The changes of a sea breeze and a daytime heat island due to land-use alteration during an 85 year period have been numerically simulated. The domain of interest is the Kanto Plain (15000 km2), including the Tokyo metropolitan area. This urban area is located in the southern part of the plain and consists of many cities in Tokyo and its suburbs. The horizontal scale of the area is about 40 km and has increased by a factor of four during the 85 year period. The simulations were conducted under a summer synoptic condition with weak gradient wind and almost clear sky. The model is based on the threedimensional anelastic equations, taking into account the hydrostatic assumption. First, it was confirmed that the simulated wind field and temperature distribution with using the land-use data for 1985, agreed with observed data. The simulations were then conducted using the land-use data for 1950 and 1900. From comparison among the three simulations, the following two major conclusions were obtained: (1) Land-use alteration modified the wind system over the Kanto Plain. In particular, the simulated sea breeze front in 1985 was more clearly defined around the northern end of the Tokyo metropolitan area. The time required for the sea breezes to reach inland areas increased by two hours. (2) The warming due to land-use alteration is found over the Tokyo metropolitan area and the northwestern part of the Kanto Plain. In particular, the area of the most prominent warming is found in the northern end of the Tokyo metropolitan area. Intensity of daytime heat island in the area were estimated as 3-4C and 2-3C during the 85 year period, and latest 35 years respectively. The above warming is confirmed to result from the enhanced sensible heat flux and the change of interaction between the boundary layer heating and sea breeze front.
Two continuous 5‐year‐long simulations over eastern Asia and the Japan islands, one for present‐day climate (control) and one for climate under doubled carbon dioxide concentration (2×CO2) are completed with a regional climate model (RegCM) nested, in a one‐way mode, within a general circulation model (GCM). The GCM is run at R15 resolution (4.5×7.5° latitude × longitude), and the RegCM is run at 50‐km grid point spacing. In the control run, both the GCM and the RegCM reproduce the seasonal migration of the westerly jet but produce too strong a monsoonal circulation, which results in a significant overestimate of summer precipitation over the eastern Asian continent. The temporal evolution of the eastern Asia summer monsoon, with steady phases separated by more rapid transitions, is reproduced, but the monsoon rain belt reaches too far north, and the occurrence of tropical storms is underestimated. Regionally averaged surface air temperatures are mostly within 1–3°C of observations. Seasonal precipitation amounts over Japan are within 10–35% of observed ones. The narrow Korea and Japan land masses, which are not captured by the GCM grid, substantially affect the simulated surface precipitation climatology in the RegCM. Under 2×CO2 forcing, warming in the range of 4–11°C is simulated, greater in winter than in summer and increasing toward high latitudes. The strength of the monsoonal circulation increases in 2×CO2 conditions, leading to a general increase in precipitation over all regions by 10–30%. The simulated precipitation change shows significant regional and, within Japan, subregional structure. The Japan and Korea land masses substantially affect the summer precipitation changes, indicating the need to capture them in numerical simulations of climate change. Because of the uncertainties in the control simulations, our 2×CO2 results are intended not to provide climate change scenarios for impact assessments, but only to illustrate the model sensitivity to different forcings. Plans are under way to further test and improve the RegCM performance over the region and to use more recent GCM simulations to drive the RegCM.
Abstract. The performance of the NCAR regional climate model (RegCM) for east Asia, where topography and shoreline are rather complex, is examined through experiments to simulate the climate during 1 month using ECMWF data as lateral boundary conditions, before its application in the nested GCM/RegCM method to predict future climate changes caused by global warming. In this study, June and January climates, in which typical precipitation phenomena in Japan such as "bai-u" and winter snow are observed, are simulated using the model with lateral resolutions of 50 km (Base case) and 25 km (High case). The resolution effects of the model are also examined using a series of sensitivity studies. The main results are as follows: (1) In June and January, while cyclones passing over the inner region tend to be more intensified in the model (sometimes unrealistically) than those observed, weak cyclones in the outer region are usually not well simulated in the model. This seems to be due to the stronger control by the lateral boundary conditions. Anticyclones are stronger in the simulation than those observed, especially in the inner region, which leads to overestimation of the sea level pressure there. (2) The model yielded a lower surface air temperature than that observed, especially in January, which may depend on the performance of BATS or the radiative transfer scheme. (3) The model tends to overestimate the regional mean precipitation in the inner region and underestimate it in the outer region in June, while it was slightly underestimated in the inner and outer regions in January. Overestimations are caused by overdeveloped simulated cyclones or by the large amount of precipitation on the unrealistic topography such as a steep slope facing a moist tongue. Underestimation is due to stronger control by the lateral boundary conditions that prevent the development of cyclones or fronts. (4) In the high-resolution models, weak cyclones in the outer region and anticyclones in the inner region are more realistically simulated, although they do not greatly improve the model results in the Base case. Precipitation is increased by ---10-15% of that in the Base case in the inner region in June and January due to enhancement of cyclones or fronts, where the high-resolution effect of topography is only ---1/5 of the total high-resolution effect of the model in June. In January, for northwestern Japan, high model resolution contributes to the correction (increase) of precipitation there. Generally, the high-resolution effect of the model on the surface air temperature does not systematically improve the results but does vary locally by the improvement of model topography. To improve the model results, the lateral boundary should be moved outward (the domain of the model should be extended to the west and south) and/or the precipitation scheme should be improved, including the adjustment of parameters. BATS or the radiative transfer scheme should be improved. More recommendations for the improvement of the model are proposed.
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