Along with rapid urbanization

urban heat island（UHI） has become one of the most serious urban problems

because of its impacts on the urban microclimate

air quality

energy consuming

public health and so on.With the advent of thermal remote sensing technology

remote observations of UHIs become the focus of urban remote sensing.While some progresses have been made

remote sensing study of UHI has been slow to improve qualitative description of thermal patterns and simple correlations.As a common indicator of UHI

magnitude is often used to reflect the degree of UHI occurrence

and it can be computed as the different indicator between the temperatures of urban surfaces and rural surfaces.However

the extent of UHI is often neglected when measuring an UHI as a whole.In order to explore a quantitative and more effective indicator for UHI dynamic monitoring at regional scale

aiming at deriving its spatial feature and facilitating the comparisons between different UHIs occur at different time or different areas based on remote sensing imageries such as NOAA AVHRR and Terra/Aqua MODIS data

this paper proposes a new parameter-UHI volume to integrate UHI magnitude with extent effectively.After the subtraction of the rural contribution in the land surface temperature（LST） image

the isolated UHI signature is suitable to use a least-squares Gaussian surface and then the UHI volume is calculated as a double integral of the UHI signature function over its footprint

to which the extent UHI happens.This study investigates the applicability of this volume model based on examination of thirty EOS-Terra MODIS level 1B imageries of Beijing city and its surrounding suburban areas

and the imageries were acquired between 2004—2006.These imageries include fifteen nighttime scenes

with their corresponding daytime scenes.Firstly

the resampled digital elevation model（DEM） data of the study area

the normalized difference vegetation index（NDVI） and the modified normalized difference water index（MNDWI） are used to extract the urban areas.Secondly

a simplified method is applied to retrieve the land surface temperature from MODIS channel 31 and channel 32

accounting for the land surface emissivity.Thirdly

four transects across the center of Beijing city at different directions including N-S

W-E

NW-SE and NE-SW are selected to detect the UHIs to ensure the application of the volume model is valid.The detections reveal that each of the UHI in Beijing City at different time has a single core and its spatial distribution is symmetrical as a whole

despite there are some small UHIs in the outskirts

so the volume model presented is appropriate for the UHI simulations.Results of the UHI simulations demonstrate that:（1）The correlation between the modeled UHI signatures and the corresponding true values is close

especially for the nocturnal UHIs.This fact indicates that the Gaussian is suitable and the volume model is valid for UHI simulation at regional scale in Beijing.（2）There is obvious UHI effect in both daytime and nighttime in the summer.In the spring

autumn and winter

there is UHI effect in the nighttime

while no UHI in the daytime;the UHI magnitude and volume shows that the diurnal UHI is intenser than the nocturnal UHI

and the difference of the volume between the diurnal and nocturnal UHIs seems to be stable.（3）Because of different dominant factors including natural and anthropogenic factors and their different influences

the changes of the nocturnal UHIs are complicated

especially the extents of UHIs.The UHI magnitude and volume shows that the nocturnal UHI effect is intensest in the winter

while weakest in the summer.The application in Beijing suggests that the volume model may be a promising routine to measure UHIs quantitatively based on remote sensing imageries.However

the precondition of this model urges that its applicability in other cities should be varified further.