博士（環境科学） 森 佳祐
学位論文題目
Evaluation of the effect of snow cover on the winter urban heat islandin Sapporo city, Japan.
(積雪被覆が札幌市の冬季ヒートアイランドに与える影響の評価)

要約
Urbanization alters the land cover from natural vegetation to artificial material, leading to the change ofsurface energy budget. As a result, urban environments form urban heat island in which urban areas are hotter
than rural areas. Urban heat islands have been investigated around the world including tropical and snowy
regions. However, the relationship between urban heat island and snow cover has not been clear. Therefore, thepurpose of this study is to clarify the snow cover effect on urban heat island in Sapporo city, Japan. This study
includes three analyses with a regional climate model coupled with urban canopy model as follow; 1) evaluating
the role of snow cover in urban street canyon on the urban heat island, 2) the assessment of snow clearance effecton winter urban heat island, 3) evaluating the sensitivity of urban heat island intensity, defined as the temperature
difference between urban and rural areas (hereafter UHI), to the snow cover.
The Weather Research and Forecasting model with 1 km grid spacing was used for numerical experiment
during continuous snow cover period in January. To represent snow cover in urban street canyon, this study
developed an urban canyon model that is able to consider time-dependent physical properties in urban areas,such as surface albedo, surface emissivity, and thermal conductivity for roof and road as a function of snow
depth and snow water equivalent. The effect of latent heat was also simply expressed by defining maximum skintemperature and minimum moisture availability.
To evaluate the role of snow cover in urban street canyon on the urban heat island, two experiments wereconducted, with and without snow cover effects in urban canyon. These experiments revealed that snow cover inurban street canyon decrease surface air temperature in urban areas, which is more strongly for daily maximum
temperature (0.4−0.6 ˚C) than for daily minimum temperature (0.1−0.3 ˚C). This result is mainly caused bydecreased sensible heat flux due to decreased net radiation at building roof because of high snow albedo. This
analysis also indicates the decrease in surface air temperature in urban street canyon by the treatment of snow
cover is comparable magnitude to the increase in surface air temperature due to anthropogenic heat release in
Sapporo.
To discuss the possible influence of local human activities, the effect of road snow clearance was examined bysensitivity experiments with artificially control of the maximum snow depth in road. The nocturnal surface airtemperature in urban areas tended to increase with intensive snow clearance, especially in suburban areas. This
result is caused by increase of ground heat transfer as a result of snow clearance.
Finally, to investigate a role of snow cover in modifying UHI, sensitivity experiments with snow cover (i.e.,realistic condition) and without snow cover in entire numerical domain were conducted for January. The
experiment without snow cover is based on assumptions that snow immediately disappears when it touches atthe land surface; hence, snow neither accumulate nor melt. The role of snow cover was to decrease surface airtemperature more in rural areas than in urban areas, which was commonly seen throughout a day. The airtemperature over snow cover decreases significantly during nighttime, resulting in increasing UHI by 4.0 °C for
daily minimum temperature. Snow cover tends to decrease net radiation, sensible heat flux, and ground heat fluxthat were more obvious in rural areas than urban areas, consistently with a fact that UHI becomes large in snowy
period or in snowy regions. Additionally, the difference of snow depth between urban and rural areas is apotential factor to determine the magnitude of nocturnal UHI because deeper snow in rural areas weakens
ground heat flux more efficiently than in urban areas.
According to the numerical experiments, the effect of snow cover on urban heat island is summarized asfollows; 1) snow cover in urban street canyon reduces urban surface air temperature, resulting in weakening
urban heat island, and 2) snow cover tends to decrease surface air temperature in both in urban and rural areas,
but more temperature decrease occurs in rural areas than in urban areas, leading to intensifying UHI.