Abstract
Despite the fact that the presence of a heat island over a city was established quite a long time ago, now there is no versatile algorithm for the determination of the urban heat island intensity. The proposed models either take into account only one or several factors for the formation of an urban heat island or do not consider physical reasons for the difference in thermodynamic conditions between a city and countryside. In this regard, it is impossible to make a forecast and determine the optimal methods for reducing the urban heat island intensity for an arbitrarily chosen city in a wide range of its characteristics and climatic conditions. This paper studies the causes for the formation of an urban heat island in order to develop the quantitative model of this process through the determination of the difference in radiation fluxes of various nature between a city and countryside (background area). A new equation allowing the intensity of an urban heat island in different seasons and different times of day, as well as under various atmospheric conditions, to be calculated from meteorological parameters measured at a stationary observation station is proposed. The model has been tested through the comparison of the results of numerical simulation with direct measurements of the heat island in Tomsk with a mobile station. It is shown that the main contributors to the formation of the heat island in Tomsk are anthropogenic heat emissions (80–90% in winter, 40–50% in summer) and absorption of shortwave radiation by the urban underlying surface (5–15% in winter, 40–50% summer). The absorption of longwave radiation by the urban underlying surface, absorption by atmospheric water vapor and other constituents, and heat consumption for evaporation are insignificant. An increase in the turbulent heat flux is responsible for the outflow of 40–50% of absorbed energy in summer and 20–30% in winter.
Highlights
The main difference between thermodynamic conditions in a city and countryside consists in an increase in the air temperature at the urban territory, i.e., the formation of an urban heat island (UHI)
It is obvious that the positive difference in the heat fluxes (B0urb + QHurb) − (B0rur + QHrur) in the daytime is the direct reason for the increased air temperature in the city
The proposed equation for the calculation of UHI intensity (UHII) was obtained with allowance for the effect of all the main factors of UHI formation and their relative contributions in different seasons and times of day and under different atmospheric conditions
Summary
The main difference between thermodynamic conditions in a city and countryside consists in an increase in the air temperature at the urban territory, i.e., the formation of an urban heat island (UHI). Direct heat emissions from the combustion of all types of fuel and the consumption of electricity are considered as the main reason for the UHI formation This internal energy source is referred to as the anthropogenic heat flux QF. The accumulated energy is directed in the opposite direction from the deeper layers to the surface and spent in the increase in the UHI temperature This fact should be taken into account when assessing the diurnal variation of the UHII
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