Progress Report
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Artificial generation of upstream maritime heavy rains to govern intense-rain-induced disasters over land (AMAGOI)[5] Weather Prediction
Progress until FY2025
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 The goal is to investigate the feasibility of weather intervention. 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 conducted reproduction and intervention experiments for various heavy rainfall events. For the Kyushu heavy rainfall event of 12–13 August 2021, we performed large ensemble simulations in which random perturbations were added to the water vapor mixing ratio below 300 m at 12 hours after model initialization.
To enable effective offshore rainfall-formation interventions, it is necessary to identify in advance where interventions are likely to induce precipitation changes. We therefore developed an intervention-effectiveness index based on the inner product of precipitation differences and lag-regression coefficients (Fig.1).
Using this index and sensitivity analysis, offshore structures were placed at suitable locations and times to promote offshore rainfall formation, resulting in a 5–8% reduction in heavy rainfall intensity (Fig.2). Using the Japan Meteorological Agency dataset of linear precipitation band events, we estimated that such effective intervention cases occur about four times per year in Kyushu and Yamaguchi.
We also examined overseeding using the WRF model for the 2014 Hiroshima, 2017 Northern Kyushu, and 2021 Northern Kyushu heavy rainfall events. Seeding was represented by modifying ice-nuclei concentrations. In all cases, precipitation was dispersed downstream, reducing localized rainfall peaks. To enable more realistic representation, we also implemented a cloud microphysics scheme with prognostic aerosols and modified it to support future seeding experiments.
In addition, during the 2025–2026 winter season, we conducted an aircraft-based cloud-seeding preliminary experiment over Toyama Bay. Through this experiment, we identified operational issues related to aircraft coordination, seeding methods, and target-cloud selection. We also established an observation site in Nyuzen, Toyama Prefecture, and conducted observations using a polarization lidar, Doppler lidar, microwave radiometer, and other instruments. These efforts have advanced both numerical and field-based knowledge toward future weather-control experiments around 2030.
3. Future plans
We will extract more than ten heavy rainfall cases from reanalysis and satellite precipitation data and apply the intervention-effect index to statistically evaluate meteorological conditions favorable for effective intervention and the occurrence frequency of potential intervention cases. We will also conduct the 2026–2027 winter field experiment over Toyama Bay.