粒度变化对城市热岛空间格局分析的影响

尺度是景观格局和生态过程研究中的关键问题。综合目前城市热岛效应研究来看,景观格局指数的引入极大推进了热岛格局的定量研究,然而其尺度效应仍未得到重视。由于热岛空间格局与形成过程的复杂性和人类认识的局限性,对其尺度问题有待深入讨论。基于Landsat TM影像反演地表温度,采用均值-标准差分类方法划分热力等级,对珠三角城市热岛格局特征的粒度效应进行了研究。结果显示:随粒度增加,弱势热力斑块类型下降,向相邻斑块转移;景观指数在类型水平和景观水平均受空间粒度影响明显,"临界粒度"现象明显;总体而言,粒度150 m是城市热岛格局特征的临界粒度,对热岛格局进行景观指数计算的适宜粒度范围为30—150 m;不同景观指数粒度效应曲线有所差异,其中斑块密度和平均分维数在两个水平指数上都有较强的规律性;根据各景观指数变化特征,研究区尺度域主要在(30 m,150 m),表明在该尺度范围内构建的热岛效应预测模型可经简单推绎后使用。分析热岛格局随空间粒度变化特征对了解热岛格局的形成机制及进行尺度推绎具有重要意义。

Impacts of grid sizes on urban heat island pattern analysis

Scaling is essential issue in understanding and predicting landscape pattern and ecological process in ecological research.As a significant indicator and ecological parameter affected by urbanization in urban ecosystem,landscape pattern and ecological process of urban heat island(UHI) and its relationship between others ecological factors are scale-dependent.The latest studies demonstrated that landscape metrics provides an effective method in quantifying UHI pattern.However,issues concerning UHI are highly complex and variable.The study of impacts of changing scale on UHI should be strengthened.A common objective of scale related research in UHI is to find the underlying mechanisms by use of the selected scales.The purpose of this study is to identify the scale characteristics and scale domain of UHI pattern,and provide basic information for pattern analysis and scaling in UHI research. In this paper,taking Pearl River Delta as an example region,we obtained land surface temperature(LST) from Landsat TM by mono-window algorithm.Then mean-standard deviation method was employed to transform LST into thematic map of thermal categories.The basic spatial unit is 30m.By the scale with pixels on side of the grid cell,2、3、4、5、6、7、8、9、10、15、20、25、30、35、40、45、50、60 and 70 basic cells were assembled using software ArcGIS.Several landscape metrics,including patch density(PD),Percentage of Landscape(PLAND),Largest Patch Index(LPI) and Shannon′s Diversity Index(SHDI),were computed to detect the UHI pattern in different spatial scales.The scale effects of spatial grain sizes were analyzed under class and landscape levels. The results demonstrated that as the spatial grain size increased,dominance of high thermal class decreased and transformed dramatically into neighbor thermal class.Scale effects appear significantly in class and landscape levels.The responding characteristics were varied by specific thermal class and landscape metrics.PD and FRAC_MN in both class and landscape levels were easier to predicted,indicating these two metrics could be treated as good methods of scaling in UHI pattern.Thresholds of grain size were obvious,such as grain size 150m of PD,PLAND,CLUMPY and CONTAG in the region.The appropriate grain extent(30—150m) was detected for calculating landscape metrics of UHI pattern.According to scale inflexions and response curves discussed in the paper,scale domains of landscape matrices were confirmed.Generally,from 30m to 150m was the scale domain to UHI pattern.That means that related ecological model of UHI can be scaled across this scale extent by ordinary transformation method.This study may offer useful information of UHI mechanism and scaling of UHI spatial model in future research.