Research Express@NCKU - Articles Digest (Volume 3 Issue 9)

The urban heat island effect is the temperature increase in urban areas compared to that in surrounding rural areas and is caused by the increased use of impervious land surfaces covered by anthropogenic material, the complexity of the three-dimensional structures of the surface, and the coincident decrease of vegetation coverage, as well as anthropogenic heat discharge due to human activities. However, the contributions of these factors remain uncertain. Since atmospheric temperature is basically affected by the surface heat balance, the contribution of these factors can be estimated by quantifying the surface heat balance. In four decades, a number of studies have been conducted in order to clarify the heat island effect in several cities. The ground measurement-based studies have provided detailed temporal variations of thermal environments, but the observation locations were restricted due to physical and economical reasons. Hence, satellite remote sensing data of various spatial and temporal resolutions have been used to investigate the urban heat island phenomenon, because the spatial pattern of the thermal emission from the land surface can be obtained over a wide area when using remote sensing data. The authors interpret the storage heat flux in urban area by remote sensing data.

For the urban surface, the absorbed net radiation R_n and the anthropogenic heat discharge A should balance the outgoing fluxes of sensible heat H, latent heat LE and ground heat G when advection is negligible,

(1)

The net radiation is the sum of the absorbed shortwave and longwave radiation. The sensible heat and latent heat fluxes are the energy transport into the atmosphere by turbulent flow. Sensible heat increases the atmospheric temperature. Latent heat is produced by transpiration of vegetation and evaporation of land surface water, contributing to limit surface and atmospheric temperature increase under given net radiation. During the day, ground heat is conducted into the ground. Heat stored in the ground is conducted to the atmosphere at night. The anthropogenic heat discharge increases the heat budget as well as the net radiation. The energy consumption due to human activities generates anthropogenic heat discharge in the form of sensible heat, latent heat, and ground heat.

In the case of urban areas, because the land surface is composed of several materials, it is very difficult to estimate the temperature gradient under the surface over a wide area. On the other hand, the anthropogenic heat is usually much smaller than solar radiation. For these reasons, it is difficult to calculate the ground heat flux and the anthropogenic heat discharge separately. Therefore, in the present study, the storage heat flux ΔG is estimated by merging the ground heat flux and anthropogenic heat discharge based on the heat balance equation (Equation (1)), which is often used in tower measurements in urban areas, as follows:

(2)

For the case in which the storage heat flux exceeds 0 W m-2, i.e., in case the downward heat flux exists, it can be interpreted as heat storage in urban canopy. In contrast, when the storage heat flux is negative, the upward heat flux is caused by stored heat in the urban canopy or anthropogenic heat discharge.

Calculation theories of net radiation, sensible heat and latent heat fluxes have already been established. These fluxes can be estimated by satellite remote sensing and ground meteorological data.

An area of approximately 890 km2 that includes the city of Nagoya, Japan, was chosen as a study area. The commercial, business and governmental districts are concentrated in the central area of the city, which is characterized by high-rise buildings arranged along a grid of roads that tend to run north-south and east-west. Suburban areas surround the urban area.

The Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) is the sensor system aboard the Terra satellite. The spatial resolutions of the sensor are 15 m for visible and near-infrared (VNIR), 30 m for shortwave infrared (SWIR) and 90 m for thermal-infrared (TIR) bands, respectively. The following data products were used: surface kinetic temperature, surface spectral emissivity, VNIR and SWIR surface spectral reflectance, and relative digital elevation model (DEM).

In order to compare the seasonal and day-night differences of surface heat balance, the authors used three ASTER data observed in daytime on July 10, 2000 and January 2, 2004, and at night on September 26, 2003.

Estimated storage heat fluxes for three days are shown in Fig. 1. A number of high storage heat lines appeared in the central part of Nagoya on July 10, 2000, most of which correspond to rivers and main roads. High-rise buildings are concentrated along the main roads and form urban canyons, where the interruption of solar radiation by buildings suppresses the temperature increase and thus enhances the storage heat flux. Since the solar zenith angle was larger on January 2, 2004, buildings blocked the solar radiation and caused larger shadow coverage on their northwestern sides. This caused higher storage heat flux in the central urban area, resulting in greater contrast between the urban center and the surroundings on January 2, 2004. The difference in the storage heat flux between the central urban and residential areas is also magnified by the large heat capacity of the high-rise buildings in the central urban area.

Fig. 1 Storage heat flux in daytime on (a) July 10, 2000 and (b) January 2, 2004, and in nighttime on (c) September 26, 2003.

The pronounced characteristic of the nighttime heat balance is huge upward heat fluxes, namely positive sensible heat and negative storage heat fluxes, on some roads including elevated expressways and overhead railways. In urban areas, general roads, together with surrounding buildings, form urban canyons, which make the sky view factor small. As a result, radiation cooling from surface of general roads is weakened. On the other hand, although the sky view factor on the elevated expressways and overhead railways is relatively large, the sensible heat from vehicles in heavy traffic should keep the temperature high, and massive elevated structures constructed of asphalt and concrete should have thermal properties that are similar to those of concrete buildings.

Water bodies such as rivers show high storage heat flux in the daytime and large negative heat storage at night. A similar pattern appears in the central urban area, where the high thermal inertia of the urban fabric causes a time lag in the surface temperature change compared to the other surface materials. The storage heat flux is high positive in daytime and highly negative at night on main roads. As in the case of the central urban area, heat stored during the daytime is released at night.

Large negative storage heat flux anomalies occur at a few locations, such as the buildings of the steel plant, where huge amounts of energy are being consumed during both day and night.

Spatial patterns of storage heat flux in an urban area were visualized using remote sensing and ground meteorological data based on the energy balance assumption.

The concentration of dense tall buildings in this area results in the storage of more heat than the surrounding low-building areas. The storage heat flux showed negative values for all analysis times including daytime, for some parts of a highly industrialized part of the city. This analysis suggests that anthropogenic heat is large in these areas and is responsible for the actual energy consumption. Meanwhile, the storage heat flux was high during the daytime, while the storage heat flux was large negative at night in the central urban area. These differences in the heat balance patterns between the urban and industrial areas were due to the existence of buildings in the urban areas and the amount of energy consumption in the industrial area.