Downwind footprint of an urban heat island on air and lake temperatures

Ann Cosgrove,Max Berkelhammer

doi:10.1038/s41612-018-0055-3

Ann Cosgrove, Max Berkelhammer

Open Access

https://doi.org/10.1038/s41612-018-0055-3

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Abstract

The urban heat island (UHI) effect was first documented ~200 years ago, making it the longest recognized anthropogenic effect on climate. Although anomalous heating in cities has been meticulously characterized, less is known about how the UHI affects surrounding regions. It is hypothesized that downwind of cities a “heat plume” forms due to the advection of urban heat. This heat transport may have impacts beyond heating of the surface, such as disrupting atmospheric convection and influencing boundary layer structure, which influences weather, air quality, and human health. Here, a lagrangian atmospheric transport model, forced with archived data from a numerical weather model, is used to generate a three-dimensional map of an urban heat plume for a major city, Chicago. We document significant heating 100–200 m above the surface and 70 km downwind of the city. Over Lake Michigan, the scale of the plume is truncated nearly in half (~40 km), suggesting the lake is acting as a sink for the exported urban heat. Using satellite lake surface temperatures, we observed a disruption of the diurnal pattern of lake temperature beneath the plume, which supports a possible role of the lake in absorbing the heat plume. The results provide unique quasi-observational evidence for a significant footprint of cities on regional atmospheric structure and potentially on adjacent aquatic bodies.

Highlights

In order to assess the regional footprint of the urban heat island (UHI), we used a three-dimensional simulation of heat transport from Chicago using a lagrangian approach
We conducted the analysis for June 2012–June 2014 using 17,400 trajectories at each of two study sites: the urban center of Chicago (UL) and a non-urban site (NUL) located in similar proximity to Lake Michigan, which served as a control by providing an opportunity to explore how the lake affects atmospheric temperatures[31] and terrestrial–aquatic heat exchange in the absence of a city
The weather dataset provides a generalized regional picture of temperature conditions but the lagrangian approach is required to isolate whether an air parcel in the region had directly interacted with the UHI

Summary

Introduction

The UHI effect is the condition that describes higher temperatures in urban areas than surrounding areas of less development.[1,2] This phenomenon is attributed primarily to the replacement of permeable surfaces with dryer impermeable surfaces which influences energy availability and leads to a higher Bowen ratio at the land surface and warmer temperatures.[3,4] The intensity of the UHI is related to regional climate and variations in urban morphology that include characteristics of density, land use type, heterogeneity, and properties of the urban surface, such as surface roughness, permeability, and albedo.[4,5,6] The UHI effect impacts energy consumption, greenhouse gas emissions, and air pollution by increasing the demand for cooling during hot summer months. Archived meteorological data was used as boundary conditions for the lagrangian atmospheric transport simulations (Fig. 1) at four time intervals throughout each day in the study period (Methods).

Results

Conclusion