Investigation of Effective Intervention Operation for Generating Offshore Heavy Rain
Due to the recent progress of global warming, occurrence of heavy rainfall is becoming more frequent around the world. In recent years, Japan has experienced an increasing number of “linearly elongated precipitation zone,” which is long in shape, stays in the same area for a long time, and cause heavy rainfall. 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 sea. Specifically, the idea is to generate rainfall upstream over the sea 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 off shore 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.
- Item 5-1 Evaluation of intervention effectiveness using weather model
- Item 5-2 Selection of events with high potential for maritime heavy rainfall generation
- Item 5-3 Ensemble weather prediction experiments
- Item 5-4 Toward better understanding of physical processes in cloud seeding and evaluating the effectiveness
- Item 5-5 Ground-based verification of weather intervention effects
- Item 5-6 Mobile and Unmanned Weather Observation for Intervention Experiments
- Item 5-7 Research on Weather Observation and Weather Intervention Using Crewed Aircraft
Item 5-1Evaluation of intervention effectiveness using weather model
Principal investigator: Kazuaki Yasunaga
Outline
Using the numerical weather prediction model (SCALE), this study confirms the effectiveness of weather intervention methods, such as dome formation over the sea, cold pool formation, sea surface cooling, microwave heating, cloud seeding. Particularly from a meteorological perspective, we clarify effective intervention operations for generating offshore heavy rain. In the later stages of the research, we plan field experiments involving airplane, ship, and weather radar. This will include arrangements for coordination, methodological considerations, and case selection.
Methods
We incorporate the proposed weather intervention methods -dome formation over the sea or cold pool formation, sea surface cooling, microwave heating and cloud seeding- into the numerical weather prediction model (assuming SCALE) in manner that closely resembles actual interventions. We quantitatively evaluate their effectiveness based on specific heavy rain events. For the intervention methods considered as promising, we construct a virtual observation system on a computer and evaluate its behavior numerically. This enables the development of plans for conducting field experiments.
At the start of the research, no specific numerical targets will be established, because the scale of heavy rainfall over the sea and the reduction in rainfall over land for disaster mitigation are expected to vary depending on the specific disaster events. By the FY 2024, we select cases with high potential to generate heavy rainfall over the sea based on the results of Item 5-2. Flood inundation and economic damage estimates for the selected cases will be calculated by Item 8-1 and 8-2 to investigate the extent to which the heavy rainfall generation over the sea and rainfall reduction over land are required. Then, we set target values for the rainfall reduction to be achieved for each selected cases at an early stage, to serve as our goals.
Importance
In terms of weather intervention, if we can apply an energy input that surpasses the flow associated with heavy rainfall, we should be able to effectively mitigate rainfall in specific areas. However, feasibility is a key word in this project, and accurately representing intervention methods close to real ones in a numerical model is one of the keys for promoting this study.
Even if promising intervention methods are found, conducting actual field experiments requires a lot of human and material resources. Creating a virtual observation system on a computer and using it for implementation plan is significant for efficient resource use.
Anticipated Challenges and Approaches
To demonstrate the effectiveness of artificial intervention, a sufficient number of experiments is necessary to allow for probabilistic discussions, even when focusing on specific heavy rainfall events. On the other hand, due to the strong nonlinear processes involved in cloud and precipitation, “a sufficient number of experiments for probabilistic discussions” is not clear. Moreover, the degree of freedom that the internal system can vary depending on the numerical experiment settings, which means that the results may also depend on factors such as the size of computational domain, resolution, and parameterization of physical processes.
For the time being, we focus on the speed of promoting project and conduct experiments as comprehensively as possible. At the same time, we aim to establish methods for setting up and structuring numerical experiments using low-dimensional models, in collaboration with the mathematical research group. We initially plan to focus on specific heavy rain events. However, effective intervention methods may vary depending on the event. Therefore, we organize intervention methods in collaboration with Item 5-2, whose study is to index heavy rainfall events using appropriate parameters.
Members
Item 5-2Selection of events with high potential for maritime heavy rainfall generation
Principal investigator: Atsushi Hamada
Outline
This study utilizes analysis data from organizations such as the Japan Meteorological Agency to select events where intervention operations are likely to be effective in generating heavy rainfall over the sea. Specifically, before selecting events, we investigate the spatial distribution and temporal changes of factors such as convective inhibition energy and the height of free convection to identify suitable events. We find the meteorological mechanisms that can be a trigger of heavy rainfall while examining potential weather intervention operations that are expected to be effective.
Methods
We utilize the mesoscale analysis provided by the Japan Meteorological Agency and ERA5 reanalysis data from European Center for Medium-Range Weather Forecasts (ECMWF) to select events of heavy rainfall that have previously caused significant disasters, and where weather intervention operations are expected to be effective. Currently, the weather intervention operations we are considering include dome formation over the sea, cold pool formation, sea surface cooling, microwave heating, and cloud seeding. Many of these methods are designed to force warm and moist air in the atmospheric boundary layer to uplift. Therefore, the effectiveness of the intervention operations is expected to be related to the atmospheric stability around the boundary layer and the water vapor flux. In selecting events where intervention operations are expected to be effective, we quantitatively evaluate the impact of convective inhibition energy and water vapor flux at the land location of heavy rainfall and upstream over the sea on precipitation. Based on this evaluation, we develop an index that combines these physical quantities to show the effectiveness of the intervention operations.
Importance
In order to evaluate the effects of the intervention operations using a meteorological model, it is important to accurately reproduce the past heavy rainfall events. Heavy rainfall events that cause disasters are diverse, and the intervention operations proposed in this project may not necessarily be effective for all events. By introducing the result of the detailed analysis of a few events into an objective and quantitative index based on physical quantities, we expect to efficiently identify events from a wide range of past heavy rainfall events where intervention operations are likely to be effective.
Anticipated Challenges and Approaches
For the selection of heavy rainfall events where intervention operations are effective and the organization of an index showing their impact, it is essential to conduct detailed analyses of numerous events to stock knowledge. However, this process may take more time than expected. Therefore, we begin to select one or two events with propriety to some extent and, in close collaboration with Item 5-1, aim to create an index that is highly related to intervention effects.
Members
Item 5-3Ensemble weather prediction experiments
Principal investigator: Yusuke Hiraga
Outline
This study utilizes the Weather Research and Forecasting (WRF) model to identify the effectiveness of weather intervention methods (dome formation over the sea, cooling pool formation, sea surface cooling, microwave heating, cloud seeding). By conducting ensemble sensitivity experiments with changing boundary conditions and initial values, we adopt an inductive approach to find the effective interventions. This will allow us to identify intervention operations that can reduce precipitation on land by generating heavy rainfall over the sea.
Methods
We conduct numerical experiments on weather intervention using the WRF model, which is widely used internationally as a non-hydrostatic meteorological model. We introduce the proposed weather intervention patterns (dome formation over the sea, cooling pool formation, sea surface cooling, microwave heating, cloud seeding) into the WRF model in a manner that closely resembles actual interventions. By conducting simulations on specific heavy rainfall events, we quantitatively evaluate their effectiveness in terms of reducing heavy rainfall on land.
At the start of the research, no specific numerical targets will be established, because the scale of heavy rainfall over the sea and the reduction in rainfall over land for disaster mitigation are expected to vary depending on the specific disaster events. By the FY 2024, we select cases with high potential to generate heavy rainfall over the sea based on the results of Item 5-2. Flood inundation and economic damage estimates for the selected cases will be calculated by Item 8-1 and 8-2 to investigate the extent to which the heavy rainfall generation over the sea and rainfall reduction over land are required. Then, we set target values for the rainfall reduction to be achieved for each selected cases at an early stage, to serve as our goals.
Importance
To achieve the goal of this project, it is essential to thoroughly assess the potential for generating maritime heavy rainfall, as well as its effectiveness. While conducting weather intervention experiments in real world requires much costs, it is possible to conduct various patterns of intervention experiments in a numerical model virtual environment. This allows us to accumulate knowledge on the most effective intervention methods, locations, and timing for generating maritime heavy rainfall, providing essential insights for efficiently conducting field experiments in the future.
Anticipated Challenges and Approaches
There have been few examples of ensemble experiments aimed at weather intervention using WRF mode, this study is pioneering. For example, when implementing artificial weather intervention, it may affect the stability of numerical computations and the computation may not proceed as planned. In such cases, the principal investigator of this study has been in contact with Prof. Shu-Hua Chen at the University of California, who is a member of the WRF model developers, and we can ask for his opinion and try to solve the problem.
Members
Item 5-4Toward better understanding of physical processes in cloud seeding and evaluating the effectiveness
Principal investigator: Tempei Hashino
Outline
We investigate the effectiveness of cloud seeding techniques for reducing rainfall amounts over land and ameliorating the localized rainfall, using a numerical weather model. The first target is summer-time isolated cumulus clouds. We aim to understand changes in the precipitation processes due to the seeding numerically. Also, we attempt to estimate sensitivity of a squall line system to the seeding quantitatively.
Methods
In this study, a numerical scheme for dry-ice seeding will be installed to Advanced Microphysics Prediction System (AMPS), and seeding experiments will be conducted with a numerical weather prediction model called The University of Wisconsin nonhydrostatic modeling system (UWNMS). We identify the suitable time and location for the seeding through sensitivity experiments.
As for model development, we seek to improve the model through collaboration with other PIs in the aircraft seeding field experiment planned in 2025 and with Fudeyasu project on cloud chamber experiments that are introduced in 2026.
Importance
The project activity of item 5 has shown that cloud seeding can be utilized to intervene rainfall events. However, quantitative understanding of temporal and spatial distribution of super-cooled water is necessary for effective seeding. Also, it is essential to understand the behavior of isolated cumulus clouds and quasi-stationary linear precipitation bands after seeding. Numerical models that simulate the dynamics and clouds are best suited for such purposes.
Generally, distribution of super-cooled water depends on not only strength of updrafts but also characteristics and amount of aerosol particles. Therefore, numerical simulation of aerosol particles is an important piece of the experiments. The numerical models commonly used in weather forecasting do not consider aerosols and assume particle size distributions and particle types for cloud-precipitation particles to reduce computational cost. The physical processes occurring in clouds and precipitation after seeding can be understood by removing such assumptions and simulating aerosol budgets with a detailed physical model.
Anticipated Challenges and Approaches
Although super-cooled water distribution depends on heating and background aerosol particles, aerosol characteristics and concentration are typically not observed except for the ground level. We need to understand how the super-cooled water distribution is affected by modulating the aerosol from typical values. The AMPS model has been used and evaluated for clouds and precipitation in polar and high-latitude regions. We plan to evaluate/improve the performance for cumulus clouds with radar and other observation datasets.
Members
Item 5-5Ground-based verification of weather intervention effects
Principal investigator: Kazuhiro Yoshimi
Outline
This project aims to establish an advanced observation framework for quantitatively verifying the effects of weather intervention through ground-based observations. Until the fifth year of the program, the focus will be on offshore cloud-seeding experiments, aiming to enhance the integrated system combining meteorological radar and surface-based instruments. In addition to the fixed observation network, the study explores mobile observation approaches, including the potential deployment of portable weather radar systems and the use of all-weather UAS equipped with precipitation particle imaging sondes, to develop flexible verification methodologies.
Methods
This research develops observation techniques that enable quantitative visualization and verification of weather intervention impacts.
In FY2025, in preparation for a winter cloud-seeding experiment, we will utilize X-band dual-polarization weather radars operated by the Ministry of Land, Infrastructure, Transport and Tourism. By optimizing key observation parameters, we will develop data visualization algorithms that support the evaluation of intervention effects. Concurrently, we will design and prepare operational procedures for in-situ observations using surface-based instruments such as rain gauges and a disdrometer, establishing a robust observational setup for the field experiment.
In FY2026, we will explore the feasibility of expanding the observation network through the possible installation of portable radar systems. Furthermore, mobile observations will be conducted by integrating all-weather UAS with precipitation particle imaging sondes. This will enable high-resolution detection of the vertical structure and localized distribution of the intervention effects. By combining both static and mobile observation techniques, the project seeks to establish a high-density and field-adaptive verification system that supports reliable effect assessment.
Importance
In the field of weather intervention, establishing a reliable observation system to scientifically evaluate intervention effects is essential. Currently, there are limited technologies capable of directly capturing the influence of cloud seeding on precipitation, making it difficult to provide convincing scientific and social justification for its effectiveness.
This study integrates static ground-based sensors with dynamic airborne platforms to create a dense and spatially adaptive verification framework. Leveraging insights from precipitation particle observations using UAS and three-dimensional precipitation structure analysis with radar, the research aims to pioneer a next-generation verification approach. The offshore field experiment is a rare initiative in Japan and will contribute to the development of core technologies for advanced weather intervention, such as targeting high-altitude clouds or mitigating localized torrential rain. This project plays a vital role in establishing the scientific credibility and public acceptability of weather intervention technologies.
Anticipated Challenges and Approaches
Technical challenges may arise in synchronizing multiple observation systems—including weather radar, surface sensors, and UAS—and ensuring consistency in spatial and temporal resolution. These will be addressed through coordinated observation scheduling based on system specifications and enhanced post-processing techniques.
Furthermore, offshore field experiments involve operational risks and meteorological uncertainties. To mitigate these issues, we will draw upon previous field experience, allocate sufficient preparation time, and implement phased trial operations to build a flexible and reliable operational structure.
Members
Item 5-6Mobile and Unmanned Weather Observation for Intervention Experiments
Principal investigator: Jun Inoue
Outline
This study aims to develop new meteorological observation methods using drones and compact sensing devices for future weather intervention and cloud seeding experiments. Direct observations of cloud microphysical properties and atmospheric conditions will be conducted using Cloud Particle Sensor (CPS) sondes and Cloud Scope systems to identify cloud environments suitable for seeding operations. In parallel, compact and lightweight meteorological sensing devices will be developed for deployment on small drones, with future applications to larger drone platforms also envisioned. The study will further establish real-time observation techniques for monitoring in-cloud meteorological conditions and aircraft icing risks, thereby providing an observational foundation to support the safety and scientific validity of intervention experiments.
Methods
Knowledge gained from previous cloud seeding experiments will be reviewed to design observation strategies based on cloud characteristics and atmospheric conditions suitable for seeding operations. Cloud and meteorological observations will be conducted using CPS sondes and medium-sized drones to monitor target cloud conditions at high temporal frequency. In addition, Cloud Scope systems mounted on large drones will be tested to evaluate the feasibility of in-cloud observations and flight safety. Compact and lightweight meteorological sensing devices will also be developed and tested for drone deployment. Through these activities, this study aims to establish observation technologies that enable both real-time atmospheric monitoring and safe drone operations.
Importance
To safely conduct future drone-based cloud seeding operations, it is essential to understand aircraft performance under low-temperature, icing, and high-altitude conditions. The meteorological observations obtained in this study will provide critical information not only for evaluating flight safety, but also for identifying cloud environments suitable for seeding experiments. By establishing an observational framework that supports both operational safety and scientific understanding of cloud conditions, this study will contribute to the future development of weather intervention technologies.
Anticipated Challenges and Approaches
In low-temperature and high-humidity environments, aircraft icing may reduce drone flight performance and affect operational safety. To address this, compact meteorological sensing devices will be used to monitor atmospheric conditions around the aircraft in real time and support safe flight decisions. Step-by-step flight tests will help define safe operational conditions, and most experiments will be conducted over the ocean to minimize risks to surrounding areas. Because flights under rapidly changing weather conditions also require advanced piloting skills and compliance with aviation regulations, this study will collaborate with experienced pilots, drone operation companies, and relevant local authorities to ensure safe and smooth experimental operations.
Members
Item 5-7Research on Weather Observation and Weather Intervention Using Crewed Aircraft
Principal investigator: Ryota Kikuchi
Outline
This research examines the feasibility of conducting weather observations and weather intervention operations using crewed aircraft. It will identify the conditions that need to be considered from technical, operational, safety, and regulatory perspectives. These include the target weather conditions and physical variables to be measured, as well as aircraft selection, installation of observation instruments and dispersal equipment, aircraft modifications and inspections, safe flight operations, applicable laws and regulations, and required permits and approvals. By considering meteorological requirements alongside the operational constraints of aircraft, the research will further examine suitable observation and operational methods.
Methods
Assessing the Feasibility of Weather Intervention Operations Using Crewed Aircraft
The use of crewed aircraft requires the location, timing, and method of an intervention to be considered in light of aircraft performance and operational constraints. This research will review conditions including the target weather conditions, materials and dispersal equipment used for cloud seeding and similar operations, flight altitude, range, payload capacity, communication methods, and requirements for safe flight operations. Based on this review, it will assess the operational feasibility of using aircraft. The research will also review the requirements and considerations associated with implementation, including the technical requirements for installing dispersal equipment, aircraft modifications and inspections, safety measures, applicable laws and regulations, required permits and approvals, airspace use, and coordination with relevant organizations. These studies will allow findings from numerical models and engineering assessments to inform planning and the selection of conditions for field verification.
Examining Aircraft Observation Methods for Assessing Intervention Effects
An objective and quantitative assessment of weather intervention effects requires a clear understanding of weather conditions before and after the intervention, together with data from aircraft observations, ground-based observations, radar, and numerical models. This research will examine the respective roles of aircraft used for cloud seeding and observation, the feasibility of operating two aircraft, and the potential use of lidar observations on the observation aircraft. It will also review observation targets, altitudes, flight paths, instruments, and approaches to using observation data, taking aircraft characteristics and operational constraints into account. This work will support the development of aircraft observation plans for field verification and help identify the observation conditions and remaining challenges involved in assessing intervention effects.
Importance
A scientific assessment of the feasibility and effects of weather intervention operations requires an approach that links findings from numerical models and engineering studies with observations and evaluations under real atmospheric conditions. Crewed aircraft are a valuable tool because they can approach target weather systems and collect the necessary observations. They may also support integrated planning of the conditions required for both observation and intervention operations. At the same time, the safe and effective use of aircraft requires expertise not only in meteorology and weather forecasting, but also in observation instruments, aeronautical engineering, flight operations, safety management, and operational planning.
The flight performance, payload capacity, maneuverability, and operational safety constraints of aircraft vary greatly depending on the type and size of the aircraft, including whether it is a fixed-wing aircraft or a helicopter. Therefore, observation and intervention conditions that are desirable from a meteorological perspective cannot always be applied directly to aircraft operations. Assessing feasibility requires a practical plan that considers both meteorological requirements and aircraft operational constraints. Planning field observation and verification requires consideration not only of technical feasibility but also of applicable laws and regulations, required permits and approvals, safe flight operations, potential effects on the surrounding environment and third parties, and coordination with relevant organizations. This research will work with aircraft operators, relevant organizations, and project research groups specializing in legal, ethical, and social issues to identify the procedures and considerations required at each stage of the research.
The knowledge gained through this work may have wider applications in weather observation, disaster risk reduction, aviation safety, environmental monitoring, and emergency response, in addition to the weather intervention operations studied here. To deepen our understanding of extreme weather and examine interventions that could help reduce damage, it is important to develop technical capabilities and knowledge in Japan that link observation, prediction, assessment, and operations. This research brings together meteorology and aeronautical engineering, while also considering safety, legal and regulatory matters, and ethical and social issues, to examine the possibilities and challenges of weather observation and intervention operations using crewed aircraft from multiple perspectives. Through this work, we aim to develop the knowledge needed to assess scientific validity, feasibility, and public acceptance.
Anticipated Challenges and Approaches
Balancing Meteorological Requirements and Aircraft Operational Constraints
When weather intervention operations are considered, the altitude, location, timing, and flight path for observation and cloud seeding are constrained by the target weather conditions and the timing of the intervention. Crewed aircraft are also subject to constraints related to flight speed, maximum altitude, range, payload capacity, available airspace, operating weather conditions, and safe flight operations. Therefore, conditions that are desirable from a meteorological perspective cannot always be applied directly to aircraft operations. To address this challenge, we will work with aircraft operators, researchers within and outside the project, and relevant organizations to refine the target weather conditions, physical variables to be measured, and conditions for cloud seeding and other interventions in light of aircraft performance and operational constraints. Multiple operational plans will be developed for different conditions, with phased goals and alternatives where appropriate. We will also consider risk management measures for adverse weather, aircraft performance or operational constraints, and situations in which the required observation data cannot be obtained. The project team will develop a shared understanding of the purpose and decision criteria for each plan and examine conditions that meet meteorological requirements while remaining feasible for aircraft operations.
Examining the Installation and Operation of Observation Instruments and Dispersal Equipment
Installing observation instruments or dispersal equipment for cloud seeding and similar operations on crewed aircraft requires consideration of equipment weight, installation location, power supply, communication methods, aircraft structure, safety, aircraft modifications, and inspections. In particular, weather intervention operations may require equipment configurations and operating conditions that differ from those used for standard weather observations. A technically feasible installation may not necessarily meet all operational, safety, and regulatory requirements. To address this challenge, we will review installation methods, technical requirements for modifications and inspections, and operational safety considerations for different aircraft types and characteristics. We will compare the features and challenges of several operational configurations: observation only, cloud seeding or another intervention only, a combination of observation and intervention, and operations involving multiple aircraft. Based on this comparison, we will develop plan options suited to different conditions. We will also review the operation of onboard equipment, observation and intervention procedures, communication and data-recording methods, and operational safety issues. The findings will be reflected in further planning for aircraft use and operating procedures.
Members
