Horizontal extent of the urban heat dome flow
Abstract
Urban heat dome flow, which is also referred to as urban heat island circulation, is important for urban ventilation and pollutant transport between adjacent cities when the background wind is weak or absent. A “dome-shaped” profile can form at the upper boundary of the urban heat island circulation. The horizontal extent of the heat dome is an important parameter for estimating the size of the area it influences. This study reviews the existing data on the horizontal extent of the urban heat dome flow, as determined by using either field measurements or numerical simulations. A simple energy balance model is applied to obtain the maximum horizontal extent of a single heat dome over the urban area, which is found to be approximately 1.5 to 3.5 times the diameter of the city’s urban area at night. A linearized model is also re-analysed to calculate the horizontal extent of the urban heat dome flow. This analysis supports the results from the energy balance model. During daytime, the horizontal extent of the urban heat dome flow is found to be about 2.0 to 3.3 times the urban area’s diameter, as influenced by the convective turbulent plumes in the rural area.
Introduction
Urban heat dome flow, which is also called urban heat island–induced circulation1,2,3, is induced by the differences between urban and rural temperatures under conditions of inversion with calm background weather. The main factors that induce these mean circulating flows are the horizontal pressure gradients between urban and rural areas and inhomogeneous plumes impinging on the inversion layer. Urban heat domes are characterized by convergent inflows at the atmosphere’s lower level, upward flows in the form of turbulent plumes4 over the urban area, divergent outflow in the atmosphere’s upper level, and a “dome-shaped” upper boundary (See Fig. 1) at the interface between the inversion layer and the divergent outflow region5.
Figure 1
The dome-shaped upper thermal boundary between the mixing layer and the inversion layer, as visualized with thermochromic liquid crystals.
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Under stable stratification and calm background conditions, the dispersion of pollutants and heat within and over the urban canopy layers (under which most human activities take place) depends mainly o n the urban heat dome flow6,7,8,9. The mixing height (i.e., the vertical extent) of the dome has been extensively examined, both quantitatively10,11,12 and theoretically13. However, the horizontal extent of the urban heat dome flow has received relatively little attention from researchers. Still, several studies have been done in which the dome horizontal extent (dome diameter) can be estimated, based on the numerical simulations and field measurements, which are summarized in Table 1.
Table 1 A summary of dome horizontal extent measurements, as extracted from existing studies using numerical models and field measurements.
Full size table
The mechanisms and factors governing the domes’ horizontal extent were not discussed in these previous studies, and no physical model was proposed to explain and predict the domes’ horizontal extent or their diameters.
When cities are grouped together to form a city cluster, each city may generate its own heat dome. Under calm background conditions, the pollutants may be transported between adjacent cities through the dome flow, and thus cause regional air pollution. Various large city clusters have arisen in Asia, such as the Beijing-Tianjin-Hebei region, the Yangtze River delta region, and the Pearl River delta region in China. The mechanisms and characteristics of the urban heat dome flows are important to consider for understanding pollutant dispersion, urban heat removal, and the transport of regional pollutants between adjacent cities under calm, stably stratified background environments.
It should be noted that the urban heat dome flow reaches quasi-steady state about 4 hours after sunrise20,21 and about 4–6 hour after sunset16,22. The urban heat dome flow is in the transition state around sunset or sunrise time14, which is not considered in this study. Therefore, in this study, the daytime refers to the time slot between the time about 4 hours after sunrise and the time before the sunset, when the daytime urban heat dome flow is in the quasi-steady state. The night-time refers to the time slot between the time about 4–6 hour after sunset and the time before the sunrise, when the night-time urban heat dome flow is in the quasi-steady state.
The energy balance model and linearized model are presented in Section 2.1 and Section 2.2 respectively. The horizontal extent of the urban heat dome during daytime is determined in Section 2.3. The implication of the results, the limitation of the models and the relationship between the dryland warming, proposed by Huang et al.23,24, and the urban heat dome are discussed in Section 3.
Results
Energy balance modelModel description
As first proposed (...truncated)