Diurnal differences between surface and canopy heat islands in major provincial capital cities of China
Investigations into the diurnal evolution differences between surface urban heat islands and canopy urban heat islands(termed Is and Ic,respectively)hold great values in enhancing our comprehension of the vertical structure of urban climates at a fine time-scale.However,the hourly surface air temperature(Ta)from densely distributed weather stations within cities and the hourly land surface temperature(Ts)that possesses a relatively high spatial resolution and that can be employed for monitoring thermal conditions of urban surfaces are largely lacking.Previous studies comparing hourly Is and Ic have mostly focused on individual cities.In this work,we utilize hourly Ta measurements from high-density meteorological stations(1544 stations)and Ts observations derived from a Diurnal Temperature Cycle(DTC)model to examine the hourly Is and Ic and the associated hourly differences(quantified as △UHI,calculated by subtracting Is from Ic)over 27 Chinese megalopolises.Furthermore,we analyze the hourly patterns of △UHI(e.g.,maximum △UHI,minimum △UHI,and duration of △UHI＞0)across cities with different climate backgrounds and city sizes.We obtain the following findings:(1)at the national scale,the annual mean △UHI remains positive throughout the diurnal cycle.The hourly △UHI pattern generally exhibits a peak shape,with the △UHI increasing from morning and reaching its maximum(1.7 ℃)at around 4:00 PM.Subsequently,it gradually decreases and reaches its daily minimum(0.1 ℃)at around 2:00 AM,with the most rapid decline occurring around sunset.(2)Across different climate zones,from subtropical to temperate cities,the maximum and minimum △UHIs follow a decreasing trend,the times at which they occur are gradually delayed,and the duration of △UHI greater than 0 ℃ gradually decreases.(3)For cities with different sizes,the variation magnitude of △UHI curve generally decreases and the time of minimum △UHI advances as city size increases.The duration of △UHI greater than 0 ℃ also increases with city size.This study can promote the understanding of the contrasting patterns between hourly differences in surface urban heat islands and canopy urban heat islands across cities with diverse background climates.The research results contribute to a deeper understanding of the vertical spatial characteristics of urban heat islands at a fine time scale.
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