Taking Flight To Seek Out The Least Understood Climate Driver

A NASA research plane spent the month of June crisscrossing the southern Great Plains in search of more detailed information on the least understood variable in long-term climate change scenarios.

Tiny suspended particles are nearly everywhere in the atmosphere, and what we see as dust, smoke, soot or haze in the sky scientists study collectively as aerosols. Some aerosols are easy to see with the naked eye, however, they have proven difficult to pin down in terms of their impact on the climate and, in the long run, climate change.

The month of flights – from a Department of Energy Climate Research Facility near Ponca City, Oklahoma – were conducted in part to help evaluate algorithms to be used in the upcoming Glory mission as well as to address NASA’s larger goal of getting a tighter grip on the important but poorly quantified impact of aerosols on climate. A team from NASA’s Langley Research Center flew the center’s B200 plane for the research flights. The B200 was outfitted with a lidar instrument which measures vertical profiles of aerosols and an instrument called a polarimeter, developed by the Goddard Institute for Space Studies, that measures polarized light scattered by aerosols to gather more accurate details about the size, shape and composition of aerosols.

Aerosols directly affect Earth’s energy budget – the balance of incoming and outgoing radiation – by absorbing and scattering incoming solar rays. That impact is understood only within a large margin of uncertainty. Aerosols also influence cloud formation. The microscopic particles help form water and ice clouds and can change cloud properties – often leading to greater cloud cover and a cooling effect. This indirect effect has been even harder to measure and model than the direct effect. It accounts for the largest uncertainty in models used for predicting future climate, according to the Intergovernmental Panel on Climate Change (IPCC) 2007 report. This finding led the U.S. Climate Change Science Program (CCSP), in a report released in January 2009, to state the case for much-needed improvements in both measuring and modeling aerosols in the atmosphere including their interactions with clouds. The commonly accepted range of potential surface temperature increase over the course of a century – assuming a doubling of atmospheric carbon dioxide – is 1.2 degrees to 4.7 degrees Celsius. Most of the temperature increase should occur in the latter part of the century. The majority of the uncertainty that leads to that wide range in the prediction of heating is due to unknowns about the impact of aerosols.

“Such a range is too wide to meaningfully predict the climate response to greenhouse gases,” the CCSP report concluded.

The flights in Oklahoma were designed to offer a closer look at aerosol-cloud interactions and see how the airborne polarimeter – called the Research Scanning Polarimeter (RSP) – and lidar – called the High Spectral Resolution Lidar (HSRL) – could work together to give a more complete picture of aerosols. Data from the flights – which covered a vast region of the southern plains, in an attempt to capture useful data over a common type of land surface – will test the algorithms to be used to process data gathered by the Aerosol Polarimetry Sensor (APS) that will fly on the Glory satellite. In addition, the flights also served as a test of the instruments’ ability to make measurements of the size, type, amount, and distribution of aerosols.

Brian Cairns, the Goddard Institute for Space Studies-based principal investigator for the RSP instrument and the Aerosol Polarimetry Sensor that will fly on Glory, said polarimetry could eventually significantly improve remote sensing measurements of aerosol size and substantially reduce the uncertainty related to measurements of amounts of aerosols.

On the more experimental end, Cairns said, scientists are using the data from the lidar and polarimeter to look at the concentration of water droplets in clouds – an important parameter that could be significantly influenced by the presence of manmade aerosols, such as pollution. Determining a suitable method for measuring droplet concentration could go a long way toward making better estimates of the influence aerosols have on cloud properties.

“You’re really just trying to get the number concentration of droplets,” Cairns said. “Over oceans, it’s not that variable within a given type of clouds. But with the addition of some pollution, that could change.”

In the long line of NASA’s ground-, airborne- and satellite-based instruments designed to observe aerosols, these flights pairing the RSP and HSRL provides another important perspective.

“The idea is we can combine data from the two instruments to get more detailed information about the aerosols,” said Rich Ferrare, a research scientist with the HSRL team at Langley. “We can combine the data to get more than either, alone, can provide.”

Ferrare also said that while satellite-based sensors provide a global view of aerosol coverage, airborne measurements allow scientists to get a closer look at the still incompletely understood processes of aerosol-cloud interactions. Studying that full range is necessary to ultimately reduce the unknowns about aerosols and their impact on climate.

“Just from the measurement standpoint, you’ve got to be able to look at the small scale, to see how those processes work,” Ferrare said. “You also need global measurements from satellites. Then you need to improve the models.

“There are a lot of things involved in reducing those error bars.”

Related Links:

> High Spectral Resolution Lidar (HSRL)
> Glory: Observing the Earth's Aerosols and Solar Irradiance