TJ4.2 Lightning Detection and Mapping: Recent Research and Future Opportunities

Monday, 23 January 2017: 4:15 PM
Conference Center: Tahoma 1 (Washington State Convention Center )
Donald R. MacGorman, NOAA/OAR/NSSL, Norman, OK

Because of the constraints of a short overview, this paper will be limited to lightning mapping research dealing with lightning physics and with the relationships of lightning with other storm characteristics.  The considerable research investigating how lightning is involved in producing various electromagnetic effects above storms, such as gamma ray flashes and the extensive family of transient luminous events, will not be included.

Since the advent of modern lightning mapping systems (VHF mapping systems, acoustic mapping systems, and ground strike locating systems), each significant increase in mapping capability has enabled paradigm shifts in our understanding of basic lightning characteristics and of relationships between lightning and various thunderstorm characteristics.  Among other things, we have learned that lightning flashes propagate from a high-field region into and throughout storm charge regions; that naturally occurring positive cloud-to-ground lightning is common in some storm situations; that the polarity of the vertical distribution of charge in some storm regions can be inverted from the usual polarity; that some storm anvils initiate lightning; that tall structures trigger many of the lightning strikes to ground in winter storms; that the overshooting tops of thunderstorms typically produce continual, very small discharges; that the very large flash rates of supercell storms are due mainly to a high rate of small flashes near the strong updraft core, probably due to the turbulent structure near the updraft; and that many, if not all, flashes are initiated by a system of positive streamers.

Besides the traditional methods for analyzing observations relevant to lightning research, promising new analysis methods have been developed in recent years, including  a method for computing the size spectra of flashes, a balloon-borne particle imager that can be used in combination with polarimetric radars to study the storm microphysics associated with electrification and lightning, and a method for combining high-time-resolution, wideband interferometer data with Lightning Mapping Array data to study the detailed processes of lightning initiation and propagation.  Furthermore, new observing platforms have become available recently or should become available soon, including phased-array radars routinely providing rapid volume scans (30 s – 90 s) and the Geosynchronous Lightning Mapper planned for launch on new GOES satellites.

One critical measurement is still missing from storm electrification studies:  the liquid water content in updrafts, particularly in the mixed-phase region, in conjunction with lightning mapping or in situ electric field measurements.  While liquid water content sensors exist, the difficulty has been placing them inside storm updrafts at high enough altitudes.  The new A-10 penetrating aircraft is capable of making these measurements, if it is actually flown into the updraft of a thunderstorm.  Obtaining liquid water content measurements should be a high priority for any field program studying lightning and storm electrification in storms that can be penetrated by the A-10.  Another option is to develop balloon-borne sensors that can be flown into updrafts; the vertical soundings would complement the data from horizontal penetrations by the A-10 and could probably be flown in the updrafts of storms too severe to be penetrated by the A-10.

Most of the results listed above need additional research to extend or improve understanding of the phenomena.  Some questions may be addressed with data that are acquired routinely.  For example:

Can we use radar data to diagnose when storms will initiate lightning flashes in anvils, and how does this affect lightning safety guidelines?

What fraction of cloud-to-ground flashes in winter storm systems strike tall structures?  Is the tendency in winter for many lightning flashes to strike tall structures due to storm charge distributions initiating flashes with downward channels to ground, which have a higher probability of striking towers than of striking surrounding terrain, or due to horizontally extensive intracloud flashes simply responding to the presence of a tall structure?

Furthering research on several other topics will require a concerted effort, probably a coordinated field program, to obtain new observations with present technology.  For example:

Are lightning flashes always initiated by a system of positive streamers, as suggested by Rison et al. (2016)?  If so, do initiations require the presence of some type or types of hydrometeor?  Also, what fraction of detected systems of positive streamers actually produce flashes, and do those that initiate flashes have any characteristics distinguishing them from streamer systems that do not grow upscale?

What can the propagation direction and distribution of positive streamers detected by wideband interferometers tell us about the finer electrical structures of storms, particularly near strong updrafts and in overshooting tops, where small discharges tend to dominate?

How well do turbulent structures in the periphery of strong updrafts translate to effects on lightning size spectra, and what do the size spectra tell us about the organization of storm charge and the kinematics of updrafts?

How much liquid water content is in the mixed-phase region of various types of thunderstorms, and how is it distributed in updrafts?  Which laboratory results are consistent with the charge gained by graupel in updraft regions having various liquid water contents?  Is having a large liquid water content required, as has been hypothesized, to invert the polarity of charge gained by graupel in storms with anomalous vertical polarity?  If so, what other storm characteristics can influence the degree to which updrafts have large liquid water contents?

And some new topics will be made possible by the launch of the GOES satellites carrying the Geosynchronous Lightning Mapper (GLM).  For example:

To what extent can assimilation of GLM data improve forecasts of downstream thunderstorms, particularly in regions near the west coast of the USA?

To what extent can ground-based lightning mapping data be used to improve the spatial resolution of flashes located by the GLM?

What storm scenarious produce the optical superbolts that military satellites have found in oceanic storms?

What structures of tropical cyclones produce lightning?  Are changes in the total lightning rates of eyewalls related either to tropical cyclone reintensification or to changes in their tracks, as has been suggested?

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