Lightning Cluster Filter Algorithm

A Review of Operational Lightning Detection: Comparison of Ground vs. Satellite-based Observations Ken Pryor PHYS 622

Background • Lightning generated by convective storms, and identified as a formidable hazard to life and property, results from strong storms lofting liquid-phase hydrometeors to high altitudes where freezing occurs and collisions between liquid drops, graupel, and ice crystals lead to electrification. • “Charge transfer-separation process”: interaction (esp. collision) between ice crystals and graupel particles results in the establishment of positive and negative charge centers within the upper and lower layers of the convective cloud, respectively.

Background • Lighter, positively-charged ice crystals are transported by the updraft to the top layer (anvil) of the thunderstorm, while heavier, negatively-charged graupel descend in the downdraft to the lower and middle layers of the thunderstorm. • This process increases the storm charging rate and the resulting electric field between the cloud, ground, and anvil.

Cloud Electrification and Lightning

Physical Concepts of Lightning Detection

Ground-based Lightning Detection: Time of Arrival (TOA) Techniquie

For a given time-of-arrival difference, the stroke that emitted the signal could be located anywhere along one of the branches of a hyperbola.

The intersection of two non-redundant hyperbola branches defines the location of the stroke at the open circle.

Courtesy of Holle and Lopez (1993)

DC Lightning Mapping Array

GOES-16 Geostationary Lightning Mapper

The GLM measures radiances at cloud top from all types of lightning (in-cloud and cloud-to-ground) during day and night, which is key to its utility because the in-cloud lightning dominates in severe storms.

The GLM LFCA

The Lightning Cluster Filter Algorithm (LCFA) performs temporal–spatial clustering of lightning event data into groups and flashes which can then be used to locate the initiation, propagation and horizontal extent of an individual flash within the GLM field of view.

Thunderstorm Microburst Model ***The presence of mixed phase precipitation (supercooled water, graupel, ice crystals) that favors storm electrification also favors intense downdraft development due to melting and evaporation. In effect, the building of an electric field within the storm also generates downdraft potential energy that is eventually realized as kinetic energy resulting from evaporation of water and melting of graupel and hail.***

Courtesy National Weather Service Training Center AWIPS Informational Series: “Operational Uses of Lightning Data” training.weather.gov/nwstc/METEOR/Lightning/Ltng_home.htm

Positive strokes from main precipitation core result from the charging of hail

GOES-16 ABI-GLM Comparison a)

b)

DC Lightning Mapping Array a)

b)

Discussion •

Ground based lightning detection systems, especially the DCLMA, that are also covered by satellite-based detection systems, such as the GOES-16 GLM, can provide a more comprehensive physical model for severe thunderstorm generation and associated hazards including hail and damaging wind.

•

For a severe downburst event that occurred in Frederick, Maryland, on 1 August 2017, DCLMA and GLM LFCA datasets were compared:

•

GLM imagery displayed lightning events in close proximity to downburst events at the time of downburst occurrence. GLM LFCA output over a one minute period (1805-1806 UTC) near the time of downburst occurrence shows a peak in flash density.

Summary and Conclusions •

Both ground and satellite-based lightning detection methods indicate an eastward displacement of maximum flash density away from the location of downburst impact at Frederick which suggests that storm structure (i.e. storm tilt and anvil development relative to the main precipitation core) influences spatial patterns of lightning occurrence.

•

The presence of mixed phase precipitation (supercooled water, graupel, ice crystals) that favors storm electrification also favors intense downdraft development due to melting and evaporation. In effect, the building of an electric field within the storm also generates downdraft potential energy that is eventually realized as kinetic energy resulting from evaporation of water and melting of graupel and hail.

•

These conditions are captured effectively by comparing plots of concurrent GLM and LMA total lightning event datasets.