My main research interest is to study the interactions between complex urban surfaces and the lower atmosphere, namely the atmospheric boundary layer. By quantifying the exchange of mass (air pollutants and moisture), momentum (wind fields altered by urban forms) and temperature between cities and the lower atmosphere, my research examines how multi-scale and multi-physics processes collectively shape the distinct urban climates. We combine computation, observation and theory to more accurately simulate these processes, which are fundamental to urban climates across scales.
Nevertheless, it still remains a challenge for numerical models at macro and meso scales to represent the relevant physical/chemical/biological processes of the microscale with a reasonable computational overhead. One approach is first to understand what emergent properties brought about by the microscale urban features are present with high-fidelity numerical models such as building-resolved large-eddy simulations, i.e. a type of computational fluid dynamics model; and the next step is to generate scalable models for the purpose of bridging the micro and meso scales as well as for fast evaluation of design scenarios.
One example of my past work is to understand how urban green infrastructure impacts the urban outdoor environment. A computational fluid dynamics model elucidates how different street tree morphologies impact the spatial distribution of air temperature and pollutants. The impacts at the city-level will be assessed by accounting for microscale effects of trees in city-scale models. The implications of this work are to inform urban planners about the tight coupling between urban thermal comfort and air quality, as well as the possible tradeoffs of ecosystem services when implementing green infrastructures.
In light of the first question about how transdisciplinary approaches would inform the design and redesign of urban-rural interfaces within the Mississippi River Watershed (MRW), I believe that there is a lot of room for transdisciplinary solutions for climate-sensitive design at both the micro and meso scales. Proactively planning and designing more sustainable and resilient communities in a holistic manner within the MRW remains a challenge. Although process-based modeling has become a second-instinct given my background in atmospheric science and engineering, I believe that the design and redesign of urban-rural systems will benefit substantially from data-driven approaches taken by data scientists. Such data-driven approaches could be combined with climate-related information in designing weather- and climate-sensitive buildings and neighborhoods. Combining the advantages of a sound physical understanding and highly scalable data-driven approaches, we will be able to account for uncertainties in climate predictions for mid-sized cities, the time-varying behaviors of physical infrastructures as well as flexibility in building codes and/or zoning requirements, while optimizing the outcomes of urban systems.
In light of the second question, one of my research interests is to connect between the scales in achieving a predictive understanding of urban climates. Currently, various upscaling and downscaling methods connect a wide range of spatiotemporal scales. However, in the near future, exascale computing will soon allow us to seamlessly connect across scales, provided we know how to do so. Since atmospheric motions and hydrologic cycles have a natural continuum of scales, especially given the increasingly powerful computational resources, no single dividing line exists between these scales. This implies that although building designs and urban planning at the neighborhood to regional scales operate at fixed targeted scales, impacts from other scales cannot be overlooked.
We first take a fundamental approach by scrutinizing what is missing in the current implementations of urban weather and climate modeling schemes when spatiotemporal resolutions increase further in the age of exascale computing. Our approach will be able to ‘screen’ factors that play a first-order effect, which require more advanced scale-aware representations. For example, when urban climate simulations are being ran at sub-kilometer resolution, mid-sized cities mingled within agricultural land cannot be simply treated as homogeneous rough surfaces as what has been done before the age of exascale computing. Nevertheless, is surface morphology a more important factor in shaping the local climates at the urban-rural interfaces? Are there any other factors such as anthropogenic heat release in these mid-sized cities that are more important for city’s thermal environment? There are still many open questions like these, especially for mid-sized cities. I envision that my research on how to better bridge the scales, which will result in exciting collaborations with both architects and urban planners.
In light of the third question, coordinated Sustainable Urban Systems (SUS) research within the MRW will advance taking a system-approach in understanding the impacts of weather and climate phenomena on the built environment. One of the pressing problems is the increasingly higher risk of flooding within the MRW, which is also a pressing problem in cities located in other watersheds worldwide. Many studies in the past in the field of urban climates have focused on the impact of urbanization on rainfall patterns as well as the implications for urban flooding, however, little is known about how a collection of mid-sized cities in a watershed impacts the regional hydrologic cycle. In addition, current research focus on urban climates, especially at the regional level, tend to focus only on megacities. Research on mid-sized cities located within a watershed is not well understood. Some particular research questions would be: how hydrologic cycles are modified by different ‘forms’ of urban-rural interfaces? How will future urban sprawls in cities within the MRW impact the hydroclimates?
With my background in urban climates and land-atmosphere interactions, I would like to collaborate with urban planners and agricultural specialists to better understand the driving forces behind the dynamic evolutions of the urban-rural interfaces. From an urban climates point of view, I would like to address questions such as how climatic factors play a role in guiding the long-term spatial planning for more sustainable and resilient communities within watersheds like the MRW. Research conducted in MRW will potentially be informative to cities located in different watersheds across the US.
Assistant Professor of Civil and Environmental Engineering