of measuring. They're all multi-angle and frequencies, but how many angles, do you do swathe sampling, what channel should you really polarize?
“There are people who can argue strongly it should be this or that way, So we now have three airborne instruments that express these different concepts. But to go forward with a satellite concept we have to be darn sure it is going to work. It may not be any of these, but we need to prove them out,” says Starr, who cautions: “It is not a shoot-out at this point. We are collecting data to enable each to progress toward these goals.”
The three instrument teams include New York-based NASA Goddard Institute for Space Studies heading the Research Scanning Polarimeter, NASA's Pasadena, Calif.-based Jet Propulsion Laboratory heading the Airborne Multi-angle Spectro-Polarimetric Imager (AirMSPI); and the University of Maryland heading the Passive Aerosol and Cloud Suite (PACS) polarimeter portion. Although the 65,000-ft. test altitude of the ER-2 is well below that of a satellite, it does provide a repeatable test environment, says Starr who adds that the “ER-2 is in some respect more challenging than the space environment. There are more vibrations and it gets wet every time you come down. We know how to build satellite instruments, right now we've got to make sure we build the right one.”
The timing of the parallel Discovery-AQ mission was a fortunate coincidence, adds Starr.
“The P-3 is measuring a lot of things we are interested in so they are a perfect match. We're trying to look at remote data and they are much more in-situ. So we are very willing to work closely with them. A big part of the air pollution out here is particulate (aerosols) and that is what we are interested in. [So we] set up flights overhead so we can be in the same neighborhood.”
A third high-altitude mission running in parallel is the Airborne Tropical Tropopause Experiment (Attrex) which is using a NASA Northrop Grumman Global Hawk unmanned air vehicle to make an unprecedented exploration of the tropopause. This is the boundary between the troposphere and stratosphere, ranging from about 8-11 mi. above the Earth's surface, depending on latitude. It marks the boundary where water vapor, ozone and other gases enter the stratosphere. “It's an extremely cold part of the atmosphere and is a 'cold trap' where water vapor condenses into ice crystals,” says Eric Jensen, Attrex principal investigator at NASA's Ames Research Center at Moffett Field, Calif.
“Water vapor is a powerful greenhouse gas, and so even a small increase in stratospheric humidity will warm the surface. We have recognized that it is very important for us to understand it and to make sure that our climate models are seeing it correctly,” Jensen adds. As changes in stratospheric humidity may have significant climate impacts, these predictions are becoming more important. Yet modeling remains uncertain because the physical processes occurring in the tropopause are not fully understood. Under Attrex, NASA will use the high-altitude capability of the UAV to carry instruments via a variety of profiles through this layer near the equator off South America.
The Global Hawk is configured with 12 instruments to measure cloud properties such as ice crystal size and water vapor concentrations in addition to trace gases and temperatures above and below the aircraft. It will also monitor meteorological conditions, radiation fields and chemical traces, “which tell us about the transport processes,” he adds.