We quite often hear the word “turbulence” when we are in an aircraft, flying 30,000 feet above the ground. However, as people are fastening their seatbelts, most of them probably do not realise that turbulent fluid flow is so ubiquitously encountered in everyone’s life.
Almost all kinds of human activities on the Earth’s surface are immersed in the atmospheric boundary layer (ABL), the layer of air approximately 1–2 kilometres above the surface of the earth, which is almost always turbulent. Working in the interdisciplinary field of micrometeorology (i.e., studying weather patterns in a short time scale, say, 30 minutes), atmospheric sciences and engineering, I study dynamics of this “thin” layer of air.
Winds in the ABL are an important source of renewable energy. The winds, or turbulent air flows, are powerful in transferring the near-surface water vapour to upper atmosphere and water returns to earth through precipitation, which are essential components of the global water cycle. In addition, any numerical weather predictions and future climate projections need to account for dynamics of the ABL in order to represent how the Earth’s surface is coupled to the atmosphere. These are just a few examples of why we are motivated to study the ABL and the land-atmosphere interactions.
To me, this field of study is interesting and important because it is through this very thin layer of air that anthropogenic (i.e., relating to humans) land use, land cover change (e.g., urbanisation), and perturbations to the air (e.g., release of greenhouse gases) ultimately impact earth’s climate system.
A schematic diagram showing modifications of the environment by cities
More than half of the world’s population now live in cities and the trend of urbanisation continues, driven by political, social and economic factors. Its impact on the earth system and climate is profound. Cities only occupy a small fraction of the Earth’s surface, but they significantly transform the local environment and their impact on the earth system extends beyond city boundaries. The urban heat island effect, which refers to the air temperature in the city core being higher than the surrounding rural areas, was first reported in the 19th century in London by Luke Howard. Records of cities’ “signature” on occurrences of summer storm dated back in the 1960s. Pollutants from the cities can also impact regions downwind from the sources.
In terms of resource consumption, cities account for 75 per cent of total energy use and 80 per cent of greenhouse gas emissions. Under a changing climate and a higher frequency of extreme weather events (e.g., heat waves, cold spells, extreme rainfalls and hurricanes), cities can be particularly vulnerable. Urban areas are also well known for many health hazards, such as degraded air quality and stressful lifestyle.
The quality of the air that every city dweller breathes has crucial health implications. As the urban population increases and as urbanisation imposes drastic anthropogenic changes to land cover, it is important to understand and be able to predict land-atmosphere interactions in spatial scales that are relevant to city dwellers. More broadly speaking, being able to quantify the exchanges of energy, momentum, water and gases within a city and with its climate system, is one of the building blocks of sustainable and resilient infrastructures in the built environment.
How are we doing so far? The answer is “in the turbulent wind”. Turbulence, as a natural phenomenon, is paramount in the study of ABL dynamics and city-atmosphere interactions. However, turbulence remains an unsolved problem in classical mechanics.
In the past few centuries, artists such as Leonardo da Vinci have delicately sketched the irregular and chaotic features of water from fountains. These complicated turbulent flow patterns, which seem to be totally intractable, are actually governed by the Navier-Stokes equations. In other words, if one can shrink oneself and tag along a fluid parcel, one will be able to predict one’s exact path using the Na- vier-Stokes equations, provided one’s initial position and other necessary information about the boundary of the flow are accurately known.
However, solutions to the Navier-Stokes equations show a strong sensitivity to initial conditions, boundary conditions and any driving forces. For example, even an inaccuracy in the molecular scale in the initial position could lead to errors in the long-term prediction of the resulting path. For example, when a puff of pollutants from traffic emission on a major road is dispersed “in the wind” and transported into the pedestrian lanes, predicting its concentration accurately after a certain period of time is extremely difficult.
In order to quantify and predict the interactions between urban sulrefavecel s and the lower atmosphere, we need to study the turbulent wind. The turbulent wind (or you can think of them as air parcels moving randomly) can be interpreted as being comprised of swirling eddies of a large range of scales. In Figure 1, the schematic diagram shows that buildings as well as trees exert a drag force that reduces the wind speed. It may require some imagination to visualise the chaotic motions of swirling eddies “superimposed” on Figure 1.
Figure 2 illustrates a snapshot of wind over an array of buildings in a vertical cross-section plotted from computational fluid dynamics modelling called large-eddy simulation. The streamlines are tangential to the wind vectors, and the colours indicate temperature distribution. The technique of large-eddy simulation, as its name suggests, computes the dynamics of large eddies while modelling the small- scale ones. The smallest of the large eddies has a dimension that is commensurate with the size of a person. This state-of-the-art technique therefore offers the unprecedented opportunity to understand and quantify the impact of turbulent wind on phenomena pertinent to “human-scale”. On the other hand, the upper bound of the large eddies is representative of the large-scale dynamics of the turbulent atmospheric boundary layer.
Snapshots from numerical simulations showing the horizontal component of the wind vector, with direction indicated by the magenta arrows. Hotter colour indicates higher wind.
Let’s use a simple example to illustrate how computational tools like the large-eddy simulation provide new insight into multi-scale inter- actions between humans, city and the atmosphere. In a tropical city like Singapore, the high level of ambient humidity often makes people feel uncomfortable. Abundant vegetation in the “garden state” can further increase the humidity level due to transpiration.
Given the high computing power we have today, it is now feasible to simulate how water vapour is transported by turbulent wind to the atmospheric boundary layer from a district as large as 20–30 square kilometres. Higher water vapour concentration means a high level of humidity. We can also assess how synoptic-scale weather events such as a heatwave impacts the water vapour distribution in this district. In addition, since we understand how turbulent wind is “stirring” water vapour at the human-scale, it is possible to explicitly estimate city dwellers’ comfort level. The comfort level can vary with one’s location, time of the day, synoptic-scale weather events, different street layouts, roads and vegetation patterns, etc.
Quantitative studies allow us to explore factors that impact the interactions between urban centres and atmosphere by dynamically simulating the physical processes in various spatial scales. This in turn enables decision-making at policy and planning levels — such as increasing vegetation coverage or demolishing older buildings to build a new infrastructure — through “hypothetical scenario studies”. It will also allow systematic cost-benefit analyses from different perspectives based on “multi-scale information” about humidity level.
For example, in addition to considering the comfort level of city dwellers, the negative impact of high moisture level to building facades can also be taken into account from the perspective of maintenance cost. Eventually, with state-of-the-art computational tools, we can quantitatively evaluate district-level green policies and environmental measures for the enhancement of urban sustainability and liveability.
Challenges still remain, both in theoretical development of turbulence analysis and practical applications to solve real-world problems in cities. Although numerical modelling tools are useful, live observations in cities with new instruments, sensors and technology are also extremely valuable. Large datasets collected on-site contain rich information such as turbulent wind measurement, temperature, humidity, pollutant concentration, greenhouse gases emission rate and building energy consumption. One may surmise that the fusion of sophisticated numerical model with large datasets presents exciting opportunities for new understanding in human-city-atmosphere interactions. This will eventually bring about innovative solutions to urban sustainability. There is a long way to go but we are motivated by the “answers in the wind”.
With a scholarship from the Ministry of Education of Singapore, Qi Li studied in Raffles Girls’ Secondary School and Raffles Junior College. She subsequently graduated from Carleton College and obtained her doctorate degree in engineering from Princeton University. She is currently a postdoctoral scholar at Columbia University and will be joining Cornell University as an assistant professor in fall 2018.