Progress Report
Artificial generation of upstream maritime heavy rains to govern intense-rain-induced disasters over land (AMAGOI)[5] Weather Prediction
Progress until FY2024
1. Outline of the project
Due to the recent progress of global warming, occurrence of heavy rainfall is becoming more frequent in Japan as well as various parts of the world. Against this background, the project " artificial generation of upstream maritime heavy rains to govern intense-rain-induced disasters over land (AMAGOI)" aims to reduce heavy rainfall on land downstream by artificially enhancing heavy rainfall over the ocean. Specifically, the idea is to generate rainfall upstream over the ocean to reduce the water vapor that serves as the seed for rain.
During the Baiu-season, the East China Sea upstream has an environment conducive to the development of cumulonimbus clouds due to evaporation from the warm sea surface and the transport of large amounts of water vapor from the southwest. In fact, one can see that precipitation tends to develop downstream, triggered by small islands west of Kyushu. Based on these facts, Item 5 "Investigation of Effective Intervention Operations for Generating Offshore Heavy Rain," considers that it may be possible to generate and enhance precipitation over the ocean with even a "small" human-induced stimulus. The goal is to clarify in which situations and with which methods effective interventions are possible. This involves reproducing heavy precipitation events with numerical weather models to test and confirm the effectiveness of "realistic" meteorological intervention methods.
2. Outcome so far
In this project, we have reproduced two heavy rainfall events—one that occurred in Kyushu in August 2021 and another in Hiroshima in August 2014—using a numerical weather prediction model. We then introduced artificial intervention methods into the model to examine their effects on each rainfall event. For the Kyushu case, we evaluated the effectiveness of an intervention method that involves deploying 21 kite-shaped objects, each 300 meters high and 200 meters wide, over the sea. In the Hiroshima case, we investigated the impact of a method known as "over-seeding," in which dry ice is dispersed from the air to enhance convective development and shift a portion of the precipitation to downstream areas.
For the Kyushu case, we conducted 25 ensemble simulations with initial times staggered by one hour and applied the kite-shaped obstacle intervention in each experiment. A comparison with experiments without intervention revealed that, on average, the total precipitation was suppressed by approximately 10% across the 25 ensemble members (Fig.1).
In the Hiroshima heavy rainfall case, the event was first reproduced using a numerical model, and virtual seeding was applied over regions where deep convection was developing. A comparison with the experiment without intervention demonstrated that over-seeding can mitigate localized heavy rainfall by dispersing precipitation downstream of the convective core. Furthermore, it was found that this mitigation effect becomes more pronounced as the upstream seeding area increases (Fig.2).
3. Future plans
The results suggest that the deployment of obstacles over the ocean and seeding of dry ice have effectiveness in mitigating localized heavy rainfall. Moving forward, further improvement in their effectiveness is expected through optimization of the timing and location of each intervention method. Additionally, the findings indicate that combining these two approaches may offer a more reliable means of suppressing heavy rainfall.