Diurnal Variations of Global Thunderstorms and Electrified Shower Clouds and Their Contribution to the

Global Electrical Circuit

Liu et al. 2010

Review By: Gus Alaka and Doug Stolz

20 February 2014ATS 780

Background• C. T. R. Wilson’s Hypotheses: Thunderstorms AND electrified shower clouds are the “batteries”

of the global DC electrical circuit. Adopted (with permission) and later tested by F. Whipple.

• Objective: To update and improve our understanding of the physical origins of the global circuit of atmospheric electricity using the most extensive collection of simultaneous satellite observations of precipitation and lightning in the Tropics (i.e., TRMM Precipitation Radar and Lightning Imaging Sensor database).

Phase lag between diurnal maximum (Carnegie-American chimney/TStorms-African chimney).

Amplitude of global thunderstorm area over land is 2x that of Carnegie Curve.

Evidence For Electrified Shower Clouds

• Mach et al. (2009) and Blakeslee and Mach (2009) documented E-fields between 500-1200 V m-1 in a tropical squall line during TOGA-COARE – the absence of lightning was noted.

• Takahashi (1973) modeled the hydrometeor charging in warm rain clouds. • Rain, drizzle, and cloud droplets can charge

positively or negatively depending on the stage of cloud development. • Charged hydrometeors account for in-cloud

charge density of about 10-2 C km-3.

Takahashi (1973)

Evidence For Electrified Shower Clouds

“…At about 2100 LT, a cumulus cloud developed east of the island and rain began to fall as the cloud moved onshore and approached the observation site… The electric potential gradient rapidly reversed in sign from positive to negative as rainfall commenced at the station. As the rain shower moved away from the station, the potential gradient slowly recovered to zero…Most of the raindrops measured were relatively small and positively charged… There was no evidence of lightning in any of the warm clouds studied in Ponape…” –Takahashi (1978)

Takahashi (1978)

Feature Partitioning• How do they segregate electrified shower clouds from shallow cumuli and

thunderstorms?

1. Invert the arbitrary 10% threshold from Fig. 1: 90% of the PF’s with the 30 dBZ echo tops at -17C (-10C) over ocean (land) DO NOT produce lightning.

2. Define: Electrified shower clouds are those with PF’s without lightning and 30 dBZ echo top temperatures warmer than the empirical temperature thresholds (consistent with Fig. 8 of Cecil et al. 2005).

10%-17C

-10C

The PF Breakdown

Thunderstorms and Electrified Shower Clouds account for an overwhelmingly small portion of the total precipitation feature population but they contribute roughly 40% of total rainfall.

Intuitive: thunderstorms are about 3x more common over land. PF’s over ocean rain more (…the role of aerosols)?Counter-intuitive: electrified shower clouds are also more common over land.

So lets investigate…

…Thunderstorms and Electrified Shower Clouds…

…as drivers of the global electrical circuit on annual and seasonal time scales.

Annual Average Diurnal CycleTotal Rainfall

Rainfall over land matches the max/min of the Carnegie electric field (CEF)Too much rain over the Maritime Continent to explain CEFRainfall heavily weighted toward ocean, which has weaker diurnal cycle

Annual Average Diurnal CycleRainfall in Thunderstorms

This is the best matchDiurnal variation is higher over land and lower over the oceanRainfall heavily weighted toward ocean

Annual Average Diurnal CycleRainfall in Electric Shower Clouds

Agreement with CEF is substantially worseDiurnal cycle of ESC’s over ocean is very weakMost ESC’s are over land-- (0.34% vs. 0.19%)

Annual Average Diurnal Cycle Rainfall Fractions, Rain/Flash

Max rainfall in the Americas from thunderstormsMin tstorm rainfall when the sun is over the PacificMax rain/flash over Maritime Continent-- Clean marine air supports bigger droplets

Min rain/flash over Africa-- Aerosol-rich continental air supports long-lasting, more mature clouds-- slightly higher for Americas

Blue arrowThis increase is only 4%!

In the Americas, rain/flash is similar to Africa, but Americas also have a higher rain fraction, so…

more flashes in the Americas??We know this isn’t true…

Seasonal Average Diurnal CycleFlash Counts

Very pronounced diurnal cycle for flash countsSame problem as in Whipple (1929) since most tstorms occur over land.Africa dominates flash counts year

Seasonal Average Diurnal CycleRainfall Over Land

A better match…Max variation (21Z) matches fairly wellAbundance of non-electrified rainfall in Maritime Continent mutes diurnal cycle-- especially DJFAmericas dominate rainfall most of the year

Non-thundestorm rainfall

Seasonal Average Diurnal CycleRainfall in Thunderstorms

Once again, best matchWeakest variation in DJF

Might be due to lower quality of electric field over Vostok due to weather

Tstorm rainfall variations are more consistent region to region

Seasonal Average Diurnal CycleRainfall in Electrified Shower Clouds

Weaker diurnal variation for ESCs in all seasonsMore ESCs in Americas than in AfricaESCs are not a good measure of the fair weather electric field

Liu et al.’s Elusive Conclusion About

Electrified Shower Clouds“Nevertheless, with all things considered, it seems unlikely that the

contribution of the electrified shower clouds to the global circuit will be either completely dominant or entirely negligible in comparison with the thunderstorm contribution. This result is in keeping with the predictions of Wilson (1920).”

Continued Uncertainty…

• The method employed by Liu et al. represents an attempt at characterizing the relative contributions of thunderstorms and electrified shower clouds to the global DC circuit, but their arbitrary partitioning scheme leaves room for improvement.• Are electrified shower clouds lumped into thunderstorm PF’s so that the populations

are inaccurately represented?

• Strength in numbers – separately electrified shower clouds may make small contributions, but together they represent an appreciable component of the DC global circuit. More observations/modeling studies of hydrometeor charging are called for.

“In a hierarchical global population of convective clouds, one expects…more electrified shower clouds than thunderclouds…”

Food For Discussion• Liu et al.’s major conclusion (or suggestion) is that the global DC circuit

responds mostly to precipitation, used as a proxy for charge separation herein. Knowing what you do about the currents associated with common lightning discharges and patterns in global rainfall,

DO YOU AGREE WITH THIS RESULT?

• How might the sampling strategy or sampling capability of TRMM LIS contribute to the first of the two main discrepancies between lightning trends and the Carnegie Curve (i.e., that the amplitude of the diurnal lightning signal is 2x that of the Carnegie Curve)?

More Take Home Points• Electric shower clouds reduce the amplitude variation of the global circuit

• Contribute negative charge to surface via precipitation• Difference from Whipple (1929):

• ESCs are more prevalent over land than ocean

• Africa dominates flash rates• Americas dominate electrified rainfall (more flashes?)• Majority of oceanic rainfall associated with non-electrified convection

• What did they miss by their methods for selecting ESCs?• How reliable is 6-hr NCEP reanalysis fields interpolated to the scale of a PF?• Lightning flashes could be from dissipating MSCs• Sub-pixel classification

• 1 thunderstorm cell is likely surrounded by several ESC cells

• Overall, the findings of Wilson (1920) and Whipple (1929) are supported.

Extra stuff

Sub-PF Scale Considerations• TRMM PR has a horizontal footprint of 4.5 km x 4.5 km which is larger than

many shower clouds that may be electrified (one caveat in its own right).• Grouping raining pixels into PF’s potentially includes electrified shower

clouds in thunderstorms.

L

L L

L L

L

= Raining

= Raining/T30dbz

= Raining/T30dbz/Lightning

THUNDERSTORM! THUNDERSTORM!

• Both of the above PF’s would are classified as thunderstorms when the example on the left is likely to contain electrified shower clouds.

Carnegie CurveAnnual Average Diurnal Cycle

Carnegie CurveSeasonal Average Diurnal Cycle

Carnegie CurveSeasonal Average Diurnal Cycle