LIS and OTD Science Accomplishments
A three year global lightning data base has been developed from the EOS Optical Transient Detector (OTD). This is the most comprehensive global lightning data base ever produced and is noteworthy for its high spatial resolution, detection efficiency, coverage, and three year period of record. The Lightning Imaging Sensor (LIS) was launched in November 1997 aboard the Tropical Rainfall Measuring Mission (TRMM). The LIS features enhanced sensitivity, higher spatial resolution, and greater location accuracy than the OTD. The excellent performance of the LIS and OTD has lead to the following scientific discoveries:
- The global lightning flash rate is on the order of 40 flashes per second (fps) as compared to the commonly accepted value of 100 fps, an estimate that dates back to 1925.
- Seventy percent of all lightning activity occurs in the tropics, with the global distribution dominated by the summertime lightning activity over the N. Hemisphere land masses.
- At low latitudes, there appears to be a bi-annual lightning peak occurring at the equinoxes.
- Over large bodies of water, circulations driven by the land-sea temperature contrast plays an important role, with peak activity occurring during winter when the land is colder than the water (Mediterranean Sea, Tasman Sea).
- There is significantly more wintertime lightning activity over the N. Atlantic Ocean than previously documented by shipboard observations (COADS data base).
- Identified inner eyewall lightning activity in hurricanes during periods of changing intensification (e.g., Hurricane Linda (OTD); Super Typhoon Paka (LIS)). Eyewall lightning observed by LIS found in association with ice scattering signatures identified by the TMI instrument (cyclone Susan).
- Identified a lightning burst signature associated with severe storm development and tornadogenesis. Signature reconfirmed during the LIS validation activity.
- Identified optical signature for long continuing currents produced by cloud-to- ground discharges.
The diurnal variation of lightning strongly peaks in the late afternoon over land and is relatively small over water. The solar flux and its associated warming dominates the seasonal variation. The inter-annual variability is much smaller than the diurnal or seasonal variations suggesting that strong convection leading to lightning is driven more by local solar flux (hence the large late afternoon maximum) rather than changes in large scale circulation patterns (See Figure 2 through Figure 7). [Note: the 97-98 El Nino event has not yet been evaluated]
OTD observations indicate that intercloud lightning activity far exceeds cloud to ground activity during initial storm growth and development, and also when storms become severe. During an overpass of a tornadic supercell thunderstorm, OTD detected a pattern of a dramatic increase in flash rate followed by a decreasing rate. A tornado formed shortly after the pass. This sequence parallels that of airmass thunderstorms where it is well established that the flash rates increase during updrafts and decrease sharply with the onset of a downdraft. These observations are also consistent with theories of tornadogenesis involving the stretching of vorticity by the updraft prior to the subsequent tornado formation at the ground (See Figure 8).
This new understanding on the interplay among the intensification of the updraft, lightning bursts, and the onset of severe weather lead to the establishment of a validation campaign to further explore relationships between lightning and severe weather. Findings to date indicate that high flash rate storms have a high probability of becoming severe (See Figure 9 through Figure 12). Further, there appears to be an identifiable signature in the flash rate (these sudden increases or bursts) associated with intensification of the updraft. The burst signature occurred an average of nine minutes before the NWS identified a storm as being severe, primarily based on NEXRAD signatures. Lightning burst signatures have been identified preceding the nocturnal tornadic storms which formed during the February 22-23 severe weather outbreak in central Florida (See Figure 13).
The continuing current signature observed by OTD was observed within a large storm complex when one pixel stayed illuminated for many successive frames for a total duration exceeding 100 milliseconds (See Figure 14). The significance of this observation is that lightning discharges with continuing current are responsible for most naturally occurring forest and wildland fires that occur in North America. The ability to provide real time continuing current warnings in areas of high fire risk potential may prove valuable for forest fire fighting operations.
With TRMM, we are now able to simultaneously observe relationships between lightning activity and the ice content of storms. We can test our hypotheses that ice formation and updrafts play the controlling roles in cloud electrification, thus providing an unique approach for remotely sensing updraft intensity and ice phase precipitation. Early review of a few cases clearly indicate that those oceanic storms that are not producing lightning (the majority) have little or no mass above the freezing level. Precipitation-sized ice is clearly indicated in those storms that are lightning producers. We are in the process of quantifying these observations (See Figure 15 and 16).
We have also been investigating the electrification of tropical cyclones and hurricanes. In 1995, all named Atlantic tropical cyclones and hurricanes produced some lightning during one or more OTD overpasses. However, throughout much of their life-cycle these storms produce little or no lightning. When lightning is present, it is normally contained in the eye wall or rain bands (See Figure 17 through Figure 19). From initial TRMM observations, we have determined that ice scattering signatures are present in all cases when lightning was detected. While there is much research to be done on lightning in tropical cyclones, early indications are that the occasional, sudden bursts of lightning that occur in these storms is associated with a change in tropical cyclone intensification.
A proposed total Lightning Mapping Sensor (LMS) in geosynchronous orbit offers significant benefits to the Nation, specifically in areas of severe convective weather warnings, and aviation weather support (See Figure 20). The LMS conceived by NASA MSFC is a follow-on to the LIS, featuring improved coverage and the ability to observe storms throughout their life-cycle. In geosynchronous orbit, the LMS would provide continuous, real-time surveillance of lightning activity over large portions of the North and South American continents and surrounding oceans (See Figure 21). It would potentially enhance operational weather forecasting capabilities as well as provide data for scientific studies of convective processes on a continental scale. In contrast to the current National Lightning Detection Network (NLDN), LMS would observe total lightning activity, including the dominant intracloud (IC) component, which is estimated to occur with order of magnitude greater frequency than cloud-to-ground (CG) lightning and may occur ten minutes or more in advance of the first ground flash in a storm. The possible operational benefits of LMS in areas of primary utility to the U.S. public:
- Improvement to the lead time and/or reliability of warnings for tornadoes, damaging thunderstorm winds and hail;
- Augmented warning capability for thunderstorm flash floods in mountainous areas where the NEXRAD weather radar network’s coverage is incomplete due to beam blockage;
- Reduced toll from cloud-to-ground lightning strikes owing to more reliable identification of electrically active storms;
- Improved efficiency and/or safety in the aviation system operation through provision of relevant information on thunderstorm phenomena, particularly over oceanic regions where current sensor coverage is limited;
- Improved forest and wild fire operations through the targeting of most probable ignition sites;
- Improved observations of rapidly evolving tropical cyclone and hurricane intensification prior to landfall.
These benefits are estimated based on assessments of LMS' ability to enhance warning or decision making capability beyond that achievable with current operational sensors.