Climate Modeling

The climate is not constant; it varies on many time-scales. Ten thousand years ago most of Europe and North America were under thick sheets of ice. On shorter time-scales periodic temperature increases in the equatorial Pacific Ocean ( El Nino ) cause year-to-year variations of climate. Recently, a potential "global warming" has become a political issue.
Numerical experiments using climate models are being carried out to understand these climate variations and to assess anthropogenic climate changes. In addition to this, transfer of solar and infrared radiation in the atmosphere, which determines in part the distributions of energy in the atmosphere and drives the whole climate system,is being investigated.

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Research subjects

Development of coupled ocean- atmosphere models
Study of ocean's uptake of CO₂ and global warming by use of simplified ocean models
Basic research on climate change on geological time scales, especially mechanisms of the glacial-interglacial cycle
Study of the energy budget of the climate system especially radiation balance
Studies of cloud distributions together with cloud properties and their influences on climate

Simulation of Climate

The first objective of climate study using models is to simulate the present climate, i.e. to compute distributions of temperature, wind, precipitation and so on as close as possible to their observed counterparts, specifying solar insolation, chemical composition of the air, land-sea distribution and so on as they are now. Full simulation of the climate by a coupled ocean-atmosphere model is a very difficult task. So far only a few research groups in the world have done full climate modelings, and they have had to include artificial corrections in boundary forcings to obtain acceptable results. Our first target is thus to develop a much better coupled model and to simulate the present climate. Once a good climate model is constructed we will carry out numerical experiments to investigate long-term climate variations and to assess global warming and associated climate change caused by the gradual increase of greenhouse gases.

Study of Global Warming by Use of a Simplified Ocean Model

Prediction of global warming corresponding to a prescribed scenario of CO₂ emission is one of the most important tasks for the coupled ocean-atmosphere models. However, coupled models are immature at present and running them requires enormous computer resources. Therefore, studies of global warming using much simpler models are necessary. In fact, in the scientific assessment of global warming by the Intergovernmental Panel on Climate Change (IPCC), predictions of future CO₂-concentration and global average temperature increase were made using an "upwelling-diffusion model" of the world ocean. However, the two models used for calculating the CO₂-concentration and temperature were different: Different parameter values were used in each transport rates were consequently different. A new simplified model, essentially a "two-layer upwelling-diffusion model", was therefore devised. The model is used to estimate CO₂-concentration. The figure shows the calculated CO₂ concentration (right) corresponding to the emission scenario (left). This scenario qualitatively represents the target of the "New Earth 21" program in which CO₂-emission is assumed to increase at the same pace until about 2050 and then decline sharply owing to new technology (clean energy and so on) to reach half of the current emission by 2100.

Radiation and Remote Sensing

Incoming solar radiation and thermal radiation emitted by the earth both drive the climate system and maintain the temperature of each part of the system. It is important to study the process of scattering and absorption of solar radiation by atmospheric constituents, such as molecules, clouds and aerosols (suspended atmospheric particles), and to study the process of emission and absorption of thermal radiation by ground surfaces, greenhouse gases, clouds and aerosols. One of our research subjects is therefore, development of models of these radiative transfer processes in numerical climate models. Simulation of global warming due to the greenhouse effect, for example, is an important study to understand the climate change.

To accomplish the climate modeling mentioned above, it is indispensable to observe the behavior of atmospheric constituents on a global scale. Satellite observation is an effective tool for these global studies. Figure 1 shows an example of a large scale change in the cloud microphysical properties caused by aerosols transported from continental area. Satellite remote sensing techniques make it possible to find such phenomena from space. We are therefore studying the global behavior of the radiative budget and atmospheric constituents using satellite remote sensing as well as modeling.

Fig. 1: Remote sensing of stratocumulus clouds observed off the coast of California. Effective particle radius (left panel) and cloud optical thickness (right panel). The cloud layer thickens with decreasing cloud particle radius affected by aerosols transported from the continental area (black-colored area at the right top corner). A similar large scale modification of cloud properties is expected from the aerosols generated by human activities.