China Meteorological News correspondent Dong Yongchun Wang Ting
Recently, the "Guangdong Province Air Pollution Prevention and Control Strengthening Measures and Division of Labor Plan" (hereinafter referred to as the "Strengthening Plan") issued by the General Office of the Guangdong Provincial Government proposed that "all localities should fully consider the long-term impact of climate change when planning, designing and approving urban infrastructure construction and renovation projects, and prohibit the construction of new high-rise buildings on urban ventilation corridors." This is also the first time that urban ventilation corridors have been written into Guangdong’s environmental protection policy.
In recent years, Beijing, Hangzhou, Wuhan, Xi ‘an, Chengdu, Guangzhou, Huizhou and other places have brought ventilation corridors into the vision of planning and decision-making. Among them, Guangzhou was listed as the second batch of national urban design pilot cities by the Ministry of Housing and Urban-Rural Development this year. The overall urban design of Guangzhou, which is being compiled, also requires further establishment of a more detailed ventilation corridor scheme.
Why do ventilation corridors attract the attention of more and more cities? How should it be planned and constructed, and will the areas along the line face large-scale demolition and construction? With these questions, the author interviewed Wang Chunlin, deputy director of Guangzhou Climate Center and a senior engineer at the research level.
Why build it?
-It can reduce air pollution and alleviate the urban heat island effect.
As early as two or three years ago, urban ventilation corridor has been included in the research field of Guangdong meteorological department. According to Wang Chunlin, the leader of the research team, the urban ventilation corridor is a passage for introducing fresh cold and humid air to the urban area with the purpose of improving the air mobility of the city, alleviating the heat island effect and improving human comfort. Generally speaking, the "ventilation corridor" is just like the "urban air duct", similar to a long and narrow ventilation duct, which can make the urban wind "pass through the hall".
"Our ventilation corridor planning is mainly based on the Technical Guide for Urban Ventilation Corridor Planning issued by China Meteorological Bureau at the end of 2015." Wang Chunlin said that many developed countries and regions have already started in this respect. Munich, Germany began the practice of introducing ventilation corridors into urban planning in the 1970s. Tokyo, Japan has long established a five-level ventilation corridor system combining mountains, valleys, sea, land and parks.
In 2006, the Hong Kong Planning Standards and Guidelines set out the definition and function of urban ventilation corridors (air ducts) in urban planning for the first time: ventilation corridors should be connected by large open areas, such as main roads, connected open spaces, beautification zones, non-construction sites, construction line backward zones and low-rise buildings. Ventilation corridors should extend in the direction of prevailing winds, and if feasible, other natural gas flows, including ocean, land and valley winds, should be maintained or guided to blow to urban areas.
Wang Chunlin believes that ventilation corridors have many advantages. "First, they can effectively alleviate the various impacts caused by urban heat island effect; second, they can help urban air flow and pollution diffusion; and third, they can greatly improve human comfort in terms of temperature and urban air quality."
Take Beijing as an example. In 2012, when the heat island area of six districts in the city center was serious, it accounted for 77% of the total area. In recent years, the trend of urban wind speed decreasing is also obvious. In this regard, He Yong, deputy director of the Planning Research Office of Beijing Urban Planning and Design Institute, said that the ventilation corridor is not to "create wind", but to make the cold air in the suburbs flow to the urban area more smoothly and "change" the hot air in the urban area, thus effectively reducing the urban temperature.
How to lay out?
-big data analysis of wind environment and planning according to local conditions
For cities with different geographical conditions, how to set the direction and route of ventilation corridors?
At present, the team led by Wang Chunlin is studying and experimenting to analyze the urban wind environment through huge data analysis and computer simulation experiments, which is an important basis for planning urban ventilation corridors. He introduced that at present, the analysis of urban wind environment mainly adopts the Mesoscale Meteorological Model (WRF), which is a new generation of mesoscale forecasting model and assimilation system jointly developed by American meteorological circles. The land surface process model in WRF is coupled with the Urban Canopy Model (UCM). By adjusting the regional urban parameters such as urban reflectivity and roughness, it has a good simulation ability for urban heat storage effect, flow field and precipitation distribution.
"To analyze the wind environment, we must first master a large number of urban underlying surface data and wind observation data." Regarding the design principle of urban ventilation corridor, Wang Chunlin said that the data of urban underlying surface include the distribution, height and geometric structure of buildings, as well as the distribution of terrain height, green space and water bodies. Wind observation data include wind force, wind direction and other elements. Taking Guangzhou as an example, the available wind observation data include the data of five national weather stations in Guangzhou in recent 30 years and about 300 regional weather stations in recent 10 years.
"Secondly, the fine-grid meteorological dynamic model is an important tool to carry out urban wind environment analysis and ventilation corridor planning effect evaluation." Wang Chunlin said that Guangzhou Meteorological Department has a large number of historical data and 400 trillion supercomputers per second, which can make statistical analysis on the output results of massive models and quantitatively compare the ecological benefits of different ventilation corridor planning schemes.
In the planning process, adapting to local conditions is an important principle. On the one hand, the existing ecological cold sources such as green space, parks, forests, rivers and lakes should be included in the wind gallery, which can better protect the ecological cold sources and make them play the role of purifying the air and reducing the urban heat island effect. On the other hand, the existing urban pattern should be fully respected, and the planning of wind corridors should try to avoid the existing large-scale buildings and avoid a large number of demolition.
"Just like urban roads have roads and paths, ventilation corridors are divided into first-class corridors and second-class corridors." Wang Chunlin said that the first-class corridor is equivalent to the main road and is the main ventilation corridor; The second-level corridor should be built along the area with great ventilation potential, and try to make up for the "broken" corridor area that the first-level corridor cannot guarantee.
At present, there are five or more main ventilation corridors in Beijing, Hangzhou, Chengdu and other cities, and there may be more secondary corridors.
On the principle of division of labor and design, the design of the first-class ventilation corridor should be as parallel or nearly parallel as possible to the dominant wind direction of the city, and make full use of the ecological cold sources such as green space, mountains and rivers with large area and good ecological environment. The setting of the secondary corridor should play the role of assisting and extending the ventilation efficiency of the primary ventilation corridor, and communicating and connecting the local ecological cold source and the areas with poor wind environment. With the help of urban trunk roads, squares, water systems and other open spaces, the final "wind corridor network" should be formed. The setting of the width, length, direction and land use control of the wind corridor should be combined with the refined wind and heat distribution characteristics and realistic construction conditions in the urban area.
What are the design principles?
-determine the direction of the wind corridor according to the prevailing wind direction in the city.
During the implementation of urban ventilation corridor planning, the stipulation that "no new high-rise buildings are built on urban ventilation corridors" mentioned in Guangdong’s "Strengthening Plan" has attracted much attention.
Does this mean that the existing buildings will be demolished and built? Wang Chunlin believes that the existing high-rise buildings should not be demolished artificially, but should be demolished by natural elimination after reaching the service life, so as not to waste resources.
It is forbidden to build new high-rise buildings on urban ventilation corridors for two reasons: first, high-rise buildings will increase the roughness of urban underlying surface and reduce the air circulation efficiency of air corridors, which is the bottleneck area of air flow. Second, the high-rise building on the wind gallery has a large windward wind speed, which has many disadvantages: a large area of quiet wind zone is formed at the back of the high-rise building, which will be extremely stuffy in summer and affect comfort; If there are pollutants in the back of it, it will cause air pollution in the near-surface layer and affect human health.
"Even if some buildings are very tall, whether they will have a great impact on the wind gallery requires specific analysis." Wang Chunlin said. It is assumed that the new central axis of Guangzhou, which extends from Tianhe North Road to flower city square, is also set as a ventilation corridor, and the towering small waist (also known as Guang Zhouta) is just in the middle below the central axis. However, due to the slender structure of the small waist building and the hollow design, it will not cause great obstruction to the wind corridor.
The prevailing wind direction in the city is the decisive factor affecting the direction of the wind corridor. Wang Chunlin said that taking many cities in the Pearl River Delta, such as Guangzhou, for example, the southeast wind prevails in summer and the northerly wind prevails in winter, and the north-south wind corridor is conducive to utilizing the ecological cold source in Conghua and other places in the north and the ocean cold source in the south.
At present, the Master Urban Design of Guangzhou, which is being compiled in Guangzhou, also clearly requires that the main corridors where the dominant wind direction enters the city should be determined, and the strategy of "making wind, gathering wind and ventilating" should be put forward to establish the urban wind corridors, water corridors, green corridors and visual corridors in Guangzhou.
How to implement the plan?
-there is still a lack of unified standards, and all localities are exploring.
When the Eight Immortals crossed the sea, they all showed their magical powers. In fact, at present, there is no uniform standard for urban ventilation corridors in mainland China, and all localities are still groping for it in practice.
Wang Chunlin revealed that Guangzhou was listed as the second batch of urban design pilot cities this year. In this year’s overall urban design of Guangzhou, the planning and design department initially planned and designed the ventilation corridor system of Guangzhou based on the numerical simulation of Guangzhou’s wind environment.
However, many cities that are more advanced in the research and planning of ventilation corridors also face some confusion in practice. For example, in many ventilation corridors planned and built in Beijing, there are still many high-rise and dense building communities, which will become the bottleneck area of air flow.
Wang Chunlin believes that the planning and improvement of urban ventilation corridors need to be fully discussed and communicated with urban planning departments to jointly promote the planning and implementation of ventilation corridors.
In fact, in addition to directly planning urban ventilation corridors, many places have also taken some measures to improve urban microclimate. For example, Shenzhen will treat the first floor of real estate as an overhead floor into the overall urban planning, and rationally arrange the layout of urban buildings; Thirty pilot cities, including Xiamen, Chongqing and Nanning, have accelerated the construction of "sponge cities".
Wang Chunlin revealed that the changes of local ventilation volume and heat island intensity before and after ventilation corridor planning can be accomplished by means of numerical simulation, field observation or wind tunnel test. Among them, the fine simulation of local wind environment can be carried out by computational fluid dynamics (CFD). He introduced that CFD is a combination of modern fluid mechanics, numerical mathematics and computer science. Using electronic computer as a tool, it applies various discrete mathematical methods to conduct numerical experiments, computer simulations and analytical studies on various problems of fluid mechanics in order to solve various practical problems. However, due to the huge demand for computing resources, CFD technology is usually applied to the simulation and analysis of wind field in residential areas, and a wider range of applications can be tried in the planning of urban ventilation corridors in the future.
Link: Controversy about Ventilation Corridor
There is a classic line in the movie "Be there or be square": Blast the Himalayas into a 50-kilometer-wide hole, let the warm and humid air from the Indian Ocean blow into the Qinghai-Tibet Plateau through Nepal, completely change the harsh ecological environment there, take off the backward hat there, and turn the Qinghai-Tibet Plateau into a beautiful and rich land of fish and rice. This line is also used to ridicule the ventilation corridor. Indeed, a new engineering design is still in the stage of scientific demonstration, which will cause a lot of controversy.
One of the focuses of controversy is whether the smog can be blown away. Opponents believe that the main reason for the smog in winter is that the cold air is weak, and the warm and humid air from the south goes north, and after the stalemate, it forms static weather. If you want to disperse the smog through the ventilation corridor, you must have wind first, but there is no wind in calm weather. Supporters believe that the existing research proves that the formation of ventilation corridor network in the central city is effective in improving the microclimate and has a certain auxiliary role in promoting the diffusion of pollutants.
Another focus of controversy is whether successful experiences can be replicated. According to reports, there is a burning wind every year in Munich, Germany, and five urban ventilation corridors are planned and built locally to let the burning wind pass through the city and take out the dirty air in the city. The effect is very good. However, some opponents believe that the wind burning in Munich is caused by the geographical factors on the northern slope of the Alps. Unlike the climate background of many cities, it is still unknown how much the ventilation corridor can play in other cities.
There are still some controversies surrounding the practical difficulties in construction. Some people think that urban construction is basically taking shape, and the landing of ventilation corridors in the future depends on urban planning, which is difficult to achieve overnight. Others question whether smog and heat waves will affect the surrounding areas if they are successfully dispersed by ventilation corridors. Supporters said, first, there is no need to "make a big move" on urban construction, just make a slight adjustment; Second, the pollutants carried away by air flow will not only spread to the open areas around the city, but also spread to the sky, and it is a process of gradual dilution, which will not affect the surrounding areas.(bian Ji)
(Source: China Meteorological News August 21, 2017 Third Edition Editor: Tang Miao)