The Global Electric Circuit Research Topic

A Barometer for Global Convection

The Global Electric Circuit permits the integrated effect of global electrified weather to be measured by a single ground station under fair-weather conditions. This makes it an attractive solutiong for long-term climate monitoring

Thunderstorms and Electrified Shower Clouds (ESCs) have global consequences for the Earth system through a complex series of electrical connections in the atmosphere known as the Global Electric Circuit (GEC). First postulated by C. T. R Wilson in the 1920s, the basic concept is that electrified weather across the globe produce quasi-steady state conduction currents (known as Wilson currets) that maintain the electrical potential of the Ionosphere at 240 kV/m relative to the Earth's surface. A downward return current is established in fair weather regions that completes the circuit. The fair-weather electric field at ground level is known to vary predictably throughout the day based on how much electrified weather is occurring at each instant. This so-called Carnegie curve (named after the research vessel that measured it) has been studied for nearly a century.

With the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellites we now have unprecedented radar and passive observations of thunderstorms and shower clouds across the globe. If we can quantify where and when electrified weather occurs, and how much DC current each storm supplies to the GEC, then we can reconstruct the global source current for the GEC. Assuming that the system is in steady-state (the source term equals the load on the circuit), then we shoud further be able to accurately reproduce the Carnegie curve from the diurnal cycle of sources.

While previous studies use proxies such as total lightning or rainfall to match the Carnegie curve, I developed a passive microwave (and optional radar) retrival algorithm to estimate the Wilson current supplied by individual storms according to their measured precipitation structure. This retrieval was developed using coincident high-altitude passive microwave and electric field measurments taken by NASA's ER-2 aircraft during multiple field campaigns. The retrival is then applied to TRMM and GPM measurements to calculate Wilson currents from electrified weather across the globe.

The passive microwave retrieval matches the Carnegie curve better than any previous proxy. It also confirms the mean Wilson currents measured by the ER-2 over oceanic (1.6 A) and land-based (1.0 A) thunderstorms, and suggests that these values are representative of global electrified weather. The retrieval predicts a global total GEC source current of 1.6 kA. 1.15 kA comes from thunderstorms while the remaining 0.44 kA comes from ESCs. These estimates are well within the range predicted in the literature (1.0 - 2.0 kA).

Additional refinement of the retrieval is needed to account for inverted polarity storms that can ‘short’ the GEC and to better represent electrified stratiform clouds, but these results are nonetheless promising. The retrieval additionally reports 20-km electric field vectors in each passive microwave pixel based on its assessment of the local 3D charge distribution. These electric field vectors have been noted to match the trajectories taken by lightning flashes with long horizontal channels - even in curved cases. Future work will include using long passive microwave satellite records to assess the long-term variation in the GEC and the GPM constelaltion to construct source maps for individual days.