中国主要省会城市地表与冠层热岛日内逐时变化特征差异研究

Diurnal differences between surface and canopy heat islands in major provincial capital cities of China

探究城市地表热岛和冠层热岛日内逐时变化特征的差异有助于更好地理解城市热环境在精细时间尺度的垂向空间多层次演变规律。然而,由于城市内部近地表空气温度观测站点分布较为稀疏,并且极轨卫星提供的中高空间分辨率遥感地表温度数据局限于日内特定时刻,这使得已有研究往往聚焦于单个或少数几个城市,从而导致不同气候背景下城市地表与冠层热岛日内逐时变化特征差异尚不明晰。本研究以中国27个省会城市为研究区,基于地表温度日循环模型对2017年—2019年MODIS地表温度产品模拟得到逐时地表温度,利用1544个高密度气象站点的气温资料获取逐时空气温度,探究地表与冠层热岛强度差值(ΔUHI)的逐时变化曲线的形态特征(含ΔUHI最大值和ΔUHI最小值及对应时刻、ΔUHI>0℃的持续时长等)。结果表明:(1)就中国这27个城市平均而言,ΔUHI日内变化曲线整体呈“山峰”形态,且ΔUHI在日内24个时刻均为正值。具体而言,ΔUHI从08:00开始迅速增大,并于16:00前后达到最大值(1.7℃),之后逐渐开始下降,其中在日落时刻下降速率最大,并于凌晨02:00达到最小值(0.1℃),之后基本保持稳定;(2)就不同气候区而言,从南亚热带到中温带,随着城市所属气候区纬度的升高,ΔUHI最大值和最小值逐渐减小,其对应的日内时刻逐渐推迟,且ΔUHI>0℃的持续时长也逐渐减小;(3)就不同规模城市而言,随着城市规模的增加,ΔUHI最大值呈现不断减小的趋势,其达到最大值的日内时刻逐渐提前,且ΔUHI>0℃的持续时长逐渐增加。本研究明晰了较大地理空间范围下不同气候背景城市的2种热岛逐时变化特征差异,有助于增进对精细时间尺度下城市热岛垂向空间特征的深入认知。

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 asΔUHI,calculated by subtracting Is from Ic)over 27 Chinese megalopolises.Furthermore,we analyze the hourly patterns ofΔUHI(e.g.,maximumΔUHI,minimumΔUHI,and duration ofΔUHI>0)across cities with different climate backgrounds and city sizes.We obtain the following findings:(1)at the national scale,the annual meanΔUHI remains positive throughout the diurnal cycle.The hourlyΔUHI pattern generally exhibits a peak shape,with theΔUHI 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 minimumΔUHIs follow a decreasing trend,the times at which they occur are gradually delayed,and the duration ofΔUHI greater than 0℃gradually decreases.(3)For cities with different sizes,the variation magnitude ofΔUHI curve generally decreases and the time of minimumΔUHI advances as city size increases.The duration ofΔUHI 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.