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Observing Lightning from Space

Lightning is rapidly emerging as a phenomenon of increasing interest to meteorologists and climate scientists. At the individual storm level, real-time measurements of lightning flash rates provide forecasters with key information about the evolution and severity of thunderstorms. Lightning data are used by forest management personnel (to assess forest fire risks), power utility providers, emergency management units, recreational facilities, and the insurance industry. At the global scale, lightning observations are used to analyze natural sources of nitrogen compounds important to atmospheric chemistry. Global lightning distributions also provide key information about deep thunderstorm convection, which in the tropics is the principal "engine" driving the global climate system.

The observation of global lightning activity is a nontrivial problem. Conventional (radio frequency) lightning detection systems have limited fields-of-view, and they are strongly biased when measuring lightning at far distances. An obvious solution is the optical measurement of lightning flashes from space, in either low-earth or geostationary orbit. This approach has been successfully developed and implemented by the GHCC Lightning Team.

The GHCC's Lightning Imaging Sensor (LIS) was launched November 1997 with other instruments on the Tropical Rainfall Measurement Mission (TRMM) satellite. The LIS builds on successful observing campaigns that used optical sensors aboard U2 aircraft and the Space Shuttle to demonstrate that lightning could be observed from cloud-top measurements. The data from these campaigns led to the design and deployment of the prototype Optical Transient Detector (OTD), which has been observing global lightning aboard a low-earth-orbit Microlab-1 satellite since April 1995. The LIS design represents a significant improvement in resolution, sensitivity and robustness over the OTD. Together, the LIS and OTD are now providing the first truly unbiased climatology of global total lightning, during both day and night.

The LIS's eye is a small, solid-state camera with special filters that admit only the peak optical wavelength emitted by lightning. Software measures the background cloud scene and records only sudden changes (transients). The camera view is 600 km on a side, and each LIS pixel covers 5-10 km on the ground, matching the smallest storm cells that might host lightning. LIS images are overlaid on visible, thermal and radar images produced by other TRMM instruments. By analyzing images from multiple instruments, LIS investigators hope to match lightning flash rates with key aspects of storms, including updrafts, total ice content, cloud top-height and total rainfall. The first LIS images were released in mid-December, and daily overviews of images are now posted on the World Wide Web a day after they are collected. Both OTD and LIS data, as well as a full suite of cross-platform analysis software, are available from the Global Hydrology Resource Center (GHRC).

NASA has also approved an advanced definition study for a Lightning Mapping Sensor (LMS) that could provide real-time observations from geostationary orbit. The LMS represents the next logical progression in lightning detection from space. The OTD sensor provided a technology proof-of-concept, a truly unbiased global climatology and an indication of the technical difficulties yet to be addressed. The LIS sensor improved on the OTD design, and is collecting high quality science data that will allow scientists to translate lightning flash rate measurements into useful forecasting, scientific and diagnostic tools. The LMS sensor is an evolution into an "operational" paradigm. Continuous, real-time LMS measurements, paired with the scientific knowledge gained from the LIS mission, will provide operational weather forecasters with critical additional information, enabling them to decide in advance whether a developing thunderstorm, squall line or hurricane will become severe. Research results from LIS and OTD data indicate that this tool may provide crucial warning time to the public.