不同类型冷却屋顶方案对城市群热环境的缓解效果
Mitigation effect of different cool roof schemes on thermal environment of urban agglomeration
ZHANG Mi ,1, MA Hong-Yun ,1, LIN Hui-Jiao1, LI Hai-Jun2, WANG Ying1
引言
IPCC第五次评估报告指出,过去30年间的地表温度呈明显的上升趋势,未来将导致海平面上升、极端天气事件增多等影响[1,2,3]。为降低气候变化的风险与影响,联合国气候峰会通过《巴黎协定》,提出将全球平均升温控制在相对于工业化前水平2℃以内,并努力将其控制在1.5℃以内[4,5]。全球气候变暖同时与城市化进程加快有关[6,7]。随着全球城市化进程的不断推进,城市下垫面由水、土和植被构成的自然表面大量转变为由混凝土、沥青等构成的人造表面,植被覆盖度降低,土壤水分减少,更易吸热升温,加上人类活动产生的热量,从而导致城市热岛效应[8,9]。城市热岛效应不仅影响区域气候环境、引发城市高温、增加城市居民的健康风险[10],还能加剧城市能源消耗和大气污染,导致城市生态环境质量下降[11]。因此如何缓解城市热岛效应引起了国内外专家学者的广泛关注和高度重视。
近几十年来,大量研究针对人类活动与城市热岛效应之间的相互作用,并进行了一系列的模拟实验。这些研究通过分析热岛效应的形成因素,结合实际案例提出了许多有针对性的缓解措施[12,13,14,15],包括强化城市绿化、增加水体面积、改进建筑材料、调整土地利用格局和减少人为热排放等。由于城区高密度建筑物占比较大,可利用的闲置土地极为有限,许多研究人员认为屋顶方案的可行性比较高[16,17]。屋顶占城区面积很大的比重,美国城市的屋顶占城市区域的面积为20%~25%,而全世界城市屋顶的面积接近3.8×1011 m2[18]。冷却屋顶方案通常包括采用高反照率屋顶和屋顶绿化,高反照率屋顶是采用直接反射太阳辐射从而达到降温的方法,而屋顶绿化是将能量的分配倾向为潜热的方向,使得城市蒸发量增大,湿度增加。
目前,冷却屋顶的设置能有效缓解城市热岛效应并降低建筑能耗已经得到了广泛的验证[19,20]。Campra等[19]对西班牙阿尔梅里亚地区气温的多年观测结果表明,由于高反照率屋顶的大量使用,该地区气温有明显的降低趋势(0.3 K/(10 a))。Courts等[16]比较了澳大利亚夏季4个实验屋顶的隔热性能和辐射收支,指出结合隔热层的高反照率屋顶可能会在缓解热岛效应和建筑传热方面提供最大的整体效益。Amir等[14]针对洛杉矶的研究指出,冷却屋顶的使用能够降低住宅建筑24%~41%的能耗。Francis等[20]对冷却屋顶的研究进行了整体评估,结果表明冷却屋顶可降低街区尺度气温在0.03~3.00℃之间,且主要集中在一年中温度较高的时段,同时能够降低年建筑能耗7%~90%。
近年来,针对冷却屋顶方案的研究大多是通过中尺度数值模式实现的[21,22,23,24,25]。多数模拟结果指出,冷却屋顶方案的降温幅度随反照率及绿化面积的变化呈线性变化[21,26-27]。Santamouris[21]的研究指出,屋顶反照率每增加0.1,平均环境温度会降低0.10~0.33 K。Li等[25]的研究指出,将30%的屋顶替换为绿色屋顶,或使用反照率为0.7的屋顶,都能使华盛顿市区地表温度降低1℃。Liu等[26]的研究指出,绿色屋顶在夜间的降温效果较强,高反照率屋顶在白天较强,但整体而言当屋顶反照率高于0.68时其平均降温效果要优于绿色屋顶。Sun等[27]对京津冀地区的模拟结果表明,使用高反照率屋顶可以有效抵消由城市扩张带来的城市热岛效应。
但不同地区的试验结果存在很大差异,例如当屋顶反照率为0.85时,在芝加哥的核心密集城区内的降温幅度达4℃[28],而在京津冀地区则仅降低了0.51℃[29]。还有研究指出冷却效果与天气条件有关,晴好天气条件下,高反照率屋顶对于热岛的缓解效果较好[30];但是在温和及寒冷的气候条件下,绿色屋顶的缓解效果更明显[31]。由此可见,不同降温措施的缓解效果会受到气候状况、城市形态特征及模拟方法的影响,因此有必要对特定研究区域来做具体的分析。其次,多数研究仅选取夏季高温热浪时段进行模拟研究[23,24,25,26],很少有研究对比夏季高温天气与普通夏日的缓解效果之间的差异,进一步量化冷却屋顶在高温热浪天气下的缓解作用,并分析其影响机制。前人的研究表明,周边环境温度越高冷却屋顶的降温效果越好[20,24]。Cao等[32]对广州地区的模拟表明,高反照率屋顶的使用,在普通夏季日能降温0.8℃左右,在高温热浪天气的冷却效果达1.2℃。同时,以往的研究多数针对单个城市进行模拟分析,而随着中国城市化进程的加快,城市群已成为一个重要特征,城市群环境下的城市气候特征不仅受城市本身影响,也和周围城市相关[7,33],因此,冷却屋顶方案在城市群区域的缓解作用有待进一步量化。
本研究采用WRF模式3.9版本耦合单层城市冠层模式(WRF/UCM),以长三角城市群为研究对象,将其2013年夏季分为高温热浪天气和普通夏日,模拟研究高反照率屋顶、绿色屋顶对城市群区域热岛效应的缓解作用,并分析其影响机制,从而为缓解城市高温、节能减排、降低气候变化的风险研究提供理论依据。
1 研究方法
1.1 模拟区域及参数化方案选取
本研究的模拟时间为2013年夏季（6月1日—8月31日),根据中国气象局对高温热浪事件的定义,即日最高气温达到或超过35℃且持续3 d以上的高温天气过程,认为2013年夏季共有4次热浪过程,分别为7月2日—7月5日、7月9日—7月12日、7月24日—8月2日和8月5日—8月18日,共32 d。据此,本研究中将这32 d认为是高温热浪天气,将其余时间认为是普通夏日,以区分两种天气造成的不同影响。
模式采用双重嵌套设置,模拟中心位于（31°N,120.5°E),格点数为111×98和76×76,网格距为9 km和3 km,其中最内层包含了长三角城市群的主要城市。初始条件和边界条件使用NCEP/NCAR提供的每6 h一次1°×1°的全球再分析资料。模拟区域的嵌套设置如图1(a)所示,图1(b)展示了内层模拟区域的下垫面覆盖类型,本研究中的地形资料已更新为ChinaLC 2010年的土地覆盖数据[34]。
图1
图1 WRF模拟的双重嵌套设置(a)和研究区域下垫面的土地覆盖类型(b)
注：黑色圆点代表30个观测站点的位置,CZ代表常州城区的位置,HZ代表杭州城区的位置。
Fig. 1 WRF simulation of double nested settings (a) and land cover type (b) of underlying surface in the study area (Black dots represent the locations of 30 observation stations, CZ represents the location of Changzhou city area, HZ represents the location of Hangzhou city area)
物理参数化方案以李海俊等[35]对长三角城市群的气候研究为基础,微物理过程采用Purdue Lin方案,长波辐射采用RRTM方案,短波辐射采用Dudhia方案,近地面层采用Monin-Obukhov方案,陆面过程采用Noah陆面模式,边界层采用YSU方案仅在最外层使用Grell-Freitas积云参数化方案。
1.2 试验方案及参数设置
文中设计了1组对照试验和8组不同冷却屋顶方案的敏感性试验,具体参数如表1所示。由于本研究中仅涉及屋顶方案的比较,因此参数的设置仅针对屋顶参数,其他参数使用模式默认方案。其中对照试验（CTRL）假设城市屋顶都由混凝土构成,其反照率为0.2,且未开启绿色屋顶方案,同时,参考Lazzarin等[36]的研究,将屋顶导热率设置为1.0 W/(m·K),热容量设置为1.0×106 J/(m3·K),比辐射率设置为0.9。高反照率屋顶方案(HR1~HR4)仅改变屋顶反照率,屋顶绿化方案（GR1~GR4）均开启绿色屋顶的分层计算方案,基于Yang等[37]在2014年对UCM屋顶方案的改进：在传统的混凝土屋顶上依次增加排水层、生长层、顶部土壤-植被层,其中叶面积指数和土壤含水的参数设置参考Sun等[38]的研究。
表1 试验方案和屋顶参数设置
Table 1
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2 模式性能检验
模拟时间段为2013年6月1日—8月31日,选取长三角城市群范围内30个自动气象站的观测数据,与对照试验（CTRL）模拟的2 m平均气温、最高气温和相对湿度进行对比,以检验模式性能。结果如图2所示：模拟的上述3个气象要素不仅与观测数据变化趋势相同,而且模拟结果与观测结果偏差较小,表明模式对长三角地区的模拟效果较好。同时,对照试验在以观测数据选取的高温热浪天气下,日最高气温也都能达到此标准。模拟的日平均气温、日最高气温和相对湿度与观测值的平均偏差分别为0.21℃、0.27℃和3.48%,相关系数分别为0.972、0.967和0.884,均方根误差分别为0.96℃、1.26℃和5.97%。
图2
图2 对照试验2 m平均气温、最高气温和相对湿度的逐日模拟值（CTRL）与观测值（OBS）对比
注：阴影部分为选取的高温热浪时段。
Fig. 2 Comparison of simulated values (CTRL) with observed values (OBS), including daily mean temperature (a), maximum temperature (b), and relative humidity (c) (The shadow parts are the selected heat-wave periods)
3 结果分析
3.1 不同比例冷却屋顶对气象要素的影响
图3显示了不同的高反照率屋顶和屋顶绿化敏感性试验模拟的温度和相对湿度与对照试验的差值。可以看出,温差及湿度差与屋顶参数之间呈很强的线性关系,前人的研究中也发现了相似结论[26,27]。总体而言,冷却屋顶方案都具有降温增湿的效果,且相同的方案在高温热浪天气下的降温增湿效果明显优于普通夏日。
图3
图3 不同比例冷却屋顶的最低气温、平均气温、最高气温和相对湿度与对照试验的差值
Fig. 3 Difference between the minimum (a, b), mean (c, d), maximum (e, f) temperature and relative humidity (g, h) of different proportion cooling roof and the control test
比较不同方案的降温效果,最好的为屋顶反照率为1.0的HR4方案,Courts等[16]对澳大利亚实验屋顶的观测结果也表明,夏季高反照率屋顶的降温效果比屋顶绿化更明显;其次,反照率为0.8的HR3方案和绿色屋顶比例为100%的GR4方案的缓解效果相近。从图中可以明显看出,最高气温的降温效果最好,平均气温次之,最低气温的降温效果最差,说明冷却屋顶在白天的缓解效果远好于夜间。比较两种屋顶方案可以发现,绿色屋顶对最低气温的降温效果优于高反照率屋顶,而高反照率屋顶对最高气温的降温效果优于绿色屋顶,这可能由于绿色屋顶方案在夜间也能通过蒸发作用产生冷却效应,而高反照率屋顶主要通过白天对太阳短波辐射的反射作用进行降温。就最高气温而言,屋顶反照率每增加0.1,在普通夏日会降温0.16℃,在高温热浪天气下会降温0.19℃;而绿色屋顶比例每增加10%,在普通夏日会降温0.07℃,在高温热浪天气下会降温0.09℃。
同时,以往的研究表明,夏季建筑物的制冷能耗与制冷度日数（一段时间内,室外平均温度高于26℃的度数累加）之间呈近似的线性关系[39],因此制冷度日数多用于估计建筑物制冷的能源需求。本试验中,普通夏日的日平均制冷度日数在对照试验中为3.76℃·d,HR4为3.51℃·d,GR4为3.56℃·d;高温热浪天气的日平均制冷度日数在对照试验中为6.31℃·d,HR4为5.38℃·d,GR4为5.62℃·d。HR4和GR4的制冷度日数在普通夏日分别降低了6.6%和5.3%,高温热浪天气下分别降低了14.7%和10.9%,大约为普通夏日的2倍,因此在高温热浪天气下应用冷却屋顶能够节约更多的制冷能耗。
比较不同方案的增湿效果,绿色屋顶方案的增湿效果平均优于高反照率屋顶,这是由于绿色屋顶主要通过增加土壤含水量、增大蒸发作用以达到冷却效果。
相比于普通夏日,高温热浪天气下的降温增湿效果更好,高反照率屋顶和绿色屋顶对最低气温的降温效果分别增大87.0%和50.0%,对平均气温的降温效果分别增大38.5%和34.9%,对最高气温的降温效果分别增大23.8%和25.0%,其增湿效果分别增大29.5%和21.9%。
3.2 冷却屋顶对城市热岛效应的影响
为了研究冷却屋顶的设置对城市热岛效应的影响,本文将城市格点平均气温与郊区格点平均气温的差值定义为城市热岛强度。同时,冷却屋顶对热岛效应的缓解作用可量化为冷却屋顶方案与对照试验之间热岛强度的差异。
图4为冷却屋顶方案缓解热岛效应的平均日变化。从不同试验中热岛强度（UHI）的日变化可以看出（图4a和图4b),不同比例的高反照率屋顶（HR1~HR4）和绿色屋顶（GR1~GR4）与对照试验（CTRL）的整体城市热岛变化趋势基本一致。在夜间,热岛效应的强度较强,且高温热浪天气的UHI明显强于普通夏日,平均高1℃左右。这是由于在白天,城市比乡村储存了更多的热量,且夜间郊区的保温作用不如城市,日落后迅速降温,温差继续扩大;在白天,热岛强度均有所减弱,且多数冷却屋顶方案在白天甚至成为一个“冷岛”,可以有效缓解城市热岛效应。
图4
图4 不同冷却屋顶方案与对照试验热岛强度的平均日变化
Fig. 4 Average diurnal variation of heat island intensity (UHI) of different cooling roof schemes and control test (CTRL)
图4(c)和图4(d)为冷却屋顶方案与对照试验的热岛强度差值。从图中可以看出,高温热浪天气下冷却屋顶方案对热岛效应的缓解效果均强于普通夏日。同时,虽然UHI在夜间较强,但大部分冷却屋顶的缓解效果均在白天更强一些。夜间绿色屋顶的缓解效果较好,而白天高反照率屋顶的缓解效果更好,这与前文的分析一致。此外,高反照率屋顶方案在12时左右降温幅度最大;其中HR4的缓解效果最好,其普通夏日的UHI最大降低1.1℃,高温热浪天气最大降低1.36℃。
3.3 冷却屋顶的城市群放大效应
由前文的分析可知,反照率为0.8的HR3方案和绿色屋顶比例为100%的GR4方案的缓解效果相近,前人对冷却屋顶的模拟研究中有相似结论[22],因此后文将重点分析这两种方案的缓解效果及其机制。图5为HR3和GR4与对照试验的平均气温差值。从图中可以看出,两种方案的降温范围和幅度相似,且均在高温热浪天气下的冷却效果更好,这与前文的分析一致。从图5中也可以看出,城市格点密集的区域,其降温效果明显强于分散的城市区域,例如,相较于杭州和绍兴地区,苏州、无锡、常州和上海地区能够产生更强的降温效果。
图5
图5 HR3和GR4与对照试验（CTRL）的平均气温差值
Fig. 5 Average temperature difference between HR3, GR4 and control test (CTRL)
为了分析冷却屋顶对城市群与单独的城区之间不同的降温效应,选取城市群内的常州城区和单独的杭州东北部城区进行对比分析。两片区域的总格点数都为132,其中常州区域的城市格点为114,杭州区域为122（略多于常州区域)。表2为常州与杭州在不同情景下的降温增湿效果,可以看出,无论在哪种情景下,常州区域的缓解效果均明显优于杭州区域,其中常州降温幅度比杭州高0.15~0.23℃,平均高32%;增湿幅度为0.6%~1.2%。因此,在建筑密集的大城市群区域采取冷却屋顶方案,其缓解效果可能优于单个城市,且未来城市面积将进一步扩大,冷却屋顶方案仍有很大的降温空间。
表2 常州与杭州的降温增湿效果比较
Table 2
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3.4 冷却屋顶的降温机制分析
高反照率屋顶和屋顶绿化虽然都有降温效应,但其作用机制并不相同。已知地表能量平衡方程为
WS1 + WL1 = WS2 + WL2 + QSH + QLH + QG。
(1)
其中WS1和WS2分别为向下和向上的短波辐射,WL1和WL2分别为向下和向上的长波辐射,QSH为感热通量,QLH为潜热通量,QG为热存储量。高反照率方案的净辐射通量由以下公式计算：
Rn = (1-α)WS1 + WL1-WL2。
(2)
其中Rn为净辐射通量,α为屋顶反照率。高反照率屋顶通过增加屋顶反照率,减少入射的太阳短波辐射以减少净辐射通量,从而达到降温的效果。
屋顶绿化方案是在混凝土屋顶上覆盖植被,使其蒸腾、蒸发作用增加（QLH增加),以降低感热通量（QSH减小),冷却屋顶。
图6描述了HR3、GR4和对照试验的城市地表能量平衡的平均日变化。由于耦合的UCM模式只作用于城市格点,因此图6仅针对城市格点的辐射通量进行分析。就净辐射通量而言（图6a和图6b),在白天,HR3的值显著低于对照试验和GR4,这是由于高反照率屋顶方案通过增大屋顶反照率,减少输入的短波辐射以减少净辐射通量;且相比于普通夏日,在高温热浪天气下净辐射通量有所增加,但是HR3的增长幅度明显低于对照试验和GR4,净辐射通量的最大值在对照试验中增加87.7 W/m2,GR4增加95.3 W/m2,HR3增加44.4 W/m2。屋顶绿化的降温机制是通过蒸发作用使潜热通量大大增加,在图6(e)和图6(f)中,GR4的潜热通量远高于对照试验和HR3,且其最大值在高温热浪天气下较普通夏日增长66.7 W/m2,而对照试验和HR3的增长不到10 W/m2。
图6
图6 HR3、GR4和对照试验（CTRL）的城市地表能量平衡的平均日变化
Fig. 6 Average daily variation of urban surface energy balance in HR3, GR4 and control test (CTRL)
冷却屋顶方案最终都是通过减小感热通量以减少地面输送至大气的热量,达到冷却效果,在图6(c)和图6(d)中,HR3和GR4的感热通量都显著低于对照试验。同时,由上文的分析可知,在高温热浪天气下的缓解效果明显优于普通夏日。前人的研究也表明,冷却屋顶方案在温度高时的降温幅度更大[20,24]。由图6(c)和图6(d)对比可知,对照试验的感热通量在高温热浪天气下较普通夏日有明显增加,其最大值增加38.0 W/m2,而GR4和HR3的变化并不明显。在高温热浪天气下,由于入射的太阳短波辐射增多,GR4的净辐射通量明显增大（图6b),同时由于蒸发作用增加,其潜热通量也大幅增长（图6f);而HR3的反射作用导致其净辐射通量变化并不明显（图6b),因此两种冷却屋顶的感热通量均无明显变化（图6d)。综上,高反照率屋顶方案在更热的天气条件下能够更大程度地减少净辐射通量,绿色屋顶方案能够释放更多的潜热,因此两种冷却屋顶方案在更热的天气条件下均有更强的降温效果。
图6(g)和6(h)显示了两种冷却屋顶的热存储量变化高度一致,其绝对值均低于对照试验,且在高温热浪天气下均高于普通夏日。根据Li等[25]的分析可知,这些热量通常储存在屋顶,要么释放至大气,要么传导到建筑物内部,然后被空调抽到室外,被抽出去的热量代表了人为热的释放量。在白天,热存储量为负值,表示从大气向屋顶传递的热量,较小的热存储量可以提供较低的冷负荷并释放较少的人为热;在夜间,与对照试验相比,冷却屋顶释放屋顶-空气界面的热量较少,这与白天较低的热存储量一致。这可以解释图4中夜间的热岛效应更强的原因,白天储存在屋顶上的能量在夜间会从地表流失,导致大气变暖,使夜间的城市区域能够保持温暖,而农村地区则迅速降温。同时,在高温热浪天气下,冷却屋顶方案与对照试验的热存储量相差更大,缓解效果更好。
4 结论与讨论
本文利用WRF模式耦合单层城市冠层模式（WRF/UCM),以长三角城市群为研究对象,将其2013年夏季分为高温热浪天气和普通夏日,模拟研究不同冷却屋顶方案（高反照率屋顶和屋顶绿化）对城市热环境、湿度和近地面辐射通量的影响,从而为缓解热岛效应、降低能源消耗以及降低气候变化的风险研究提供理论依据。结果表明:
(1)冷却屋顶方案的缓解效果与屋顶参数之间呈很强的线性关系,同时,高反照率屋顶的降温效果更好,屋顶绿化的增湿效果更好。高温热浪天气下,HR4和GR4的制冷度日数分别降低了14.7%和10.9%,大约为普通夏日的2倍。
(2)在夜间,高温热浪天气的热岛强度明显高于普通夏日,平均高1℃左右;虽然热岛效应在夜间较强,但大部分冷却屋顶在白天缓解效果更好。其中HR4的缓解效果最好,其普通夏日的热岛强度最大降低1.10℃,高温热浪天气最大降低1.36℃。
(3)屋顶反照率为0.8的HR3方案和绿化面积为100%的GR4方案的缓解效果相近,比较其与对照试验（CTRL）的平均气温差值,可以明显看出,城市格点密集区域的降温幅度高于分散的城市区域,如在城市群中的常州区域较相对独立的杭州区域的降温幅度高0.15~0.23℃,由此可见,城市群对冷却方案的降温效应有一定加强作用。
(4)冷却屋顶方案均在高温热浪天气下缓解效果更好,分析辐射通量的日变化可知,高反照率屋顶方案在更热的天气条件下能够更大程度减少净辐射通量,绿色屋顶方案能够释放更多的潜热通量,因此两种冷却屋顶在高温热浪天气下其降温幅度均高于普通夏日。
据统计,2018年建筑耗能占全球能源消耗的32%,并具有很大的节能潜力[40]。从基础设施角度来看,绿色公共建筑的大规模推广是节能减排的重要途径,虽然高反照率屋顶和屋顶绿化可以有效缓解城市高温,降低夏季制冷能耗,但在实际情况下完成大范围的冷却屋顶还有很大的局限性。本文中由于资料等限制,模拟设计存在很多理想假设,如屋顶绿化方案中的土壤湿度设置为定值,一般情况下土壤湿度会随着时间变化而降低,从而导致潜热通量及降温作用减小。在未来的工作中,需要设计更合理的冷却屋顶方案,进行更符合实际情况的模拟。同时,本文的研究仅针对2013年夏季,存在一定不确定性,未来应进行更长时间尺度的模拟,为国家制定城市规划、推广绿色公共建筑提供更可靠的理论支撑。
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