NTB: You mentioned infrared spectra. What kind of work is being done there?
Dr. Wielicki: One of the more exciting things with the infrared is we’re getting into about half of the whole infrared radiation that the Earth is emitting out to space, seeing it for the first time in a spectrum, out there beyond 15- to 50-micron wavelengths. It’s called the far infrared. It’s where most of the water vapor greenhouse is. The atmosphere holds more water vapor. That itself is a greenhouse gas, and it therefore amplifies the carbon dioxide warming we get almost by a factor of 2.
This will be the first time we really see this globally everywhere in spectra as well at high accuracy. The transform spectrometer has about a 25 kilometer field of view at nadir. One of CLARREO’s visions here is to not only work as a sort of a transfer radiometer in orbit to calibrate other instruments, but also our own spectra by themselves. Unlike our normal remote sensing instruments, where we might try very accurately to get our instrument noise down to do a retrieval at a single point, this is a very different way of going at it. You go for a very high-accuracy spectra, averaged over very large time and space scales, and then you’re looking at climate change anomalies over years to decades and pulling climate change signals out of those spectra. We typically call it spectral fingerprinting or spectral benchmarking.
NTB: What are the radio occultation teams working on?
Dr. Wielicki: Current GNSS (global navigation satellite system) radio occultation meets CLARREO accuracy goals for temperature changes at altitudes between 5 and 20 km. Improvements that the CLARREO team is studying are advances in sampling by adding receivers for not only the U.S. GPS system, but also for the European and Russian GPS systems. Second, we are studying ways to reach the CLARREO accuracy at both lower altitudes below 5 km and higher altitudes above 20 km. The lower atmosphere issues are signal attenuation by water vapor, horizontal inhomogeneity, as well as sharp vertical temperature inversions. Upper atmosphere issues include the ionosphere, local multipath, and upper boundary condition.
NTB: What has the CLARREO been able to determine about how the Earth is changing?
Dr. Wielicki: The very process of deriving more rigorous observing requirements for a diverse set of high-accuracy decadal climate change observations has led to new methods of understanding climate observing system requirements and how to derive them.
CLARREO has pioneered new ways to use climate models to simulate future observing system simulations. We call these climate OSSEs: Observing System Simulation Experiments. This concept has been common in weather prediction observations, but rare in climate observations. The reason is that the problems are very different. Weather is a challenge of observing an instant of time to start the weather model: what is called an "initial value" problem. Climate is almost independent of initial value, and is instead a "boundary condition" problem. So climate OSSEs take very different approaches than weather OSSEs.
We ran a climate model for 100-year climate simulations to understand what CLARREO could provide to test climate predictions. This gets us closer to more rigorous hypothesis testing for climate change. The climate model is the hypothesis, and the climate OSSE shows us how CLARREO can make that test in the future, including what tests might take 5 years, 10 years, or 20 years. These lessons developed in defining the CLARREO mission are being written up now in journal papers and will be applicable to all climate observations in the future. So yes, CLARREO is already discovering new things and we haven't even launched. Science is not about knowing the answers, but is about finding the right questions to ask.
NTB: Are there similar missions that are providing complimentary data?
Dr. Wielicki: Not right now. This is really a new concept. There are other groups that have tried to propose similar things. For example, there’s a UK group, led by Nigel Fox, at their National Physical Lab, and we proposed one called TRUTHS, and that was to do pretty much the reflective solar part of our CLARREO mission. That was proposed to the European Space Agency (ESA) about 4 or 5 years ago, and so we’ve been working actively with them. And ultimately what you want really want is for the US to put up one of these, and then the international community to put up their own version of it, and then just like metrology labs for international standards, you “shoot it out” between those different laboratories for who really has the accuracy on climate change. And given the critical nature of climate change accuracy, this is just kind of fundamental, basic scientific practice to do this.
NTB: What would you say is the biggest challenge from a technical perspective when you’re developing these instruments in the lab?
Dr. Wielicki: The biggest challenge is not to get too fancy on the technology, so in general we are not using cutting-edge detectors or other things that you might have, like huge optics. The optics on our instruments are about an inch — they’re tiny. So what you do instead is focus on crossing the t’s, dotting the i’s, and putting independent verification methods on everything that can change on an instrument in orbit. With instruments like CLARREO, when you’re looking at your calibration sources, you want the entire optical path to be identical to when you look at the earth. Otherwise now you’ve got all these things you haven’t calibrated out that are kind of sitting there in uncertainty; that’s really the challenge.