An investigation of urban heat island intensity (UHII) as an indicator of urban heating

The objective of this paper is to evaluate the reliability of urban heat island intensity (UHII) as an indicator of urban heating. The diurnal patterns of air and surface-temperature based UHII and variations in urban and rural area heating were analyzed and discussed. The detailed air-temperature based UHII patterns were determined in one urban and four suburban areas of Hong Kong. UHII was determined as spatially-averaged air-temperature difference between an urban/suburban area and its surrounding rural area. Additionally, reported air and surface-temperature based UHII patterns were integrated in the discussion to carry out a comprehensive analysis. The urban and rural area heating variations (i.e., the diurnal variations in net radiation, sensible heat flux, latent heat flux, and heat stored by an area) were examined in the light of UHII patterns to validate UHII as an indicator for urban heating. It is noted that the air-temperature based UHIIs were higher and positive in the night-time but lower and negative during the daytime. On the other hand, most of the surface-temperature based UHIIs, investigated through satellite data were positive during both the daytime and night-time. It is revealed that UHII can well reflect urban heating during night-time and early morning. However, the lower and negative UHII during solar peak time (daytime when solar radiation is the dominant source of heating) has seemingly not been representing urban heating.