Chapter 11: lightning

this girl is charged !! Source: Halliday, Resnick, and Walker, Fundamentals of Physics

Lightning is due to in-cloud electric charge separation. This separation can only be understood in view of cloud processes.

Lightning Conductor Source: Viemeister, The Lightning Book large electric potential small electric potential

textbook, Fig lifecycle of a lightning strike pilot leader stepped leader return stroke

Lightning Strike Frequency Ground strikes /km 2 /yr Source: Ackerman and Knox, Meteorology

lightning occurs mostly over land reasons: 1. CAPE and convective updrafts are weaker for marine thunderstorms 2. the marine environment has fewer CCN and fewer ice nuclei.

review Final will cover –chapter 7 global winds –chapter 9 airmasses and fronts –chapter 10 extratropical cyclones –chapter 11 thunderstorms and tornadoes Final is similar in format and value to the two mid-semester tests

Summary of chapter 7: how winds blow around the globe General circulation model –Hadley cell, Ferrel cell, Polar cell –trade winds, mid-lat westerlies, polar easterlies –Polar front; mid-latitude heat transfer by transient airmass advection –jet stream above the polar front (thermal wind) –effect of continents and seasons ocean currents, and upwelling

Summary of chapter 9: airmasses and fronts airmass classification –cT –cP –mT –mP frontal classification –cold fronts –warm fronts –stationary fronts –occluded fronts

Summary of chapter 10: how a mid-latitude frontal disturbance works Fronts exist, may strengthen, and may evolve –Polar front (initial) –Developing phase (open wave) –Mature phase (occlusion first forms) –Dissipating stage (low far into the cold air) This evolution is tightly connected to the jet stream

Evolution of a frontal disturbance

Chapter 10 review cont’d The jet stream is consistent with a large horizontal temperature gradient (the atmosphere is baroclinic). The jet stream has waves, called Rossby waves These waves may first form in the lee of mountains (lee cyclogenesis) These waves propagate, and are unsteady The shorter waves are important for weather at the surface, because –UL divergence occurs ahead of the Rossby trof UL divergence causes uplift, and cyclogenesis near the surface. These waves, in turn, are affected by the low-level cyclogenesis. –Warm advection ahead of the surface low builds the UL ridge –Cold advection behind the surface low deepens the UL trof. The evolution of midlatitude frontal disturbances is understood by the synergy between UL wave evolution, and LL cyclone evolution (baroclinic instability). Finally, the raison d’étre of these frontal disturbances is to transfer heat poleward …

slow fast Note the advection of cold and warm airmasses

Chapter 11: review All thunderstorm types require static instability –Low-level warm, humid air –Upper-level cool air The more instability, the stronger the updraft, and the more severe the storm can be Three types exist: –Ordinary (air mass, single-cell) –Multicell –Supercell The distinction between these types is based on wind shear –Ordinary storms: little shear  short-lived –Supercell storms: strong shear -> long-lived Thunderstorms are sometimes organized on the mesoscale as large storm clusters (maybe squall lines)

Chapter 11: review cont’d Supercell storms are marked by –Rotating updraft Due to the tilting of horizontal spin (from the wind shear) Strengthened by vortex stretching –Separate downdraft –Sometimes also a hook echo a bounded weak-echo region A v-notch Most tornadoes, and all severe ones (F3-F5), are spawned by supercell storms The supercell ‘mesocyclone’ may spawn a tornado by the same mechanisms: –Vortex tilting –Vortex stretching Tornadoes are classified from F0 to F5, according to intensity Lightning results from charge separation in a thunderstorm –supercooled droplets need to collided with graupel/hail