Detecting Lightning
NSSL scientists know that field observations of electricity in thunderstorms are critical to our understanding of lightning. Scientists are looking for ways to maximize the use of existing observing systems in forecast operations, and have also developed their instruments to target specific types of observations.
Multi-Scale Observing Systems
The following three systems allow scientists to make observations on both the storm scale and the mesoscale.
Mobile Ballooning Laboratories
In the late 1970s, NSSL and its collaborators began to develop mobile facilities that could measure meteorological parameters both at the surface and aloft while stopped or moving. They are called "mobile labs" and have transitioned from small vans to an ambulance-style vehicle configuration for increased safety and increased observational and data collection capabilities. The mobile lab concept includes mobile ballooning. In this mode, crews launch instrumented balloons in the desired environment, including into active storms. Data are received and recorded via radio link into the mobile lab. The balloons are always instrumented with a radiosonde (a small instrument package with sensors that measure profiles of pressure, temperature, and relative humidity, and radio the information back to a receiver on the ground). In some projects, other instruments are added. A common addition is the electric field meter (see below). Others include x-ray detectors, lightning sensors, and other specialized probes. To make mobile ballooning in storms possible, NSSL scientists and their collaborators developed a high-wind balloon-launching apparatus for inflating, handling, and launching the fragile, helium-filled balloons. (read more)
Electric Field Meter
In collaboration, NSSL researchers developed the present generation of balloon-borne electric field meter (EFM) that are flown in conjunction with the mobile laboratories to determine the electric field vector beneath and in thunderstorms. This makes it possible to determine the electrical structure in context of the hydrometeor and wind field structure from radar. These are also being linked to structure of lightning that can now be mapped in very high time and spatial resolution.
OK-LMA
The Oklahoma Lightning Mapping Array (OK-LMA) detects and measures all lightning, including flashes that stay within clouds and flashes that reach the ground. More specifically, it provides three-dimensional mapping of lightning channel segments over west-central OK and two-dimensional mapping of all lightning over most of OK. Thousands of points can be mapped for an individual lightning flash to reveal its location and the development of its structure. The OK-LMA was an integral part of the 2003 and 2004 Thunderstorm Electrification and Lightning Experiment (TELEX). (read more)
Lightning Mapping
Operational Lightning ground strike mapping systems help detect thunderstorms by monitoring cloud-to-ground lightning activity across the continental U.S. The network consists of more than 100 remote ground-based lightning sensors that detect the electromagnetic signals given off when lightning occurs. The system can process this information in real time to determine if the lightning strikes or remains in the sky. For the cloud-to-ground flashes, the system determines the location, polarity, and strength of lightning.
NSSL scientists are looking into the amount of lead-time to be gained by adding the detection of intracloud and cloud-to-ground flashes. The U.S. National Lightning Detection Network (NLDN) being used by the NWS is now capable of detecting roughly 10-20% of all cloud flashes, in addition to a much larger fraction of cloud-to-ground flashes, over the contiguous United States.
What we have learned
The first flashes produced by a storm are usually cloud flashes, and if detected, can signal the initiation of a thunderstorm.
The ratio of cloud flashes to cloud-to-ground flashes is typically greater than 2:1. This means that systems that map either cloud flashes or all types of flashes will most often detect storms more quickly and reliably than those that just detect cloud-to-ground flashes.
The length of time between the first cloud flash and the first cloud-to-ground flash varies greatly amoung different parts of the country. Results showed in north TX and OK, 50% of thunderstorms produced a cloud-to-ground flash within 5-10 minutes of their first cloud flash. However, in roughly 10% of storms, no cloud-to-ground flash occurred within an hour of the first cloud flash. Behavior was much different in storms over the High Plains of NW KS and NE CO. There it required 30 minutes after the first cloud flash for 50% of storms to produce a cloud-to-ground flash, and 20% of storms produced no cloud-to-ground flash within their first hour of lightning activity. This trend may indicate differences in cloud microphysical processes operating in different regions or perhaps differences in the environments (e.g., shear and instability). Ongoing research is aimed at quantifing the differences.
Trends in the flash rate and location of cloud-to-ground lightning relative to the storm can help identify the development of thunderstorms, the growth of updrafts, and the formation of precipitation and downdrafts.
Lightning mapping techniques show that some supercell thunderstorms have "lightning holes" where updrafts are located and precipitation is scarce, much like the "inflow notch" noticed by early radar meteorologists. If these holes form, as suspected, just before a storm becomes severe, this information could alert forecasters to developing severe conditions.
Satellite
Geostationary satellites are another useful tool to diagnose and predict severe storm formation and intensification. Studies clearly indicate the different capabilities that satellites have in storm observation when compared with ground-based radar.
What we have learned
Satellites provide a view of the cumulonimbus cloud top structure, but they also can detect shallow cloud lines atop thunderstorm outflow boundaries, which may serve as zones for new storm formation, but are not yet visible to radar due to the absence of precipitation-sized hydrometeors with in them. This is particularly true in the visible light spectrum where the first stages of storm development are seen as thin lines of puffy cumulus clouds.
GOES-R Lightning Mapper
An optical lightning mapper is being planned for GOES-R and is expected to detect 80-90% of cloud flashes. Some research mapping systems can detect essentially all but the smallest flashes throughout their coverage region. NSSL scientists are looking at ways to maximize this information.
Doppler Radar
NSSL scientists are looking for ways to detect and determine systematic changes of flash activity and to relate these to satellite and radar observations of thunderstorms. This will contribute both to our understanding of lightning formation and indicators of impending storm severity.
Polarimetric Radar
Polarimetric radars can provide valuable information about the types of precipitation occurring in a storm (rain, hail, graupel). When combined with mapped lightning flashes, we can explore what happens to the electrical structure of the storm when these charged precipitation particles bump into each other.
What we have learned
In a study of a multi-cellular storm occurring during TELEX, scientist found a new polarimetric signature at the cellular/updraft scale preceding the lightning flash in the storm. The polarimetric signature in question is associated with lofted raindrops with serve as nuclei for riming graupel upon glaciation. The signature was absent in nearby cells, which did not produce lightning.
