1. Land processes in the climate system

In addition to the atmosphere and ocean, the land where we live is one of the key components of the climate system. Rainfall over land is temporarily retained on the surfaces of tree leaves, infiltrates into the soil, and then flows into rivers and lakes. Some of this water returns to the atmosphere through evaporation from water retained on tree leaves, soil, and water surfaces, as well as through transpiration from vegetation. In cold regions, it remains on the ground surface for a relatively long period of time as snow. Frozen soil or permafrost may form when soil temperature drops below zero. Furthermore, solar and atmospheric radiation reach the ground surface through the canopy. It is known that surface reflectance of solar radiation varies significantly depending on ground surface conditions. Reflected sunlight returns to the atmosphere, and the ground surface also emits radiation according to its temperature. In addition, sensible and latent heat from the surface are transferred to the atmosphere. As described above, water, energy, and biogeochemical cycles on land are highly complex and closely interconnected. Their behavior differs depending on surface conditions such as vegetation, soil, terrain, and land use. These processes interact with the climate; therefore, understanding land processes and their interactions is important for understanding the climate system and projecting climate change. Highly accurate land simulations are also important for addressing socially critical issues, including the prediction of floods and droughts.

2. Land modeling

The development of land models began in the 1960s. Manabe's bucket model (1969), which is one of the representative models in the early days, defined available soil moisture as a bucket with a certain depth and provided atmospheric models with latent heat and sensible heat fluxes, which are lower boundary conditions for atmospheric models. Since then, land models have evolved into ones that contain more complex processes, such as water-energy balances of multi-layered soil and snow, vegetation, land use and land cover, rivers, frozen soils, and human activities. Land models have become vital tools for understanding the role of land in the climate system.

MATSIRO, a land model of the climate model MIROC developed in Japan, has been developed over a long period of time. MATSIRO6, adopted in the latest climate model MIROC6, has achieved improved reproducibility of land processes by adopting parameterizations for simulating high-latitude wetlands and snow cover fractions considering the distribution of subgrid snow depths. However, it has been developed as part of the atmospheric model, and thus calculations have generally been performed using the same spatial resolution as that of atmospheric models. Actual land, however, is relatively more heterogeneous and complex than the atmosphere and ocean. Therefore, it has been difficult to fully simulate such complexity using only a land model as part of an atmospheric model.

3. Development of Integrated Land Simulator

Given this background, we have been developing a new land model called the Integrated Land Simulator (ILS) in cooperation with researchers both in Japan and overseas. ILS is a new land simulator that handles a multiple land component models related to water, energy, and biogeochemical cycles on land and provides a framework for reproducing complex and heterogeneous land processes. ILS aims to grasp the behavior of the entire land surface in a manner closer to reality by combining land component models that simulate various land processes.

One of the features of ILS is that multiple land component models are integrated by general-purpose couplers. This enables coupled simulation without requiring each model to use a common grid system. For example, a land model with a high-resolution basin-shaped grid system suited for land can be coupled with an atmospheric model or an ocean model as it is. Furthermore, biogeochemical models and human activity models can be combined more flexibly. In addition, it becomes easier to incorporate the latest versions of component models that are being developed on a continued basis. ILS can be used as a part of the climate model while the complexity of land is maintained.

4. Current initiatives

In the current ILS, MATSIRO, a land physics model, and CaMa-Flood, a river dynamics model, are coupled. This allows users to calculate land processes, such as those related to the ground surface, soil, snow, and lakes, and river flow in a unified manner. The developed model has been adopted in the next-generation climate model MIROC7 as a land model. Our goal is to contribute to better climate simulations through the continuous development of ILS.

Another key point is that it has become possible to develop land models jointly with researchers who have a broad range of backgrounds. Land processes that have not been fully considered, including cities, crop growth, irrigation, plant hydraulics, hillslope hydrology, water isotopes, groundwater, and water temperature, are being coupled. Furthermore, the latest boundary condition datasets, data assimilation, AI-based parameterizations, implementation in a hydrological simulation system, as well as coupling with atmospheric models other than MIROC are underway. As we are moving forward with initiatives to encourage interested researchers to take part in model development, such as hosting hackathons and developing documents, it is expected that land modeling research will become even more active.

Tomoko Nitta／Associate Professor, School of Science and Engineering, Chuo University
Area of Specialization: Climate and Hydrologic Cycle Studies

Tomoko Nitta was born in Hiroshima Prefecture. She graduated from the Department of Civil Engineering in the Faculty of Engineering, Hokkaido University in 2007, completed the Master’s Program in the Graduate School of Engineering, The University of Tokyo in 2009 and the Doctoral Program in the same graduate school in 2012. She holds a Ph.D. in engineering (The University of Tokyo). After serving as Specially-Appointed Researcher at the National Institute of Polar Research, Assistant Professor and Specially-Appointed Lecturer at the Institute of Industrial Science, The University of Tokyo, etc., she assumed her current position in 2025.

Her current research themes include the development of land models, land water cycles in cold regions, and interactions between the atmosphere and land. Her research papers have been published in academic journals such as the Journal of Climate, the Journal of Hydrometeorology, and more.

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