CAREER: Multi-Scalar Transport and Similarity in the Urban Boundary Layer

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More than half of the global population live in urban areas, which nontrivially modify the atmosphere through two broad pathways: urban form (i.e., changes in surface properties) and urban function (i.e., anthropogenic activities emitting heat and mass). These two pathways occur via turbulent exchanges of momentum, energy and mass in the atmospheric boundary layer and carry ‘distinct fingerprints’ of a city’s form and function. For increasingly fine-scale climate and numerical weather prediction (NWP) models, it is a persistent challenge to reflect these ‘distinct fingerprints’ of different cities across the world, yet in a manner that is generalizable and computationally tractable. Due to incomplete understanding of multi-scalar transport, whether different scalars of anthropogenic origins obey similarity relations in the urban surface layer remains unclear. This is also one of the key stumbling blocks to generalize urban land-atmosphere exchanges for multiple scalars. In particular, incorporating the effect of urban function on surface-atmosphere exchanges into climate and NWP models is almost completely missing. Therefore, the overarching goal is to improve basic understanding of multi-scalar transport and inform physically realistic, generalizable estimates of the surface-atmosphere exchanges, especially for less explored scalars. The project will lead to findings necessary for the next-generation urban climate modeling tools, which can be implemented to develop more precise (i.e., city or neighborhood-specific) mitigation and adaptation measures with changing climates. To achieve the overall project goal, the approach of this project is motivated by a critical comparison between flow and transport in the urban canopy versus vegetated one, which has been extensively studied. Such an approach will generate new insight into the transferability of theories and models between the two, informing development of urban-specific models for surface-atmosphere exchanges based on the existing ones for vegetation counterpart. To advance basic understanding of multi-scalar transport and departure from similarity, the mechanisms responsible for scalar dissimilarity will be separately investigated at the micro- and local scale of heterogeneities in urban form and function. Understanding of multi-scalar transport at both the micro- and local scales will be systematically studied by first deriving a ‘city profile generation’ module to generalize urban form and function (Aim 1). Then, hypotheses regarding multi-scalar transport and their similarity will be tested to advance basic understanding (Aim 2). The new understanding will help improve urban surface-atmosphere exchange modeling and interpretation of observations that rely on scalar similarity theory (Aim 3).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.