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File Cabinet and Bookshelf
GHCC Scientists Study Lightning From Space
Introduction
Lightning is known to occur in many types of storms and storm systems (e.g., airmass thunderstorms, oceanic storms, storms having vigorous updrafts, severe supercell thunderstorms, stratiform regions of mesoscale weather systems, some hurricanes and tropical cyclones, and even some snowstorms). Improved characterizations of the relationships between meteorological processes that lead to cloud electrification and the subsequent production of lightning should improve our interpretation of global cloud measurements. NASA's contribution to the global observation of thunderstorms is the Lightning Imaging Sensor (LIS). The Lightning Imaging Sensor (LIS) is an Earth Observing System (EOS) instrument flight of opportunity on the Tropical Rainfall Measuring Mission (TRMM), scheduled for launch in November 1997. LIS will provide one of the "24 EOS Measurements" of key physical variables needed to advance understanding of the entire Earth system and to develop a deeper understanding of the interaction among Earth system components.
The LIS is a small, highly sophisticated optical remote sensing system which, from low Earth orbit, is able to detect and locate lightning over large regions of the globe. An early prototype version of the LIS, also developed by the LIS Instrument Team, was flight qualified and launched in April 1995 to demonstrate the LIS instrument and science concepts as a small satellite payload. This instrument, named the Optical Transient Detector (OTD), has provided continuous global lightning data since its launch.
In recent years, scientists have become increasingly aware of the key role played by lightning in the dynamic interplay of forces occurring in the Earth's atmosphere. Research has indicated that lightning may be a very good indicator of the strength of convective storm systems. The OTD has contributed to the discovery of potential lightning indicators for application to more timely hazardous weather and tornado warnings, and for improved forest fire and wild-land fire management; to the use of lightning as a proxy for detecting intense atmospheric convection; to the production of the most complete and detailed maps of the global lightning distribution; and to the discovery that the global flash rate is approximately 40 flashes per second, less than half of the widely accepted estimates dating back to 1925.
System Description
The OTD is a highly compact combination of optical and electronic elements. It was developed as an in-house project at NASA's Marshall Space Flight Center in Huntsville, Ala. The name, Optical Transient Detector, refers to its capability to detect the momentary changes in an optical scene which indicate the occurrence of lightning. The OTD instrument is a major advance over previous technology in that it can gather lightning data under daytime conditions as well as at night. In addition, provides much higher detection efficiency and spatial resolution than has been attained by earlier lightning sensors.
At the heart of the system is a solid-state optical sensor similar in some ways to a TV camera. However, in overall design and many specific features, OTD had to be uniquely designed for the job of observing and measuring lightning from space. Like a TV camera, the OTD has a lens system, a detector array (serving a function somewhat analogous to the retina in the human eye), and circuitry to convert the electronic output of the system's detector array into useful data.
The sensor system (camera) is approximately 8 inches in diameter and 15 inches high, while the supporting electronics package is about the size of a standard typewriter. Together, the two modules weigh approximately 18 kilograms (40 pounds). The total weight of the satellite placed on orbit is 75 kilograms (165 pounds).
OTD Operations
Under an agreement between NASA and the Orbital Sciences Corporation, the Optical Transient Detector was carried as a secondary payload on a Pegasus, an Orbital Sciences Corporation air-launched rocket. The Pegasus launch on April 3, 1995, delivered the OTD into an Earth orbit of approximately 710 kilometers (446 miles) altitude, with an inclination of 70 degrees. With that orbit, and OTD's wide 100-degree field of view, it will be able to survey virtually all areas of the globe where lightning normally occurs.
OTD was expected to be in operation for only two years, but has continued recording the occurrence and worldwide distribution of lightning beyond the expected mission lifetime. The data are transmitted on a daily basis from OTD to a ground station in Fairmont, W.Va. From there the data are sent to the Global Hydrology and Climate Center in Huntsville, Ala., for processing, analysis and distribution to the scientific community.
The OTD data have provided past field experiment support to the NASA PEM-TROPICS chemistry mission. The OTD data was used to support mission planning for the 1997 NASA SONEX mission. The OTD data and associated scientific research has confirmed its importance in contributing to the understanding of atmospheric and precipitation processes. This contribution has reinforced the the need for similar observations from geosynchronous orbit, where storm morphology can be monitored continuously.
Education
Two doctorate degrees have been completed making use of OTD data, and a total of 10 masters's and doctorate degrees are now under way which incorporate OTD observations into experimental and theoretical studies. OTD data has been used in a cooperative project between the Marshall Center and the Huntsville City Schools involving five high school students.
Using a unique vantage point in space, the Optical Transient Detector promises to expand scientists' capabilities for surveying lightning and thunderstorm activity on a global scale. At the same time, it will help prepare the way for the future, when more systematic monitoring of a wide range of indicators will allow scientists to better track our Earth's vital signs.
Results
This section showcases some of the important results from the Optical Transient Detector mission and related observations of lightning from space. The data used to produce these products are available from the LIS data archive at http://ghrc.nsstc.nasa.gov.
1. Global Distribution of Thunderstorms (yr_ltg_map.qt) 
Between September 1995 and August 1996, the OTD observed nearly 1 million lightning flashes worldwide. The lightning flash density (flashes per square kilometer per month) is shown in the movie. The movie depicts the seasonal shift of thunderstorm activity between the summer and winter. There is a much greater frequency of winter-time thunderstorms in the North Atlantic and Mediterranean Sea than expected from earlier thunder day and lightning climatologies. The satellite is in a precessing orbit where the time of observation varies from day to day. Thus, the OTD distribution is acquired intermittently throughout the diurnal cycle. This diurnal sampling bias was removed by applying a moving average filter to the data. Most of the lightning is in the intertropical convergence zone (ITCZ) over the continents. There is far more lightning over the land masses than over the oceans. This results from the stronger vertical motions in continental clouds than in oceanic clouds.
2. OTD Observation Over Africa (otd_africa.gif) 
OTD views an instantaneous cloud scene of 1300 km x 1300 km for up to 3 minutes, depending on the orientation of the satellite. During this time interval the individual lightning transient events are recorded every two milliseconds. At approximately 80 second intervals, the OTD also records an image of the cloud scene. Multiple fields of view can be spliced together to construct a complete orbit swath of the lighting activity and their parent thunderstorms. Lightning observations over Africa are extremely rare. New insights into the nature of storms in the African continent are now routinely available through the use of the OTD.
3. OTD Calibration and Validation (otd_nldn.gif) 
The LIS/OTD science team is collecting coincident observations from the OTD with other satellites and ground-based systems to validate the OTD algorithms and instrument performance. One such example is shown. The distribution of 23,549 lightning flashes (in-cloud and discharges to ground) observed by OTD over N. America during May 1995 is derived from a total of 354 orbits (left-hand image). The cloud-to-ground flashes detected by the National Lightning Detection Network (NLDN) that are coincident in time with the OTD observations are shown in the right hand image. Comparisons with independent data sets such as this help determine the on-orbit performance of the OTD and LIS instruments.
4. Hurricanes (opal.gif)
Recent evidence suggests that lightning observations can be used to understand the changing intensity of hurricanes. Land-based measurements of lightning in the core of hurricanes have been observed to be precursors to a period of intensification combined with the presence of an eye wall cycle. This is very useful information to a forecaster, because the dense cirrus overcast typically hides the underlying convective cells when cloud-tops are only observed in the visible or infrared. This potential use of lightning was evident in Hurricanes Andrew and Opal which had large outbreaks of lightning in the storm core during periods of deep intensification as they approached land in the U.S. All 19 named Atlantic hurricanes and tropical storms were observed by OTD at various times in their lifecycles. All produced at least some lightning during OTD overpasses with Hurricane Opal the most active. The image shows Hurricane Opal near the time of landfall. In this example, the lightning event data are superimposed on a composite image of the weather radar and GOES infrared weather satellite. Most of the lightning is over Florida in the outer rainband, yet some infrequent lightning is also occurring in the eyewall. Cloud-to-ground lightning is also occurring in the outer rainband, yet not in the eyewall at the time of this observation.
5. Tornadic Storms (otd_ok_storm.gif) 
Analysis of OTD observations of a tornadic and non-tornadic storm during an overpass over Oklahoma on 17 April 1995 supports an important connection between lightning and storm dynamics. During the 3 minute sampling of the tornadic storm, OTD observed total flash rate rates in excess of one flash per second with a high percentage (94 percent) being intracloud flashes. The total lightning stroke rates in this storm decreased just prior to tornado touchdown, as confirmed by a storm chase team. This result is analogous to the lightning precursor observed with wet microbursts in the Southeast U.S. where the total flash rates increase and then decrease rapidly a few minutes prior to storm collapse and microburst onset. These preliminary results indicate that total lightning rates are more indicative of the dynamic changes occurring within a storm than the cloud-to ground lightning rates alone. (The satellite's gravity gradient boom identified in the left-hand image, which keeps the OTD sensor oriented to the Earth, partially obscures the OTD field-of-view).
6. Lightning and Precipitation Studies (otd_precip.gif) 
On May 19, 1995, OTD observed a complex of storms in the Tennessee Valley following a significant severe weather outbreak. The total lightning flash density map (left-hand image) corresponds to the regions of heaviest rainfall rate shown in the right-hand image. Various studies have showed a direct proportionality between total rain volume and the number of flashes in storms that produced lightning. More recent results clearly indicate that the coupling between lightning and rainfall is highly variable and is strongly dependent on geographical location. Systematic variations in rain yields (ground discharges divided by kilograms of rainfall per month) have been found for mid-latitude and tropical continents, tropical oceans, and tropical island and coastal rainfall regimes. This latter result suggests that the storm environment is an important factor in the study of the relationship between lightning and rainfall. By acquiring coincident OTD lightning and precipitation measurements, we provide a unique opportunity to examine and understand the mechanisms that influence the relationship between lightning and rainfall. An improved understanding of these mechanisms is needed to maximize the benefit of incorporating lightning measurements into surface rainfall algorithms. Space based rainfall estimation is especially important to the developing nations in the tropics and Southern Hemisphere, where land-based weather radars are uncommon.
7. Lightning Observations of the March 13, 1993 Superstorm (ols_ssmi.gif)
An illustrative example of the proposed use of lightning data to corroborate and complement microwave brightness temperature observations is shown in this example. This figure shows the DMSP F10 multi-sensor observations of the March 13 superstorm that produced up to four feet of snow as it progressed up the east coast of the U.S. The nighttime visible-infrared composite image shown at left is from the Optical Linescan System (OLS) while the 85 GHz microwave brightness temperature image is shown at right. Lightning flashes produce the horizontal streaks that show up on the visible image channel. City lights show up as bright spots in the image. The lightning flashes follow the curvature of the squall line highlighted by the lowest 85 GHz brightness temperatures. . The lowest brightness temperatures result from the scattering of radiation by large ice particles and hail within the cloud. Observations such as this suggest that future lightning observations on GOES geostationary weather satellites will be useful in identifying the most intense regions of storm systems.
8. Monthly Lightning Distributions as Determined by OTD and OLS (ols_otd.gif) 
The global distribution of lightning has been estimated previously using thunder days (based on a human observer hearing nearby thunder), local flash counters (with a typical range of tens of kilometers), and Schumann Resonance measurements. Satellite observations have been made using optical detectors and high frequency radio receivers. However, the OTD has produced the highest spatial and temporal resolution direct measurement of global lightning activity to date.
In this example, the OTD global lightning distribution for the month of August, 1995 is shown in comparison to the DMSP OLS distribution for the same month. The OTD orbit allows storms to be sampled throughout the diurnal cycle. The OLS can only detect lightning during nearly complete darkness. The fewer lightning flashes in the DMSP OLS distribution (top panel) can be attributed to the following:
- Observations are made at one time of the day (near local midnight).
- The observation of a single storm is very limited (lightning is only detected as the OLS scans across track and intercepts lightning within its instantaneous field of view (approx. 2.5 km).
- The time of year the observations are made.
At high latitudes in the northern hemisphere summer (June-August), the period of darkness is relatively short, lasting less than two hours above 55 degrees latitude. Thus, the absence of lightning in northern Asia is primarily a consequence of the OLS sampling.
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