Global Climatology (Endowed by Itochu Group)

The climate changes occuring on global to regional scales are due both to natural causes and to the artificial effects of human activities. In order to clarify the mechanisms of the climate change on various scales and assess the resulting effects, it is important that research on regional aspects as well as on global impacts be done.

In this section, the problem of regional climate changes resulting from human activities is being investigated mainly with numerical experiments using a regional climate model. Furthermore, the dynamical role of small-scale disturbances in the global atmospheric circulation is being studied to obtain basic knowledge and information for understanding the mechanisms of global climate change.

Human activities cause climate change not only on a global scale but also on a regional scale. In fact, regional climates respond more sensitively to human activities than does the global climate.

In this study, the climate in and around urban areas is treated as an example of regional climate change. In urban areas, the covering of the ground surface changes through the increase of buildings, the enlargement of pavement, the decrease of green zones and so on as the population increases. Furthermore, anthropogenic heat in the urban area is increasing due to energy consumption for maintaining a comfortable living environment, industrial activities, propulsion of vehicles, etc. As a result, the characteristics of the urban climate differ from the climate of the surrounding area.

In order to understand how climate in an urban area changes through urbanization, many numerical experiments are being done using a regional climate model for cases with and without alteration of the ground surface by urbanization, with and without anthropogenic heat emission and with and without factors to characterize urbanization. The regional climate is also expected to be affected by the direction and velocity of the background wind controlled by the large-scale atmospheric circulation. The effect of the background wind is being investigated by numerical experiments with and without it.

Urbanization has an effect on the climate not only in the urban area itself but also in the surrounding area. Using numerical experiments, we can understand the influence of human activities in urban areas on the climate in the surrounding area. The knowledge obtained will be useful for the consideration of the relationship between the climate changes on different scales.

There are very small waves and turbulence having time scales shorter than one day in the atmosphere. Although these phenomena are generally energetically weak, their importance is widely recognized: the momentum deposition by upward propagating small-scale waves, mainly generated over topography, is essential in maintaining the large-scale structure of the subtropical jet stream. Moreover, small-scale waves associated with strong convection control the quasi-biennial and semi-annual oscillations observed in the tropical middle atmosphere. It is, therefore, necessary to elucidate their generation mechanisms and propagation characteristics and to examine quantitatively their effects on the global circulation.

In the last decade, recently developed powerful tools such as radars and lidars for measuring winds and temperature in the atmosphere have brought significant progress in our understanding of the dynamics of small-scale phenomena, particularly in middle latitudes. Figure 3 shows the propagation direction relative to the subtropical jet axis of waves with periods of 10-20 hours called inertia-gravity waves. The analysis was done based on wind data monitored for three years by a VHF Doppler radar at Shigaraki (35/, 136&). In winter, the inertia-gravity waves propagate from higher latitudes where polar cyclones are dominant, and in summer from tropical regions where convective activity is high.

However, the distribution of observatories, even in the middle latitudes where the current observation network of radiosondes is densest, is too sparse to examine the detailed horizontal structure of the small-scale phenomena. Numerical simulations will supplement the shortage of observations. Furthermore, in tropical regions where the observation network is much more sparse, it is important to undertake comprehensive studies using satellite data as well as conventional radiosondes.