The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan

Dynamical Downscaling of Global Climate Change for Urbanizing Kanto Plain
Fujio Kimura* **, Sachiho Adachi**University of Tsukuba, Ibaraki, Japan**Advanced Atmosphere-Ocean-Land Modeling Program, Research Institute for Global Change,
Japan Agency for Marine-Earth Science and Technology, Kanagawa, JapanAbstract
This study estimates the local climate change around the Tokyo Metropolitan area by a new dynamical
downscaling method, i.e., the pseudo global warming method, which allows to reduce the model bias of theCGCM projection. First, the results of hindcast are compared with the observed temperature. Then the dynamical
downscaling is carried out using the boundary conditions given by the CGCM projection of the future climate
assuming IPCC SRES-A1B scenario for the emission of the greenhouse gases. Results show that the uncertaintyof the temperature projection caused by GCMs is very large. The range of temperature change by global warmingis larger than the temperature increase caused by change in land-use.
Key words: global warming, downscaling, regional climate model

1. INTRODUCTION
By the global warming caused by increase of the greenhouse effect gas will further enhance the heatdiscomfort of the citizen living in urban heat islands. Mitigation of the urban heat island will be more necessary.
However, the heat contrast between urban and rural may change by the global climate change. This study intends
to quantitative estimation of the urban heat island effects and warming by the increase of greenhouse gas aroundTokyo metropolitan area by a dynamical downscaling method using a regional climate model.

2. METHOD AND MODEL DESCRIPTION
The numerical model for downscaling is TERC-RAMS (Inoue and Kimura, 2007), which is a modified version of
the Regional Atmospheric Modeling System (RAMS). Parameterization of urban surface is given by a single layerurban canopy model presented by Kusaka and Kimura (2004). The horizontal grid interval is 3 km in the fine grid
system (77 X 72 grid points), which is nested in the coarse grid system that covers central Japan with 15 km
resolution. Six numerical experiments are carried out including a hindcast run and four downscaling runs. Theseare listed in Table 1, where CTL indicates the hindcast run for August 2006 assuming reanalysis data
JRA25/JCDAS for boundary conditions and the actual land-use based upon the Fine Land Cover Data (FLCD)
provided by Ministry of Land, Infrastructure, Transport and Tourism. In the run LAND, land-use are modified after
a simple assumption that the decadal increment of the total urban area in the Kanto Plain will be constant and isequal to a half of that between 1987 and 1997. Figure 1a indicates distribution of the rate of urban area in 1997,
while 1b shows assumed rate in 2070s. Runs BCCR, CCCMA, MIROC and MRI shown in Table 1 aredownscaling runs that estimate the regional climate in 2070s based on the coarse grid projections given by four
GCMs assuming SRES A1B emission scenario. Names of runs are given by Model IDs of GCMs defined in IPCC
report (AR4, Chap.8). In these downscaling runs, land-use is assumed to be the same as the present.
A new downscaling method, Pseudo Global Warming Downscale (PGW-DS) was adopted (Kimura and Kitoh
2007, Sato et al., 2007, Kawase et al. 2009). The PGW-DS is almost the same as the conventional dynamical
downscaling method but the boundary conditions is obtained by the reanalysis data adding the monthly meandifference between the future and present climates simulated by CGCMs.
Table 1. Descriptions of numerical experiments.
Numerical experiments for Aug. 2006
CTL
Hindcast using reanalysis
LAND

Same as CTL, but modified land-use

Downscaling of projected climate in 2070s by four GCMs
BCCR
CCCMA
MIROC

BCCR-BCM2.0
CGCM3.1(T63)
MIROC3.2(hires)

MRI

MRI-CGCM2.3.2

The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan

3. RESULTS
Mean screen height temperature wasoverestimated up to 1.0 C at Otemachi,
Kumagaya
Kumagaya and most other stations
during daytime, while the shapes ofProbability Density Function (PDF) ofhourly temperature and the diurnal
variation were simulated well. The
maximum observed temperature was27.5 C at Otemachi, while simulated one
was 28.5 C. More comparison withobservation is reported by a separate
report by Adachi and Kimura (2009).
Figure2ashows
horizontal(a)(b) (%)
distribution of monthly mean screenheight temperature in August 2006
Figure 1. Distribution of the ratio of urban area (a) in 1997,
given by CTL, while 2b indicates the(b) in 2070s.
same figure but obtained by run MRI,i,e., the downscaled temperature in
2070s using the CGCM projection by MRI-CGCM. Temperature in the central part of Tokyo (Otemachi) is about
28.5 C in the CTL, but it was estimated to be over 31 C in 2070s based upon MRI. Temperature contrast between
the central part of Tokyo and rural areas tends to be enhanced. Figure 2c indicates monthly temperature of therun LAND. Temperature clearly increased in the most part of Kanto Plain, particularly in the northeastern part ofthe Plain. However, the temperature difference between CTL and LAND is much smaller than the difference
between CTL and MRI that means change caused by the global climate change.

(a) CTL      b MRI     c) LAND Figure 2. Horizontal distribution of monthly mean screen height temperature of (a) CTL, (b) MRI, and (c)LAND.

4. DISUCUSSIONTemperature change until 2070s strongly depends
on the GCM which gives boundary condition of theregional climate models. Figure 3 indicates
Probability Density Functions (PDFs) of hourly
surface temperature at Kumagaya in August 2006.
Black line and blue line indicate PDFs of surface
temperature given by observation and CTL,respectively. Simulated present PDF given by CTL
agrees well to that of observation. Red line indicatesthe mean of four PDFs given by downscaling
temperature from four CGCMs. The red PDF entirely
shifted to higher temperature. Particularly themaximum temperature increases by nearly 8 C andreaches to the extreme value of 42 C. Four thin lines
indicate individual PDFs estimated by each CGCMs.
These lines widely disperse showing that theuncertainty of the projection is quite large. TheFigure 3. Probability Density Functions (PDFs)
of hourly surface temperature at kumagaya.

The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan

temperature change is much larger than the estimated change in temperature caused by future change in landuse, which is shown by the difference between blue line (CNTL) and green line (LAND).

5. CONCLUSION
Dynamical downscaling is carried out using the boundary conditions given by four CGCM projections of thefuture climate assuming IPCC SRES-A1B scenario. The intensities of the regional scale warming at the surface
level are individually estimated for the increase of the greenhouse gases and for the change in land-use. Results
show that the uncertainty of the temperature projection caused by dependency of GCMs is very large and therange is larger than the estimated temperature increase caused by future change in land-use.

Acknowledgment
This work was supported by the Global Environment Research Fund (S-5-3) of the Ministry of the Environment,
Japan.
References
Adachi, S. and F. Kimura, 2009: Numerical Studies on Temperature Change and Urbanization in Recent 30 years
in Kanto Plain, Submitted to Extended Abstract for The 7th International Conference on Urban Climate (ICUC7), Yokohama, JapanInoue,T. and F.Kimura, 2007: Numerical experiments on fair-weather clouds forming over the urban area innorthern Tokyo, Scientific Online Letters on the Atmosphere, 3,125-128.
Kawase, H., T. Yoshikane, M. Hara, B. Ailikun, F. Kimura, and T. Yasunari, 2008: Downscaling of the climaticchange in the rainband in East Asia by a pseudo climate simulation method. Scientific Online Letters on theAtmosphere, 4, 073-076.
Kimura, F., and A. Kitoh, 2007: Downscaling by pseudo global warming method, The Final Report of ICCAP, 4346.
Kusaka H. and F.Kimura, 2004: Coupling a single-layer urban canopy model with a simple atmospheric model:
Impact on urban heat island simulation for an idealized case, J. Meteor. Soc. Japan, 82, 67-80.
Sato, T., F. Kimura, and A. Kitoh, 2007: Projection of global warming onto regional precipitation over Mongoliausing a regional climate model. J. Hydrol., 333, doi:10.1016/j.jhydrol.2006.07.023.