This week from AGU: Cell tower rain gauges, lightning channels, North Sea storm surge

A shaky cell phone connection during a rainstorm can be an annoying nuisance. But now scientists are showing that these weakened signals can be used to monitor rainfall in West Africa, a technique that could help cities in the region better prepare for floods and combat weather-related diseases, according to a new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

The usual cloud-to-ground lightning occurs when a large negative charge contained in a "stepped leader" travels down toward the Earth's surface. It then meets a positive charge that comes up tens of meters from the ground, resulting in a powerful neutralizing explosion that begins the first return stroke of the lightning flash. The entire flash lasts only a few hundred milliseconds, but during that time, multiple subsequent stroke/return stroke sequences usually occur.

Some researchers have proposed that a residual current must flow through the ionized channel of air left by the first stroke to allow the propagation of the next stroke, called a dart leader, but the prevailing—though unproven—view today is that the current decays to zero amperes before subsequent dart leaders and their following return strokes occur.

To test the zero-current hypothesis, Ngin et al. triggered lightning strikes using rockets and wires. The authors find that rather than decaying to zero, residual currents between 2-8 milliamps remained prior to dart leaders. However, aspects of the residual current remain unknown, including its source. The authors note that the sensitivity of their measurements is unprecedented and may help to improve models describing the evolution of the ionized lightning channels and the process known as current cutoff.

Storm surge impacts on low-lying coasts threaten vulnerable human communities on a global scale. On 5-6 December 2013, the margins of the southern North Sea experienced the most significant storm surge in 60 years. Detailed ground surveys immediately following the surge documented considerable variability in inundation levels, with implications for surge forecasting and societal responses to extreme coastal floods.

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