typical thunderstorm often illustrates that lightning and rain go together
but not always. The more important partnership may be between lightning and
ice in a storm cloud. New observations recently confirmed that lightning follows
clouds ice content, potentially providing climate scientists with a new
method of measuring water in the atmosphere worldwide an important component
of global climate models for forecasts.
This satellite image of Hurricane Katrina, created by combining radar and radiometer data together from the moment when the storm made landfall, shows the hurricanes increased precipitation as it grew to a Category-5 storm. Little white spikes mark where the Tropical Rainfall Measuring Missions Lightning Imaging Sensor detected flashes of lightning. Note that the detected lightning occurs in the outer spiral bands of the hurricane, and not at the center. That difference may be because the clouds forming Katrinas eye lacked precipitation-sized ice, a conclusion that requires further research. In the meantime, a new study has shed light on the role of ice in the occurrence of lightning. Image by Dennis Boccippio, NASA Marshall Space Flight Center.
It rains hard usually when you have lightning, says Walter Petersen of the Earth System Science Center at the University of Alabama in Huntsville, but lightning can also happen without rain. However, lightning always occurs in the presence of ice particles, something determined with observations and lab experiments in the 1940s and 1950s. The experiments, Petersen says, showed how three water phases present in a cloud could make graupel pellets slightly smaller than hail but bigger than ice crystals transfer an electrical charge.
The more graupel, the more lightning, Petersen says, likening a thunderstorm cloud to a battery. As water vapor is blown up into the cloud, it cools and condenses to form small cloud droplets and then larger raindrops. These bits of water become supercooled and eventually freeze as they move up, accreting together with other ice particles to make graupel pellets, which tend to fall back down. At the same time, thunderstorm updrafts move ice crystals up in the cloud. As passing ice crystals collide with the falling graupel, they exchange electrons. The movement creates an electrical current, basically making one end of the cloud positive and the other negative perfect for making lightning.
With that knowledge in hand, Petersen and colleagues examined satellite data from NASAs Tropical Rainfall Measuring Mission (TRMM) to look at lightning flashes around the world, along with the locations of water and large ice particles. The team compared ice density in the clouds with the amount of flashes produced, and found that the correlation of the amount of ice with the amount of lightning stayed steady, whether over land masses or oceans: Increasing the amount of ice increased lightning flashes proportionally.
If you look at the oceans or the continents or coastal areas, all these different areas have different ways to make rainfall, Petersen says. The thing that is consistent in those areas is the mechanism to make lightning: the presence of this ice mass.
The new research, published in Geophysical Research Letters on July 26, is excellent and probably the best documented if not the first confirmation of the steady correlation between lightning and precipitation-sized ice in thunderstorm clouds, says Ken Pickering of the University of Maryland in College Park.
Because ice is part of the water budget, Petersen says, it is an integral piece of information for weather and climate modeling. But just how much ice contributes to the system remains unknown. The research team suggests that lightning measurements could be a good indication for how much water-ice is present in clouds on a global scale.
Pickering also suggests that the findings could also help in determining the chemical impact of lightning on the atmosphere, which catalyzes the production of ozone by creating nitric oxide a possible climate feedback loop. Current satellite instruments, he says, cant get an exact count of all lightning flashes around the world, and chemical modelers must make estimates based on meteorological variables. We may be on a better track to predicting global flash rates better, Pickering says, for the next generation of climate models.
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