Lightning and climate: A review

1. Lightning and climate: A review E.R. Williams 23 February, 2017 Trent Davis

2. Lightning and Climate • Every second, around 45 flashes over Earth, or ~1.4 billion flashes per year (Christian et al.,2003) • Deep, convective thunderstorms pump ice crystals to upper troposphere • Sublimate to become water vapor • Water vapor = greenhouse gas at high altitudes • Produce most of lightning on Earth • Satellite era largely influencing a detailed study of global lightning • Close link found between global lightning and upper-tropospheric water vapor (UTWV) (Price 2000) • Stronger convection = more water vapor transport & increased lightning

3. Lightning and the General Circulation • Walker circulation link to lightning • Upwelling portions of Hadley Cell and rainfall Williams and Stanfill(2002) Christian et al. (2003)

4. Lightning and the General Circulation • Continental thunderstorms • Increased sensible heat flux over land • Stronger, broader updrafts = deeper water vapor transport into mixed phase regions • Higher cloud bases • Increased lightning production • Maritime thunderstorms • Lower sensible heat flux over oceans • Weaker, narrower updrafts • Warm rain processes dominate Williams and Stanfill (2002)

5. Christian et al. (2003)

6. Tropical Chimneys • Africa • Most continental • Warmer, drier surface, greatest SH flux • Aerosols • South America • Dominates ionospheric potential (IP) and global DC circuit • Changes in aerosol concentrations important • Maritime Continent • Most maritime • Most rain, least lightning • Least SH flux • Convective invigoration Thunder Days (Land) Carnegie SH Flux (W m-2) Williams and Stanfill (2002)

7. Lightning and Temperature • Price and Rind (1992) found increases in lightning with warming due to CO2 increase • Increase in positive CG flashes found with increased θe • Convective adjustment remains largest uncertainty in future predictions – does lightning increase level off? • CAPE is crucial to water budget – circular problem

8. Lightning and Temperature (ENSO) • Understanding global lightning response to temperature on this timescale helps understanding connections to water vapor feedback and convective adjustment. • Consistency in enhanced lightning activity in warm ENSO phase • 1 case found lightning enhancements in tropics • 2 cases found lightning enhancements in extratropics • Little change in Schumann resonance during 1997-1998 El Nino Goodman et al. (2000)

9. Upper Tropospheric Water Vapor Israel • Water vapor extremely important here due to radiative absorptivity • Thunderstorms act as ice factories • Price (2000) shows relationship between Schumann resonance and UTWV • Schumann resonance isn’t too hard to record, also a globally “uniform” parameter • Relationship implies a 25% increase in ELF signal for every 0.25 mm increase in UTWV California Price (2000)

10. Upper Tropospheric Water Vapor • Maritime Continent • Least lightning activity (most maritime chimney) • Most extensive high-level cirrus • More frequent high-top convection • Africa • Most lightning activity • Least high-level cirrus, what gives? • Higher Bowen Ratio and stronger diurnal signal less frequent, more intense thunderstorms with deep updrafts Annualized Total Lightning Activity Christian et al. (2003)

11. Climate Extremes • Climate models suggest more frequent extreme weather events in a warming climate • Lightning activity is generally enhanced with these events • Natural Questions • Are mean t’storm flash rates larger in a warmer world? –No • Are there more t’storms in a warmer world? –Yes • Are there more MCCs in a warmer world? –Yes • Are there more Schumann resonance Q-bursts with extreme positive CG flashes in a warmer world? –No • Lightning activity appears to increase due to more storms, not increased flash rate (1 & 2) • Convective adjustment could weaken these trends on longer timescales

12. Convective Invigoration (Recap) Rosenfeld et al. (2008)

13. 1989–2000 Mean Flashes (km-2 yr-1) Lightning and Aerosols • Houston, TX paradox • 3-fold increase in CG flash density in 12-year period • Precipitation enhancement east of town • Strong afternoon peak • Strong cut-off along TX-LA coast also suggest heat-island effect • Debate over aerosol role between Williams et al. (2002) and Andreae et al. (2004) • Stoltz and Rutledge (2016) seem to agree with aerosol importance. • Smoke incursion from fires in Mexico positive CG flashes • Studies of cases in Brazil oppose this though • Instability at play? 1989–2000 Positive Flashes (%) Steiger et al. (2002)

14. Long Range Variability • Correlations between IP and galactic cosmic rays (GCR) • Possible relationship between GCR and global cloud cover • Columnar resistance increasing ionospheric potential • Harrison (2002) claims a factor of 2 drop in global electrical circuit over 20th century • Global temperature was on the rise over this period though • Improvement in UK pollution a likely explanation

15. Conclusions • Global lightning and the electrical circuit = window into climate analysis, especially water vapor • Global rainfall and lightning maxima don’t overlap • Lightning prevalence = deep convection and ice crystal transport to upper troposphere • Could induce warming, then more lightning… • Not a direct correlation between high-level cirrus and flashes • Big question: What precisely is the response of lightning and global circuit to warming? • Convective adjustment, surface characteristics, aerosols, and general climatic response all important

16. Sources Christian, H.J., Blakeslee, R.J., Boccippio, D.J., Boeck, W.L., Buechler, D.E., Driscoll, K.T., Goodman, S.J., Hall, J.M., Koshak, W.J., Mach, D.M., Stewart, M.F., 2003. Global frequency and distribution of lightning as observed from space by the optical transient detector. J. Geophys. Res. 108, 4005. Goodman, S.J., Buechler, D.E., Knupp, K., Driscoll, D., McCaul Jr.,, E.E., 2000. The 1997–98 El Nino event and related wintertime lightning variations in the southeastern United States. Geophys. Res. Lett. 27, 541– 544. Price, C., 2000. Evidence for a link between global lightning activity and upper tropospheric water vapor. Nature 406, 290-293. Rosenfeld, D., U. Lohmann, G.B. Raga, Colin D. O’Dowd, Markku Kulmala, Sandro Fuzzi, Anni Reissell, Meinrat O. Andreae, 2008. Flood or drought: How do aerosols affect precipitation? Science 321, 1309-1313. Steiger, S.M., Orville, R.E., Huffines, G., 2002. Cloud-to-ground lightning characteristics over Houston, Texas: 1989–2000. J. Geophys. Res. 107. Williams, E., Stanfill, S., 2002. The physical origin of the land-ocean contrast in lightning activity. C.R. - Acad. Sci., Phys. 3, 1277–1292.