Japan's record 2020 rains renew debate over engineering rainfall

In early July 2020, a stationary front stretching from China to the waters east of Japan settled over the country and stayed there for nearly a month, according to a study using NASA satellite data. The front first struck southern and western Japan on July 3, and continuous rain set a historical 24-hour rainfall record for Kyushu of 496 millimeters, more than triple the intense rain rates that satellites had tracked just days earlier.

NASA's IMERG system, which combines data from a constellation of international satellites, measured rain accumulations across southern Kyushu climbing rapidly from July 3 to July 5, reaching over 40 centimeters across the southern half of the island and as high as 60 centimeters in some areas. Devastating floods and landslides swept through the region on July 4, and media reports at the time cited over 40 deaths and evacuation orders for hundreds of thousands of people. A later accounting found the full month of rain, which persisted with only minor shifts in position until July 28, ultimately caused 84 deaths and destroyed thousands of houses.

The rains fell during the Meiyu-Baiu season, an annual rainy period that arrives in East Asia from June to mid-July and is named for the Chinese and Japanese words for "plum rains," because they typically arrive as plum trees ripen. Climate scientists have linked the east-west rain bands typical of this season to warm air traveling eastward from the Tibetan Plateau combined with moist tropical air moving in from the southwest.

The heavy rain did not stop at the shoreline. Researchers modeling ocean circulation around Japan found that the huge volume of freshwater draining from rivers into the sea altered coastal salinity and currents, effects that most ocean forecasting models are not built to capture because they rely on average monthly river flow rather than real conditions during a storm.

To better track this, the team fed hourly river discharge data covering 368 rivers in Japan into an existing ocean forecasting system, replacing the system's usual reliance on climatological averages. The heavy rainfall also produced measurable land subsidence of 1 to 2 centimeters in the flooded region, which later recovered once the floodwater drained into the ocean, according to separate research cited in the study. The work adds to evidence that extreme precipitation events have become more frequent in Japan in recent years, linked to warming air and ocean temperatures.

Separately from what happened in Japan, researchers in several countries are studying whether clouds can be deliberately manipulated to increase or decrease rainfall, a practice known as cloud seeding. The technique dates to the 1950s, when scientists found that dry ice and silver iodide, a compound that can encourage water droplets to form ice crystals, could trigger precipitation inside a cloud. It saw wide use in the late 20th century before losing momentum, but interest has returned as countries confront prolonged droughts, heatwaves, flooding, and fires.

One method under study by the U.S. Department of Agriculture sprays electrically charged water particles into clouds from agricultural aircraft, using nozzles that charge the droplets as they are released. Because the droplets carry a charge opposite to that of the water already in the cloud, they attract surrounding water molecules, and researchers say the droplets collide and merge with them until they grow heavy enough to fall as rain, a process reported to take 15 to 20 minutes from seeding to precipitation. A researcher on that project said conventional flare-based seeding generates 10 to 15% additional rainfall, while their charged-particle method showed a 25 to 30% increase in early testing, though he cautioned this comes from a limited data set.

Cloud seeding is also framed as a tool against water scarcity more broadly, according to a research group focused on extreme heat and air pollution, which noted that the same technology can be adjusted to suppress rainfall or hail rather than boost it. That group pointed to countries such as Ethiopia and Thailand adopting cloud seeding to address water shortages, and it also noted the technique's potential relevance during floods, citing the previous year's flooding in Pakistan as an example of the kind of disaster the field aims to eventually address.

Independent of any specific country's plans, a Columbia University researcher who studies weather modification cautioned that cloud seeding's real-world effects are more modest than sometimes assumed. Because natural weather already varies so much, isolating cloud seeding's precise contribution to a given rain or snow event is difficult, but she said most studies suggest an increase in precipitation of only 5 to 15% compared with what would have fallen anyway. In the western United States, the most common approach seeds clouds with silver iodide from airplanes or ground-based cannons specifically to boost snowpack, according to an unpublished dataset compiled by the researcher and a colleague.

The same researcher distinguished cloud seeding from larger-scale climate interventions known as geoengineering, such as injecting reflective particles into the upper atmosphere or brightening ocean clouds to cool the planet. Those approaches remain confined mostly to modeling and laboratory studies and have not been deployed in the atmosphere at meaningful scale. She said geoengineering research is worth pursuing to improve scientific understanding of the atmosphere, but that it is not yet ready for large-scale use and should not distract from cutting greenhouse gas emissions. China, by contrast, runs a national weather modification program employing an estimated 35,000 people, compared with only a couple hundred in the United States, where each state runs its own separate program.