More than a dozen global climate models are in widespread use around the world, but only a handful go well beyond the weather layer of the troposphere into higher atmospheric realms. NCAR’s Whole Atmosphere Community Climate Model (WACCM) is pushing the vertical envelope. Version 3 of WACCM, released in July 2008, simulates climate up to 140 km (87 mi).
WACCM recently simulated global temperature trends across various altitude bands spanning the stratosphere (bottom plot, 13–22 km; top plot, 38–52 km). The model runs (black) compare well with satellite-derived temperature anomalies (blue). For more on this image, click here. (Image courtesy William Randel.)
A couple of other centers have similar high-top models, including the Max Planck Institute and Environment Canada. The extra height isn’t critical for modeling day-to-day weather events, because the bulk of atmospheric energy is focused in the troposphere. However, high-top models are invaluable for examining how phenomena such as solar storms and ozone depletion affect the middle and upper atmosphere and how their influence can, at times, percolate to Earth’s surface.
“To accurately reflect the recovery of the ozone hole, you need a high-top model,” says NCAR’s Daniel Marsh. Stratospheric ozone loss above Antarctica, for instance, has been linked to upper-level circulations that help focus cold air over the continent. “If you’re not resolving the stratosphere with a high-top model that includes chemistry, you might get a different evolution of climate in the troposphere,” Marsh says. High-top models are also finding use in “world avoided” simulations—for example, studies of what might have happened had the Montreal Protocol not been implemented.
Last year marked the launch of the CCSM’s Whole Atmosphere Working Group, which is focused on WACCM. “We want to increase the user base for WACCM and make it more of a community model,” says Marsh, who co-chairs the working group with Aaron Ridley (University of Michigan). For the next IPCC runs, the WACCM team hopes to incorporate an interactive ocean, which could be a first for global models that extend into the thermosphere.
The developers have their sights set still higher. An experimental version called WACCM-X now extends through the upper thermosphere and ionosphere (the ionized region of the thermosphere) to about 500 km (310 mi). It’s the first global climate model to reach such altitudes. When WACCM-X is complete, scientists will use it to study space weather events, such as the impacts of coronal mass ejections on Earth’s atmosphere. They hope to eventually couple WACCM-X to magnetosphere and plasma models, which simulate atmospheric processes at even higher altitudes.
The Space Weather Modeling Framework, developed at the University of Michigan by Ridley and colleagues, now simulates processes from the low corona of the Sun to Earth’s upper atmosphere. “When plans are completed to couple WACCM-X to the SWMF, we will truly have the first-ever Sun-to-mud model,” says Ridley. “This model can be used to accurately predict the state of the near-Earth space environment many days in advance, which is the goal for many operational forecasters.”