Cirrus Clouds Hold Clues to Climate
Research flights will obtain most comprehensive data set to-date for climate models
Cirrus clouds can blanket the entire sky, so they obviously affect how the sun’s energy is transferred between Earth and space. But how much? That’s an important question for scientists grappling with future climate change scenarios.
In January, scientists sponsored by the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility began a five-month aircraft campaign to gather data from cirrus clouds in the skies above Oklahoma. Using an instrumented research aircraft equipped with various probes and sensors, their goal is obtain a new and comprehensive set of in-cloud, or “in situ,” measurements about the size and number of ice crystals that make up cirrus clouds.
“It’s amazing that the state of the climate system depends on tiny amounts of ice near the top of the atmosphere, but these clouds are like the atmosphere’s blanket. A larger concentration of ice crystals in cirrus clouds is like adding another blanket. The thickness of the blanket and what controls it are what we need to understand,” said Dr. Gerald Mace, a professor of atmospheric sciences at the University of Utah and the lead scientist for this study.
The ARM Climate Research Facility obtains climate data from fixed observation sites around the globe, including its flagship Southern Great Plains site, centered near Lamont, Oklahoma. Since the early 1990s, ground-based instruments at this site have been collecting continuous data about clouds and other atmospheric properties for the climate modeling community.
Ice crystal data collected during the Small Particles in Cirrus, or SPARTICUS, campaign will be compared against the data obtained at the Southern Great Plains site to evaluate the accuracy of the various measurement methods. The campaign may also help resolve some lingering concerns related to discrepancies in ice crystal measurements obtained during previous studies. These discrepancies lessen confidence in the climate models that use the data to simulate future climate.
Doctor Freeze, In the Wind Tunnel, With a Probe
When compared to satellite and ground-based measurements of cirrus cloud properties, evidence from previous aircraft campaigns indicated that the probes were recording an inordinate amount of tiny—less than 50 microns—ice particles. Other measurements and modeling studies suggested that the concentrations of small crystals were actually much smaller.
Without a ‘smoking gun’ to explain why the data were so inconsistent, plenty of theories emerged as to the source of the mystery. That is, until videos from wind tunnel experiments revealed the culprit as “forward scattering.”
The wind tunnel allowed researchers to simulate what happens when ice particles from a cloud come into high-speed contact with the aircraft probe— they shatter. These smaller particles would then deflect through the path of the laser beam that takes the measurements. This clearly would skew the data towards a larger number of smaller particles.
Scientists have developed new probe designs and faster electronics to help resolve this problem. One of the new probe designs includes slanted tips that direct the shattered particle remnants away from the laser beams, decreasing the odds for erroneous measurements.
In the electronics department, post processing of the data will determine the “inter-arrival time” or local clustering of the particles. Clustering is a key factor in determining the likelihood that concentrations are enhanced by shattering.
As particles pass through a laser beam between the probe’s two “struts” the instrument stores information about the size of the particles based on the changing intensity levels of the laser beam. Particles that have shattered send through many closely clustered remnant particles that should have anomalous inter-arrival times compared to the particles that have not shattered.
Because the number of remnant small particles is dependent on the concentration of large particles, improvements to these measurements would result, at best, in an estimated correction factor for the actual concentration of small particles measured during past campaigns. That said...
“If anybody has a chance at gathering the data needed to calculate a statistically significant correction factor it will be the SPARTICUS field campaign,” said Jason Tomlinson, an aircraft operations leader for the ARM Aerial Facility, which coordinates all the logistics for ARM aircraft campaigns. “The five months of flying will generate a large data set covering a variety of atmospheric conditions that will allow the correction factor to be more representative across various types of cirrus clouds.”