日本ヒートアイランド学会論文集 Vol.10 (2015)
Comparison of daytime air heating amount between city and surrounding areas
Radiosonde measurements were taken in Tokyo and its surrounding rural area in summer and autumn 2011. Simultaneous launch of radiosondes at four site clarified distribution of air heating amount. In calm fine daytime, air heating in the cental part of Tokyo is 93 Wm-2 (50%) larger than that of rural area. The mixing layer height is also ca. 50% larger in the urban area. Heat budget analysis revealed that urban-rural difference of the heating amount was mainly caused by the difference of surface sensible heat flux.
Estimation and sensitivity check of the anthropogenic heat in Nagoya city
In this study, we estimated the monthly averaged daily variation of anthropogenic heat (AH) in Nagoya city, where the frequency of the extremely hot day with the daily maximum temperature exceeding 35℃ is the highest among the three largest cities in Japan. The hourly AH was estimated with a horizontal resolution of about 500 m. In addition, we investigated the effect of the AH on surface air temperature by numerical simulations using a hydrostatic mesoscale model. As a result, the estimated AH explained 83% of the total energy consumption in Nagoya city in 2008. The peak values of the AH in downtown and industrial area were beyond 100 W/m2. The impact of AH on the surface air temperature was greater at night, and it was found that AH slightly modified the wind flow and convergenece zone in the city.
Evaluation of Long-term Temperature Trends at Urban Meteorological Stations in Japan with Respect to Time of the Day and Weather Conditions: Analysis Using the JRA-55 Data.
Data at 106 surface observation stations of the Japan Meteorological Agency and the Japanese 55-year reanalysis (JRA-55) for 52 years (1961 to 2012) were analyzed in order to evaluate urban-induced temperature trends with attention to the dependence on time of the day and weather conditions. Stations were categorized into four groups according to the population of municipalities in which they are located. For the nighttime of fine and weak wind days, the departure of observed surface temperature from the reanalysis 850hPa temperature, δT, has a positive trend which tends to increase with population, with values of 0.02-0.05 °C/year for the group of cities with population over a million. On the other hand, δT trends are generally small in the daytime and on days without sunshine. These features agree with the climatology of urban heat islands, which are enhanced in the nighttime under a fine and calm condition, indicating the influence of urbanization on long-term temperature trends at cities in Japan. However, δT trends have seasonal differences with larger values in spring and summer in comparison to autumn and winter, irrespective of time of the day and city population. This fact implies the contribution of some other factors than urbanization on δT trends, although their attribution will require further research.
Development of a Surface Wetting Passive Cooling Louver System with Hydrophilic and Water Absorbing Coating Film and an Evaluation of its Fundamental Performance by Outdoor Experiment
A Passive cooling louver system (PC louver) coated with hydrophilic and water absorbing film is developed in order to form cool microclimate in outdoor and semi-outdoor spaces. The PC louver was designed to shade direct solar radiation, provide radiation cooling and cool airflow. The results from the PC louver's outdoor experiment show that when more than 1 kg/h of water evaporated per vertical surface area (m2) during the day, PC louver's surface temperature was kept approximately to the ambient wet bulb temperature throughout the day, and air temperature passing through the PC louver decreased by -3℃ at most when enough water was supplied to wet the whole PC louver's surface.
Water mist of average droplet diameter near 20μm can be used to create pedestrain “cool spots” or help improve cycle efficieny of outdoor air conditioner units. Pump waste heat, sunlight and other factors may cause the supply water temperature to become warm. The effect of water temperature over the range from 8℃ to 92℃ on mist mass flow rate, droplet diameter, mist evaporation rate and cooling effect were measured. The temperature of air in the mist cone was also measured. It was found that an increase of water temperature from 27℃ to 60℃ yields 7% lower spray mass flow, 5% smaller Sauter mean diameter, and 6% faster evaporation. Temperatures inside the mist cone dropped below ambient within 20-40cm of the nozzle even when 92℃ water is sprayed. Increasing temperature of mist spray supply water could have use to speed evaporation in tight spaces, as a heat sink, or as a legionella countermeasure.
Evaluation method of cooling energy savings by the high reflectance paint based on field observation
Cooling energy savings in a building with the roof coated by high reflectance paint are examined based on field observation. It is diffcult to recognize the energy savings by using the date observed every hour. It is assumed that factors affecting to cooling energy load are the internal heat generation, the set temperature, weather conditions, ect. From the analysis on the relationship between in-outdoor air temperature difference and electric power consumption for air conditioner, which are averaged and integrated into every day, the reduction of electric power consumption by high reflectance paint coating is estimated. Analysis methods of in-outdoor air temperature difference and internal heat generation are discussed for more accurate estimation of cooling energy savings.
Urban heat island observation in Himeji city: Seasonal variation of the horizontal and vertical temperature profile
To study the urban heat island, we observed temperature at six sites in the city of Himeji over 21 months. The temperature was compared with those observed by the Automated Meteorological Data Acquisition System in Himeji (urban area), Fukusaki (rural area) and Ieshima (island). The vertical temperature profile was simultaneously observed on a tower at 18-m and 70-m heights. The results show that under land breezes, the average urban-rural temperature difference in early morning was 1.7°C in January and 1.4°C in August. The average urban-seaside temperature difference under sea breezes was 0.7℃ at 12:00 in August. Temperature varied with downwind distance from the upwind edge of the Himeji urban area, under land or sea breezes. Potential temperature differences between the urban surface (18-m height), urban upper level (70-m height) and rural surface (1.5-m height) were small during January daytime. However, these differences were large during August daytime because of strong solar radiation.
Influence of small green park to surrounding building area
To the question that how long are air temperature reduction effects in the surrounding urban area affected by a small green park, horizontal air temperature profile in the leeward area of small green park are examined based on measurements results carried out in Kobe city and the numerical model to represent the advection and diffusion effects. Measurements results conducted in Tokyo and Osaka are also compared with the calculation results. It is considered that air temperature is dominated in accordance with the heat budget conditions in the urban area in more than about 50m from the green park.
Prediction of Cool Air Distribution with the Consideration of Wind Direction Change by Superimposing CFD Analysis Results
－Application to a Semi-enclosed Space Consisting of Evaporative Cooling Walls －
This paper proposes a method for the prediction of cool air temperature distribution and the appearance frequency of cool air under breezy conditions, considering the changes in the wind direction. It is seen from the measurement results in a semi-enclosed space consisting of evaporative cooling walls, that the appearance frequency of cool air is an important factor in the prediction of air temperature distribution during a breeze. In order to predict the air temperature distribution of cool air, CFD simulation results of mean flows in 8 wind directions were superimposed, considering the appearance frequency of upper wind in these directions. The results of the prediction of cool air appearance frequency showed good agreement with the measurement results, and from the superimposed image, the characteristics of the air temperature distribution could be visualized well.