What kind of clouds have lightning?

Observing storms from space

Riming electric charge separation Takahashi and Miyawaki, 2002, JAS Keys : Graupel Temperature Super-cooled liquid water Why there is a relationship between radar reflectivity and lightning flash?

Define precipitation features using TRMM Precipitation Radar Radar Precipitation Features (RPFs) Contiguous area with rainfall Properties of precipitation features Flash rate Temperature at 20, 30, 40 dBZ echo tops Maximum reflectivity at temperatures Area of 20, 30, 40 dBZ at temperatures Volume of 20, dBZ at -5 o C o C IWC of 20, dBZ at -5 o C o C PR echo top height (km)

Population of RPFs and those with lightning LandCoastOpen oceanAll 36S-36N RPF3.7 million6.4m million11.8 million22.2 million With flash 11.5%2.6%0.5%3%

2-D histogram of RPFs and probability with lightning Temperature of 20 dBZ echo top vs. Temperature of 30 dBZ echo top

Regional variation of lightning probability in precipitation systems

Correlation coefficient to flash rate in precipitation features maximum reflectivity at altitudes vs. Flash rate

Echo top temperature vs. Flash rate Correlation coefficient to flash rate in precipitation features OceanCoastLandAll 36S-36N Maximum 20 dBZ echo top Temperature Maximum 30 dBZ echo top Temperature Maximum 40 dBZ echo top Temperature Maximum reflectivity does not have good correlation with Flash rate neither ( < 0.5 )

Correlation between radar reflectivity vs. flash rate in precipitation features

Correlation coefficient to flash rate in precipitation features area of reflectivity at temperatures vs. Flash rate Solid : land Dotted: ocean

area of reflectivity at temperatures vs. Flash rate auto-correlation among different part of storms removed Solid : land Dotted: ocean

Regional variation of correlations (I) Correlation coefficients

Regional variation of correlations (II) Slope of linear relationship

Summary Flash rate is best correlated with the volume of the high reflectivity (> 30 dBZ) in the mixed phase region, confirming the importance of the presence of large particles in the charging process. More flashes are generated over land than over ocean given the same volume of dBZ in the mixed-phase region. There are large regional differences in the correlations between radar reflectivity properties vs. flash rate. Even with the similar radar reflectivity profiles to the oceanic systems, it is still easier to have lightning flashes over Amazon. Other factors must play important roles besides the graupel at mixed phase region in the lightning generation.

Relative Contributions of Electrified Shower Clouds and Thunderstorms to the Global Circuit: Can 10 Years of TRMM Data Help Solve an Old Puzzle?

Fair Weather Charge In fair weather there is a natural separation of charge in the atmosphere Upper troposphere is positively charged. Ground is negatively charged.

The atmosphere normally has a voltage gradient of 100 volts/meter… … which may sound like a lot, but what happens when you stand one meter from a 110 volt outlet ?

Old Puzzle / Hypothesis 90 years ago “A thundercloud or shower-cloud is the seat of the electromotive force which must cause a current to flow through the cloud between the Earth’s surface and the upper atmosphere... In shower-clouds in which the potentials fall short of what is required to produce lightning discharges, there is no reason to suppose that the vertical currents are of an altogether different order of magnitude.” Wilson (1920)

Carnegie Curve vs. thunder days 80 years ago Diurnal variation of electric field seems reproduced by the thunder days (after add an arbitrary uniform oceanic storms to bring the amplitude down)

Thunderstorms observed by TRMM Thunderstorms over different regions have different lightning flash rates AsianAfricaAmericas

Carnegie curve vs. flash count Good correlation in phase, but much higher amplitude Same as pointed out by Bailey et al. 2007

Carnegie curve vs. rainfall in thunderstorms Good correlation in both phase and amplitude

Diurnal variation of flashes Asian Africa Americas

Diurnal variation of rainfall in thunderstorms Asian Africa Americas

Vostok electric field vs. rainfall in thunderstorms Good correlation between Thunderstorm rain vs. electric field in different seasons DJF Vostok electric field might be contaminated by weathers

End of story? It seems that the diurnal variation of rainfall in thunderstorms has a very good correlation with the Carnegie curve both in the phase and the amplitude. However, this is not the end of the story. What about the shower clouds without lightning as mentioned by Wilson 90 years ago?

ER-2 overflight of Emily

In-situ storm current observations Storm current from 850 ER-2 overflights Courtesy of Mach et al. 2010

A VERY rough way to identify the electrified shower clouds 30 dBZ echo top colder than -10 o C over land, -17 o C over ocean

Rainfall from thunderstorms, electrified show clouds and others

Carnegie vs. electrified shower clouds

Thunderstorm + electrified shower?

A different approach define convective cells Three “cell” definitions: Convective pixels (red color fill) 40 dBZ pixels in any levels (black line) 30 dBZ pixels at 6 km (white line)

Global distribution of convective cells

(Solid line) Rain in cells defined by 30 dBZ at 6 km has the best correlation

Electric field vs. Rainfall from convective cells defined by area of 30 dBZ at 6km

Summary of the Carnegie curve With more observations available today, we have better tools to play the same game as Whipple (1929) played 80 years ago: relating the diurnal variation of thunderstorms to that of the electric field. Diurnal variation of rainfall from thunderstorms has a good correlation with the Carnegie curve both in the phase and the amplitude. The role of electrified shower clouds is still hard to describe due to the difficulty of identifying them and quantifying the electric field that they contribute. A different approach of adding convective cells gives more hints to this puzzle.

Let’s talk about the final project Written reports have to be submitted to me in before end of Tuesday Feb 26. Late report is not accepted. If you insist to give me a paper report, you have to hand the report to me on Monday Dec 25. We have 8 students Each one have about 10 minutes of presentation and a few minutes for questions. So we need 2 class time. We do a lottery on who present first. If you have presentation on the day, come to classroom 5 minutes early to upload your slides onto my laptop.