Lightning: Charge Separation Mechanisms, Detection and Applications Kaitlyn Suski May 29, 2009 SIO 209

Outline Background Charge Separation Mechanisms Detection Applications

Lightning Background Occurs in cumulonimbus clouds. Lightning is an electrical discharge. Contained in a volume bounded by -5 to -40  C and has a radius of ~2 km. Charge separation creates centers of charge in the cloud. When insulating ability of air is exceeded or the charge reaches it’s breakdown potential it is discharged. Charge separation can be as large as a hundred million volts and can transfer 10 coulombs of charge to the ground. Accompanied by thunder. Rapid expansion and compression of heated air.

Charge Separation Mechanisms Two proposed mechanisms of charge separation: Convection charging theory Precipitation related charge separation Figure 11.2, Cotton.

Convection Charging Theory Pocket of positive ions in lower troposphere due to fair-weather electric field. –Convective updrafts carry the charge into the cloud. –Cosmic radiation produces ions in the ionosphere. Clouds attract negative ions to cloud tops resulting in cloud polarity. –Downdrafts carry negative ions down resulting in discharge. Figure 11.3, Cotton.

Precipitation Charging Theories Induction Charging Theory –Collision of droplets and ice with precipitation particles results in a negatively charged larger particle and a positively charged smaller particle. –The positive particles are carried up in updrafts, while negative particles are carried down by gravity. –Need a high frequency of collisions and rebounds. –Speculated that charge separation is better for ice- ice collisions than water-water, however, relaxation time is long and significant charge may not be transferred.

Precipitation Charging Theories (2) Charging of Graupel and Hail –Rapid charge transfer between graupel/hail and vapor grown ice crystals. –Requires a large concentration of ice crystals and large, rimed graupel or hail. Figure 11.7, Cotton.

Electrical Double Layer and QLL

Long Range Lightning Detection Network (LLDN) Continuous, real-time data coverage over the Pacific Ocean. –PacNet –Lightning strikes emit electromagnetic waves, which are detected by magnetic direction finding or time of arrival methods with 2 or more sensors. –Sensors spaced every 100 km. Pessi Presentation

Applications Can be used to track convection and tropical cyclones when high clouds block satellite coverage. –Lightning rate is correlated with convective strength. This is verified by radar reflectivity. –Can lead to more accurate intensity forecasts. Proxy for latent heat release in deep convective clouds. –Ratio of lightning to rainfall rate is stable over Pacific Ocean. Used to estimate a moisture profile by using a lightning- rainfall-moisture profile relationship. –Need to know the temperature because moisture has a nonlinear relationship with temperature.

References Businger, Steven. “Assimilation of Long-Range Lightning Data over the Pacific.” Cotton, William. “Chapter 11: Cloud Electrification,” Atmospheric Thermodynamics and Microphysics of Clouds, AT 620 Lecture notes, Fall 1996, revisions Summer Pessi, Antti. Long-Range Lightning Detection over the Pacific: Applications of the Data Stream for TCS-08 Presentation, ftp://ftp.soest.hawaii.edu/businger/outgoing/PacNet

Questions?