Landscape Elements in the Lhasa Residential Area: Comparison
Please note this is a comparison between Version 2 by LIXING CHEN and Version 6 by Rita Xu.

Landscape elements have a direct impact on the thermal environment and have become an important means to improve the quality of life of residents. However, the selection and configuration of landscape elements have different effects on human thermal comfort under different climatic conditions. Lhasa is located in the middle of the Qinghai Tibet Plateau, with an altitude of 3650 meters. Due to the particularity of its geographical location, the climate conditions in Lhasa are significantly different from those in Beijing, Xi'an, Lanzhou and other cities in the same climate area. At the same time, the local residents are mainly Tibetans. Religious culture and climate conditions have led to significant differences in the living habits and the needs for thermal comfort of local residents compared with those of residents in other cities. Therefore, researcher focuses on special climatic conditions, taking the typical residential areas in Lhasa, Tibet as the research object, in order to better understand the impact of residential landscape elements on the outdoor thermal environment and thermal comfort in the plateau cold climate area.

景观元素因其对热环境的直接影响而成为提高居民生活质量的重要手段,但景观元素的选择和表征在不同气候条件下对人体的热舒适度有不同的影响。本研究以西藏拉萨典型居民区为研究对象,采用正交试验方法制备实验方案,利用ENVI-met进行模拟,探索高寒气候条件下绿色植被、水域、地面折射的侵入度,以及它们与冬夏热环境的相互作用。以生理当量温度(PET)为优化指标,确定了住宅景观元素协同效应的最优设计方案。结果如下。(1)景观构象因子的顺序如下:绿色构象>水域面积>夏季叶面积指数>地面折射。在冬季,目>水域面积>地面反射率>叶面积指数(LAI)进行绿色构象。(2)通过正交试验和数值模拟的结合驱动,确定了景观构件的最优方案,即“灌木草坪,水域面积比16%,地面折射率为0.5,LAI = 3 m2/m3”。(3)最后,针对拉萨居民区典型户外活动空间提出了景观构象的最优设计策略。

  • high altitude
  • landscape elements
  • outdoor thermal comfort
  • orthogonal experiment

1. Unique Geographical Location and Climatic Conditions of Lhasa

1. Background

Lhasa is located in the middle of the Qinghai Tibet Plateau, with an altitude of 3650 meters. The plane of the city of Lhasa is distributed in a belt from east to west. Two mountains (South Mountain and North Mountain of Lhasa) sandwich the city, and a river (Lhasa River) passes through the city. It is a typical axial development city, as shown in Figure 1. According to the Chinese standard meteorological data, the hottest month is June, and the coldest month is January. The monthly average maximum temperature is above 20°C, the monthly average minimum temperature is below −3°C, and the annual temperature difference is 18.1°C . Lhasa belongs to the cold region in China's architectural climate zoning, but because it is located in the Qinghai Tibet Plateau, the climate conditions of Lhasa are significantly different from those of Beijing, Xi'an, Lanzhou (cold climate region), and other cities. By comparing the enthalpy and humidity diagrams of the three cities (Figure 2), it can be seen that the annual temperature and the humidity of Lhasa are relatively lower, and the annual temperature difference is also relatively smaller. Beijing and Xi'an also have coexisting heat and humidity, but Lhasa has a dry climate, so there is no such phenomenon.

According to the sixth report of the Intergovernmental Panel on Climate Change (IPCC), global warming caused by human activities is 1.0 °C higher than before industrialization, global warming is continuing, and the risk of extreme weather events is further increasing [1,2]. The urban heat island effect (UHI) caused by climate change and anthropogenic changes in rapid urban development has directly led to the emergence of urban problems such as high temperature, drying, poor ventilation, and pollutant accumulation. These changes reduce the urban outdoor thermal environment and the thermal comfort of residents [3,4] and increase the energy consumption of urban buildings [5,6]. They also endanger human health, induce various health problems [7,8], and can even lead to death [9]. The China Climate Change Blue Book released by the China Meteorological Administration in 2021 pointed out that: “China is a sensitive area and a significant impact area of global climate change, and the heating rate is significantly higher than the global average in the same period [10]”. It has become crucial to take strong measures to improve the urban environment and improve the quality of the human living environment. There is already a large amount of research trying to find optimal solutions to mitigate the many challenges posed by the growing climate problem. From the perspective of urban planning, this is mainly reflected in two aspects: urban building layout and urban landscape (vegetation, water body, underlying surface, etc.). As a nature-based solution, urban landscapes are believed to play a key role in improving microclimates and mitigating heat island effects [11]. In addition to this, a good landscape environment can enhance residents’ well-being while improving residents’ comfort [12,13].

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2. Literature Review

Figure 1. City map of Lhasa.

确定如何基于自然的解决方案创造健康的户外热环境已成为一个热门的研究课题,改善居民区的室外热环境尤为重要,因为它们是许多户外活动的基础。与住宅区室外热环境和热舒适性密切相关的景观环境因素主要包括绿化、水体和地表。在绿化方面,在炎热和干旱地区,选择能够减少太阳辐射的树木是降低生理当量温度(PET)最有效的城市设计策略[14]。Middel等人的研究[15]证明,紧凑的建筑布局在白天对气温的影响大于绿化,在紧凑的低密度建筑区使用树木使温度降低了2°C,高密度建筑区的树木可以有效地将空气温度降低0.2-0.4°C[16],但这在开罗的低密度建筑区无效[17]。在炎热潮湿的气候中,Srivanit等人[18]和Zhuang等人[19]的研究证实,树木可以影响太阳辐射并改变风环境,显着降低平均辐射温度(Tmrt)和PET。树冠的结构以及叶子的大小,形状和颜色影响了太阳辐射的水平[20]。在寒冷气候下,除了改善夏季的室外热环境[21]外,树木还可以通过调整树木的布局来有效改善冬季的室外风环境[22]。在温带气候下,Lee等人[23]证实,树木和草坪的组合形式对Tmrt和PET的影响比单独的草坪更显着。在水体方面,Lu等人[24]以潮湿炎热的气候为背景,证实水体在白天变暖阶段具有降温作用。此外,晚间降温阶段起到了保温作用,具有加湿和调节风速的作用。在寒冷的气候条件下,滨水居民区湿热环境的影响规模很大程度上取决于体积比、路堤高度和绿化[25]。此外,地面的特性是影响居民热舒适度的重要因素[26]。在潮湿炎热的气候下,Kwan等人[27]、郭等人[28]和宋等人的研究[25]发现,不透水表面的温度高于透水面的温度,天然的低垫面具有较好的冷却效果。在温和的气候下,Santamouris等人[29]表明,具有高反射率的材料可以显着影响人体的热舒适度,但Taleghani等人的研究表明,Tmrt和PET都随着反射率的增加而增加。

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研究人员发现,景观元素对不同气候带区域热环境的影响之前已经研究过很多次,其研究结果也各不相同,但对各种景观元素之间协同作用机制的研究还比较缺乏。每个城市都处于特定的区域大气环境中,城市气候的差异影响景观要素对室外热环境的改善机制。中国西藏地区受到其特殊地理位置、气候条件和宗教文化的影响。景观元素对热环境的影响与其他气候带不同。当地居民每天进行大量的户外活动,频繁和长期的户外活动对户外环境提出了更高的要求。然而,对高海拔寒冷气候带的研究相对较少,很难为改善极寒气候下的生活环境质量提供理论支持。

Figure 2. Enthalpy humidity diagram and solar radiation intensity comparison diagram of Lhasa, Beijing, and Xi’an.

2. Characteristics of Residents' Behavior Habits

The Lhasa area is affected by strong radiation, and the demand for shade space in summer is greater than that in winter. There is also a greater objective demand for the comfort of the outdoor space thermal environment in summer and winter. Through a questionnaire survey, researchers examined the behavioral characteristics of outdoor activities of the local residents in Lhasa. The content of the questionnaire mainly included two parts: (1) basic information about respondents (gender, age, name, and ethnicity); and (2) respondents' behavior habits (activity time, activity form, and preferred activity venue).

3. The Impact of Landscape Elements on Outdoor Thermal Comfort under Different Climatic Conditions

Determining how to create a healthy outdoor thermal environment based on nature based solutions has become a hot research topic, and it is particularly important to improve the outdoor thermal environment in residential areas because they are the basis for many outdoor activities. The landscape environmental factors that are closely related to the outdoor thermal environment and thermal comfort of a residential area mainly include greening, water bodies, and underlying surface. In terms of greening, in hot and arid areas, choosing trees that can reduce solar radiation is the most effective urban design strategy for reducing physiological equivalent temperature (

因此,本研究为了更好地理解高海拔寒冷气候区住宅景观要素对室外热环境和热舒适性的影响,以拉萨为例,基于正交实验设计方法,对具有不同景观要素构象的居民区热环境进行了数值模拟。以PET) . The research of Middel et al. proved that compact building layouts had a greater impact on air temperature during the day than greening, the use of trees in compact low-density building areas reduced the temperature by 为优化目标,采用直观分析和方差分析对仿真结果进行分析。主要目标如下:

(1)阐明高海拔和寒冷气候条件下各种景观元素对室外热舒适性的主要和次要影响和优势。

2 °C, and trees in high-density building areas could effectively reduce air temperature by 0.2–0.4 °C, but this was ineffective in Cairo’s low-density built-up areas . In hot and humid climates, studies by Srivanit et al.  and Zhuang et al. confifirmed that trees could affect solar radiation and change the wind environment, signifificantly reducing the average radiation temperature (Tmrt) and PET. The structure of the canopy, as well as the size, shape, and color of the leaves, affected the level of solar radiation . In cold climates, in addition to improving the outdoor thermal environment in summer , trees can also effectively improve the outdoor wind environment in winter by adjusting the layout of trees . In temperate climates, Lee et al. confifirmed that the combined form of trees and lawns had a more signifificant effect on Tmrt and PET than lawns alone. In terms of water bodies, Lu et al. used a humid and hot climate as the background to confifirm that a water body had a cooling effect in the daytime warming stage. Additionally, the evening cooling stage played a role in heat preservation and had the effect of humidifying and regulating the wind speed. In cold climatic conditions, the scale of inflfluence of hot and humid environments in waterfront residential areas depends largely on the volume ratio, the height of the embankment, and the greening . In addition, the characteristics of the ground are an important factor that affect the thermal comfort of residents . In a humid and hot climate, the research of Kwan et al., Guo et al., and Song et al.  found that the temperature of an impermeable surface was higher than the temperature of a permeable surface, and a natural lower pad surface had a better cooling effect. In mild climates, Santamouris et al. showed that materials with high reflflectivity could signifificantly affect the thermal comfort of the human body, but the study of Taleghani et al. showed that both the Tmrt and PET increased with increasing reflflectivity. Researchers found that the impact of landscape elements on the regional thermal environment in different climatic zones has been studied many times before, and that their research results were also different from each other, but studies of the synergy mechanism between various landscape elements are still relatively lacking. Each city is in a specifific regional atmospheric environment, and the differences in urban climate affect the mechanism of improvement of landscape elements for the outdoor thermal environment. China’s Tibet region is affected by its special geographical location, climatic conditions, and religious culture. The impact of landscape elements on the thermal environment is different from those of other climatic zones. Local residents carry out a large number of outdoor activities every day, and frequent and long-term outdoor activities put forward higher requirements for the outdoor environment. However, there are relatively few studies on high-altitude cold climate zones, and it is diffificult to provide theoretical support for improving the quality of the living environment in extremely cold climates.

Therefore, in order to better understand the impact of residential landscape elements on the outdoor thermal environment and thermal comfort in a high-altitude cold climate area, taking Lhasa as an example, the thermal environment of a residential area with different landscape element confifigurations was numerically simulated based on the orthogonal experimental design method. The PET was taken as the optimization objective, and the simulation results were analyzed with intuitive analysis and variance analysis.)阐明高海拔和寒冷气候条件下景观元素及其与室外热舒适相互作用的协同关系。

(3)提出高海拔、寒冷气候条件下居民区景观要素配置优化的试验方案和策略。