Thermal Environment And Its Related Influencing Factors Of Urban Parks And Their Vicinity Area In Beijing,China

Rapid urbanization has induced urban heat island effect,which is the cause of several severe environmental issues and puts great threats on public health.While the construction of urban green spaces has been widely proved as an effective approach to mitigate adverse effects of urban heat island.Related realms are still in need of further empirical evidence,especially on the distribution of microclimate within urban parks,park cool island intensity and its influence on local climate etc.Field measurement of air temperature and relative humidity was conducted in several urban green spaces and their vicinity areas located at the core area of Beijing.Microclimate characteristics and influencing factors within urban parks,park cool and wet island intensity and the contribution of urban greening were thoroughly investigated.This empirical research aims at providing reference for urban planning and design of urban green spaces.Main results are listed below.(1)Microclimates within Zizhuyuan Park and Tiantan Park are highly heterogenous.Significant air temperature and relative humidity gradients are detected within two parks.Vegetation plays potent cooling and humidifying effects.In Zizhuyuan Park,for every 10%increase of vegetation coverage within 50m buffer zone,air temperature at night in summer shall drop 0.18?.In Tiantan Park,different vegetation types have different effects on microclimate.Deciduous trees have significant cooling and humidifying effects at noon in summer(?=-0.67,?=0.65).Evergreen trees,though have little cooling and humidifying effects in summer,have a significant humidifying effect in winter(?=0.58).Grassland is found to have an effect of increasing air temperature at noon in summer and winter(?=0.48,?=0.52).The shape of vegetation coverage represented by averaged weighted mean shape index is not correlated with microclimate parameters.Among all influencing factors,radiation condition above sampling points plays the most prominent role at noon in summer and winter.Increasing vegetation coverage and using trees shading,especially deciduous trees,may effectively regulate microclimate within urban parks in Beijing.(2)The selected 10 urban parks within the 4^th Ring Rd.of Beijing are cool and wet island.Park cool island intensity ranges 0.11-2.54?at noon and 0.36-3.22?at night in summer,with an average of 1.09?and 1.69?respectively,while their park wet island intensity ranges 1.64-4.69%at noon and-5.79-12.11%at night in summer,with an average of 3.63%and 6.52%respectively.Park cool and wet island intensities are significantly correlated with park size.For an increase of 100ha of park size,park cool island intensity may increase 0.9?,while park wet island intensity may increase 2.9%.However,no threshold of park size is detected.Moreover,neither the shape of urban park nor the composition represented by the percentage of different land cover types are found correlated with park cool or wet island intensity.Local scale climate of around 400-1000m radius is affected by urban parks.Multiple linear regression models show that at night in summer,influence of park size on air temperature and relative humidity out of urban parks is the most prominent,while relative distance to urban park and the vegetation coverage around sampling points have similar contributions.Large urban parks have strong influence on local scale climate,while enhancing vegetation coverage in urban area out of parks is also effective on regulating urban climate.(3)Significant air temperature and relative humidity difference is detected along 2^ndRing Rd.of Beijing,with significant higher air temperature and lower relative humidity detected along the west 2^nd Ring Rd.Urban vegetation has a significant cooling and humidifying effect.For every 10%increase of vegetation coverage within 1000m buffer zone,air temperature at night in summer shall drop 0.27?,while relative humidity shall increase 1.66%.Complicated shapes of vegetation may facilitate the cooling and humidifying effect at night in summer and winter.Different local climate zones have different contributions on local scale climate.Zones with compact buildings and low vegetation coverage(LCZ 1 and LCZ 2)may significantly increase air temperature and decrease relative humidity,while zones with scatter buildings and high vegetation coverage(LCZ 3 and LCZ 4)have the opposite effects.Increasing urban vegetation,especially green spaces in commercial and residential areas may effectively regulate local scale climate and eliminate the adverse effect of urban heat island.