Editorial Feature

Does Lightning Affect the Global Climate?

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Lightning is more than just light and noise: it is an intense chemical factory that affects both local air quality and global climate. But how big is its effect? Researchers are in the process of researching further on the matter.

Since the early to mid-2000s, researchers have been developing a new technique to estimate the factory's output; the method was applied to the Geostationary Lightning Mapper (GLM) that began to monitor the Western Hemisphere from a next-generation weather satellite launched in 2014. The GLM was placed on a GOES-16 satellite that promoted increased severe storm warning lead time, allowing researchers to predict damaging winds, lightning, and other natural phenomena before they strike.

Interests in Trace Gases Produced by Lightning

In an interview with William Koshak, a lightning researcher at NASA's Marshall Space Flight Center, the link between lightning and the global climate is explained.

Atmospheric chemists are very interested in trace gases produced by lightning, particularly nitrogen oxides (called 'NOx' for short).

William Koshak, Lightning Researcher, Marshall Space Flight Center, NASA

NOx includes nitric oxide (NO), a toxic air pollutant produced by automobile engines and power plants, and nitrogen dioxide (NO2), a poisonous reddish-brown gas with a sharp odor.

We know that lightning is the most important source of NOx in the upper troposphere, where our weather takes place. NOx indirectly influences our climate because it partly controls the concentration of ozone (O3) and hydroxyl radicals (OH) in the atmosphere. Ozone is an important greenhouse gas, and OH is a highly reactive molecule that controls the oxidation of several greenhouse gases.

William Koshak, Lightning Researcher, Marshall Space Flight Center, NASA

Lightning is a Wildcard for Global Climate

While output from the automobile and similar industries can be measured, lightning is a wildcard in models of regional air quality and global climate because it is difficult to realistically model several important lightning characteristics such as lightning energy and the thermochemical yield of NOx produced by a lightning stroke. Hence, the global production rate of lightning NOx is still under study, but researchers were able to pinpoint that it  ranges anywhere from 2 to 20 teragrams per year, with 1 teragram equating to 1 trillion grams.

More Work Needs to be Done to Better Model Lightning

Fortunately, space-based atmospheric chemistry measurements made by NASA's Aura satellite provide a 'top-down' constraint on global chemistry and climate models. With these new constraints, the best estimate to date is closer to 6 teragrams per year. However, more work must be done to better model lightning and other chemical processes before full confidence in this estimate is achieved.

William Koshak, Lightning Researcher, Marshall Space Flight Center, NASA

To better understand lightning flash energy–a critical parameter in lightning NOx production–Koshak and his colleagues are using data from the Lightning Imaging Sensor (LIS) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite and two arrays of ground instruments at NASA's Kennedy Space Center in Florida. LIS is a special camera that uses a very narrow spectral filter and other techniques to detect the lightning optical emissions during both day and night. The filter is centered near 777.4 nm, which is just below the deep red limit of human vision.

Their results were reported in Lightning Charge Retrievals: Dimensional Reduction, LDAR Constraints, and a First Comparison with LIS Satellite Data, a paper that was published in the Journal of Atmospheric & Oceanic Technology of the American Meteorological Society. His co-authors are E. Philip Krider, Natalie Murray, and Dennis Boccippio.

Correlation Between Flashes and Lightning Measurements

According to Koshak, "The idea is to investigate what correlation may exist between the optical characteristics of flashes seen by LIS versus ground-based lightning measurements at Kennedy. The ground-based sensors allow us to probe deep within the thundercloud to determine the geometry of the lightning channel, the charges deposited by the flash, and the energy of the flash. The key is to see if the space-based optical measurements can be related to the ground-based flash energy estimates. If this can be done, it would be possible to use sensors in space to remotely retrieve flash energetics over a much larger region of the globe.”

"It's a formidable task, and this is just a preliminary look," he says of the forthcoming paper. The cloud medium is variable and therefore scatters the light emitted from lightning in complex ways. Energetic flashes embedded deep within an "optically thick" thundercloud could appear relatively dim to a space sensor, while weak-energy flashes occurring near cloud-top could appear relatively bright. All these complexities must be unraveled, and it is "tricky business."

Better Understanding of One of the Key Pollutants in the Atmosphere

With ongoing research, key players in the industry hope to develop more knowledge about the area, as well as techniques that will use GLM data to estimate lightning flash energy.

In practice, we'll do it in a statistical fashion. We would like to give the atmospheric chemistry modelers a realistic probability distribution function for lightning flash energies that they can use in their models to better simulate lightning (be it ground or cloud flashes).

William Koshak, Lightning Researcher, Marshall Space Flight Center, NASA

This article was updated on 10th April, 2019.

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