Abstract The thermally induced local circulation over a periodic valley is simulated by a two-dimensional numerical model that does not include condensational processes. During the daytime of a clear, calm day, heat is transported from the mountainous region to the valley area by anabatic wind and its return flow. The specific humidity is, however, transported in an inverse manner. The horizontal exchange rate of sensible heat has a horizontal scale similarity, as long as the horizontal scale is less than a critical width of about 100 km. The sensible heat accumulated in an atmospheric column over an arbitrary point can be estimated by a simple model termed the uniform mixed-layer model (UML). The model assumes that the potential temperature is both vertically and horizontally uniform in the mixed layer, even over the complex terrain. The UML model is valid only when the horizontal scale of the topography is less than the critical width and the maximum difference in the elevation of the topography is less than about 1500 m. Latent heat is accumulated over the mountainous region while the atmosphere becomes dry over the valley area. When the horizontal scale is close to the critical width, the largest amount of humidity is accumulated during the late afternoon over the mountainous region.
This paper analyzed the downscaling products for Japanese climate using five regional climate models (RCMs) with horizontal meshsize of 20 km with boundary conditions given from JRA-25 reanalysis. The RCMs successfully reproduced the temporal variations and geographic distribution of temperature and precipitation. Skill scores for the surface temperature were improved by the downscaling. The JRA-25 underestimated the precipitation amount for summer and winter seasons, and the RCMs reduced the error, especially in winter. The RCMs showed common features, such as a warm bias in the areas with a monthly-mean temperature lower than the freezing point, an overestimation of weak rainy days, and an underestimation of heavy rainy days. The comparison among five RCMs suggests that the warm bias is due to the lack of model resolution and the precipitation bias is related to the convective parameterization. The multi-RCM ensemble mean has considerable advantages over the individual RCM in regards to the bias and skill scores of surface temperature and precipitation, although it still showed the warm bias in snow areas in winter. The data set of the multi-model dynamical downscaling is expected to contribute to impact studies on the forthcoming climate change in Japan.
Abstract In this study, the impact of global climate change and anticipated urbanization over the next 70 years is estimated with regard to the summertime local climate in the Tokyo metropolitan area (TMA), whose population is already near its peak now. First, five climate projections for the 2070s calculated with the aid of general circulation models (GCMs) are used for dynamical downscaling experiments to evaluate the impact of global climate changes using a regional climate model. Second, the sensitivity of future urbanization until the 2070s is examined assuming a simple developing urban scenario for the TMA. These two sensitivity analyses indicate that the increase in the surface air temperature from the 1990s to the 2070s is about 2.0°C as a result of global climate changes under the A1B scenario in the Intergovernmental Panel on Climate Change’s Special Report on Emissions Scenarios (SRES) and about 0.5°C as a result of urbanization. Considering the current urban heat island intensity (UHII) of 1.0°C, the possible UHII in the future reaches an average of 1.5°C in the TMA. This means that the mitigation of the UHII should be one of the ways to adapt to a local temperature increase caused by changes in the future global climate. In addition, the estimation of temperature increase due to global climate change has an uncertainty of about 2.0°C depending on the GCM projection, suggesting that the local climate should be projected on the basis of multiple GCM projections.
The formation mechanism of the Baiu front, which appears in early summer and gives a typical rain band around Japan, was reporduced using a regional atmospheric model. The initial and boundary conditions of the model were derived from the European Center for Medium-Range Weather Forecasts (ECMWF) data. In the "realistic simulation", temporary variable boundary conditions derived from twice-daily ECMWF data were utilized. In the "zonal mean simulation", zonally uniform and temporally constant atmospheric fields obtained from ten-day averaged global zonal mean ECMWF data was utilized as the initial and boundary conditions. The simulated Baiu front in the zonal mean simulations, as well as that in the realistic simulations, has similar structures to the real Baiu front, i.e., the Low-Level Jet (LLJ) runs parallel to both the precipitation zone and the upper-level jet in the eastern part. Basically, the simulated Baiu front is formed by the deformation of the zonal mean field due to the Land/Sea contrast and topography. Although the distribution of the simulated rainfall over the Baiu front depends on the cumulus convective parameterizations, the fundamental structure of the Baiu front does not depend on them. A comparison between the zonal mean simulations of early and late June indicates that the Baiu front is formed at a higher latitudes in late June, when the upper-level jet is weak and located northward. The location of the Baiu front is quite sensitive to the zonal mean field. The Baiu front accompanied by the LLJ is also represented by numerical experiments without topography, which suggests that the Baiu front could be reproduced by two factors alone, the zonal mean field and the Land/Sea contrast. The orography, including the Tibetan plateau, intensifies the LLJ and the precipitation over the Baiu front. The LLJ also appears in the zonal mean simulation without a condensation process. However, the LLJ is formed along the eastern coast of the Eurasian continent and locates in the northern side of the upper-level jet eastward of Japan, which is a different feature from the zonal mean simulations with a condensation process. Accordingly, it is speculated that the condensation process acting on the atmospheric field is necessary to keep the LLJ in the southern side of the upper-level jet as in the real Baiu front.
During the winter monsoon, a convergence line called the Boso Front often appears in the area between the Kanto Plain and the Izu Islands, Japan. Two typical cases of high-level Precipitable Water Vapor (PWV) are observed along the convergence line in the lee of Chubu Mountains by the aid of latest techniques of Global Positioning System (GPS).Statistic analysis of GPS derived Precipitable Water Vapor (GPS-PWV) indicates that the prevailing wind direction controls the position of the high-level PWV in the lee of Chubu Mountains. A numerical model simulates the behavior of water vapor, namely, the temporal variation of simulated PWV and surface wind, which suggest the reasons for the high PWV near the convergence line. The convergence of surface wind gathers moisture, and trapped moisture by reduced wind velocity may also contribute the high-level PWV.
The formation mechanisms of both the South Pacific convergence zone (SPCZ) and the baiu front are investigated using a regional climate model. Some idealistic numerical experiments are carried out assuming zonally uniform and temporally constant atmospheric fields obtained from ECMWF analysis data as initial and lateral boundary conditions. A rainfall zone similar to the SPCZ is reproduced using a zonal mean atmospheric field of the Southern Hemisphere (SH) summer. The simulated SPCZ in the idealized model framework is highly sensitive to the variation of SST during 1997–98 in a manner similar to observation. The SPCZ is extremely weak in an experiment under the zonal mean field of the Northern Hemisphere (NH) early summer. Experiments with a different intensity of zonal wind speed and baroclinicity suggest that a mild zonal wind (weak baroclinicity) weakens the precipitation of the SPCZ and even occasionally suppresses precipitation when it is too weak. The heat contrast between the Australian continent and the South Pacific Ocean contributes to form another rainfall zone when the zonal flow is very weak. Under these conditions, the SPCZ becomes unclear, and the rainfall zone appears from the southeastern part of Australia to the east of New Zealand. The latter rainfall zone will be intensified if the orography in the Australian continent is magnified. This rainfall zone is formed by the heat contrast between land and ocean and is somewhat similar to the baiu front. A continent as large as Eurasia creates a better-defined rainfall zone, even under stronger zonal flow. The baiu front seems to be a rainfall zone caused by the heat contrast between land and ocean that differs from the SPCZ in its formation mechanism.
The two-stage numerical model developed by Kimura (1983) is extended to three-dimen-sional and used to study the relation between local winds and photochemical air pollution on the Kanto Plain. The first stage is a three-dimensional, timedependent local winds model which gives the wind velocity and vertical diffusion coefficient. The second stage is a photochemical air pollution model which uses the results of the first stage as input data.The calculated results are compared with observed surface wind velocity and pollutant concentrations when the large scale wind are light (≤3m/s). Good agreement between calculated and observed wind fields is obtained. The calculated concentration of ozone also agrees with the observed oxidant concentration, which is expected to be almost equal to ozone concentration. The simulation also suggests that the mountainous area of Central Japan is very important for the wind system over the Kanto Plain, forcing pollutants from the Tokyo metropolitan area to be more often transported westward than eastward.Previous air pollution models have needed a lot of meteorological data but some of these, such as upper level wind velocities, are difficult and expensive to obtain. The two stage model will be very useful for preliminary estimation of photochemical air pollution effects where detailed meteorological data are not available.
