Natural Lights from Lightning

Cities and urban infrastructure are not the only light sources that can be seen looking down at the Earth from space. Short-lived bursts of light from lightning flashes can also be noted in satellites and International Space Station imagery. The places where these natural lights from lightning are considerably different than those produced by humans and vary throughout the day and over the course of the year.

These natural lights from lightning flashes are visualized using a Monte Carlo approach to distribute real satellite lightning flashes observed by satellite across the globe based on the multi-sensor satellite climatology. The result is a collection of lights that resembles a terrestrial analog to the stars in the night sky.

Global Currents from Electrified Weather

TRMM Current Carnegie Curves

The Global Electric Circuit (GEC) is a complex series of electrical connections in the atmosphere. First postulated by C. T. R Wilson in the 1920s, the basic concept is that electrified weather across the globe (thunderclouds and electrified shower clouds) produce quasi-steady state conduction currents that maintain the electrical potential of the Ionosphere at 240 kV/m above the Earth’s surface. Meanwhile, a downward return current is established in fair weather regions, thus completing the circuit. Fair weather electric fields at the surface are known to vary predictably throughout the day based on where electrified weather is occurring at that instance. This so-called Carnegie curve after the research vessel that measured it in the early 20th century has been studied for nearly a century.

With the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellites we now have unprecedented observations of thunderstorms and shower clouds across the globe that even include radar in addition to passive microwave measurements. Therefore, if we can quantify where and when electrified weather occurs, we should be able to precisely reconstruct the diurnal Carnegie variation of the GEC. Rather than use rainfall or other proxies to do so, my approach to this problem is to develop a retrieval algorithm that uses observations of the structure and intensity of each individual electrified cloud to estimate the electric fields that would be observed overhead. A passive microwave algorithm has been developed with NASA ER-2 overflight data that includes direct measurements from the passive microwave and electric fields at 20 km.

After validating this algorithm with the aircraft data, it is then applied to TRMM and GPM observations to calculate electric fields above electrified clouds in any region of the globe. These electric field values can be used to compute total conduction (Wilson) currents, directly, and they are found to match the Carnegie curve better than any proxy in literature. Moreover, this approach also predicts the observed ocean (1.6 A) and land (1.0 A) Wilson thunderstorm currents described in literature and a total source current for the GEC of 1.6 kA - 1.15 kA from thunderstorms and 0.44 kA from shower clouds - well within the range presented in literature (1.0 - 2.0 kA).

Additional refinement of this method is needed to account for inverted polarity storms that can actually ‘short’ the GEC and to better represent electrified stratiform clouds, but these results are nonetheless a promising start. Electric field vectors have also been noted to predict the course of propagating optical lightning flashes, even around curves. Additioanlly, if the algorithm is capable of predicting Wilson currents for individual storms and reconstruct the observed diurnal behavior, it may also be useful for probing variations in the GEC at other timescales (i.e. MJO, ENSO, long-term clinate). These are just a few of the potential applicaitons of this algorithm.

TRMM/GPM Mean Total Current Map

Optical Characteristics of Lightning

Twenty years ago the Optical Transient Detector (OTD) was launched that provided a bird’s eye view of lightning around the world. Unlike most ground-based sensors, optical sensors like the OTD can capture both intracloud and cloud-to-ground flashes consistently across the world’s continents and oceans. To date, every optical lightning sensor has flown in low-Earth orbit, providing only a snapshot of lightning activity in a given storm. However, with the deployment of the Geostationary Lightning Mapper on GOES-R, it will be possible to monitor the life history of lightning for individual storms using optical measurements.

As the launch of GOES-R and the LIS on the International Space Station (ISS-LIS) project near, significant attention has been places on the value that these measurements can provide to the lightning science and meteorological communities. A particularly interesting direction is to consider sub-flash optical lightning features and optical flash characteristics to see what additional information can be teased out from OTD/LIS/GLM observations. Though I have published on this topic and lightning locations relative to storm morphology in the past, I am particularly excited about a new development: by considering the evolution of LIS groups (the optical analog to something akin to strokes), it is possible to identify flashes that propagate horizontally. Two examples are shown below.

These flashes are clearly not localized discharges confined to the convective core. They likely result from complex charge structures (i.e. in stratiform regions) and terminate a long distance from where they initiated. Some may be spider lightning flashes, others may even be bolts from the blue. Coincident Lightning Mapping Array (LMA) observations will be particularly useful to gain a better understanding of the breakdown structure of these flashes and tie them to our understanding of lightning physics. This work will benefit from the global domains of past and future optical lightning sensors and may have broader applications, for example, in thunderstorms classification by charge structure and implications for the Global Electric Circuit.

The other side of my work in this area is to explore why optical flashes look they way they do from radiative transfer considerations. Unlike ground-based radio observations, optical lightning measurements can be heavily influenced by the properties of the surrounding cloud region. A flash may be large because it has a complex branching structure as in the previous example or simply because it is very bright and its radiance is scattered across a larger region of cloud at levels that can be detected by the sensor. The rededicate aspects of optical flashes can be seen in the fact that LIS detection efficiency is better at night than during the day when the background radiance is higher.

It also complicates the observed trend of oceanic flashes being larger and brighter than their land-based counterparts. Are they exceptional because they truly are stronger discharges, or are they simply viewed better by the sensor? To explore this issue, I have created a database of clouds that are illuminated by LIS flashes that includes a combination of their radar, passive microwave, and infrared properties. This database can be used, for example, to identify clouds that are otherwise similar between land and ocean and then compare the optical properties of the flashes in each region on an equal footing. If we do this for a large sample of similar clouds, we find that these land/ocean differences are robust and not just due to radiative transfer concerns. There also appear to be structural differences in land and ocean flashes, for example in group count and prevalence for propagation, that imply that there are physical differences in oceanic discharges compared to their land-based counterparts outside of energetics.

It is important to be able to account for these factors for the next generation of lightning imagers, as ISS-LIS and GLM will not have the same set of coincident observations available natively that LIS enjoyed on TRMM. .