Net radiation is one of the important indicators to climate changes. This study demonstrated how much the atmosphere condition response to human activity or the environment. Conducted in Metropolitan City of Hong Kong, the technique used is useful for any city with similar environment worldwide.
Radiation is a natural phenomenon; all bodies are capable of emitting radiation. A body, which emits the maximum amount of radiation at a given temperature over a unit surface area called as black body, which has an emissivity equal to unity. The emissivity defined as the relative ability of a surface to emit energy by radiation. In other words, it is the ratio between energy emitted by a particular material and the energy emitted by a black body at the same temperature. This emissivity defined by the intrinsic property of the material. Another property related to material is albedo, which defined as the ratio between reflected radiation from the surface and the incident radiation upon it. A perfect black body assumed to emit perfectly and albedo becomes zero. The albedo and emissivity along with other properties are responsible for net radiation over the surface. The Net radiation defined as the difference between the total incoming radiation and outgoing radiation at the measured surface. Net radiation is one among the important indicators of global climate change, it empower to distinguish the changes in local meteorological condition due to local anthropogenic activity or global climate change.
Net radiation is a vital component in urban energy balance, which may be disturbed due to urbanization. In order to estimate and understand urban energy balance properly – cautious estimation of net radiation component is necessary. The present study estimates net radiation over Hong Kong using ASTER (Advance Space borne Thermal Emission reflection Radiometer) level 1B satellite image. The high spatial resolution and wider spectral resolution of ASTER is very helpful for net radiation assessment but at the same time, atmospheric correction may distracts the accuracy of net radiation estimation. Hong Kong possess unique climatic condition where predefined models in global atmospheric correction algorithms may not be appropriate, so in this study radiosonde and sun photometer values were used to replicate local atmospheric and aerosol conditions. The atmospherically corrected Visible near Infrared (VNIR) images and Short Wave Infra-Red (SWIR) images used to calculate emissivity and albedo; simultaneously, ASTER Thermal Infra-Red (TIR) image was atmospherically corrected using MODTRAN code – Band 13 used to calculate land surface temperature. This study was unique because several studies in the past measure net radiation either by remote sensing or by field measurement, the integration of both techniques is new in Hong Kong.
Fig. 1 Study area and net radiation field measurement site
1. Hong Kong
Field measurement – Ta Kwu Ling, HK
The study conducted in Hong Kong, Even though Hong Kong located in the Tropic of Cancer it has humid sub-tropical climate, hot and humid during summer with occasional rain and typhoons with the highest and lowest temperatures ever recorded in Hong Kong were 36.1°C and 0.0°C respectively..
ASTER measured net radiation over Hong Kong
The technique used in this study is useful for any complex build urban environment in the world. The results may further improved if emissivity values derived directly from satellite image at finer scale. Assuming common meteorological conditions over large surface may also cause difference in satellite measured net radiation value and the field measured net radiation. The accuracy of the instrument used to measure net radiation and the spatial variation between two measurement techniques (measured area) possess possible variation in the value. Overall, the result in this study based on the maximum utility of the available data and techniques in Hong Kong. In addition, this is the first study conducted in Hong Kong for net radiation with validation using field measurement. The estimated net radiation values range between 35 – 975 watts/meter square and some pixels within the city possess high net radiation when compared to rural areas, which was evident that the anthropogenic activities have impact on net radiation value. The method used here will help for the appropriate estimation of net radiation component for energy balance. In general, previous study about net radiation either by remote sensing or by field measurement and the idea of integrating both of the techniques is innovative. The methods used here can be adopted for any city possess similar complex urban environment and climatic conditions of Hong Kong.