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Circular cities benefit from more rainfall

Circular cities benefit from more rainfall

After noticing that weather data from roughly circular cities like Dallas and London often show more rain than triangular cities like Chicago and Los Angeles, Dev Niyogi and his colleagues at the University of Texas at Austin decided to investigate the link between the shape of an urban area and its rainfall. The authors argue that how wind and weather interact with shape in urban environments should be taken into account when building future urban spaces that must be more resilient to the effects of climate change.

Circular cities, square cities, and triangular cities are rated in order of rainfall amount and intensity from largest to smallest. The findings are important for city development that is both sustainable and resilient, especially for those that are expanding.

This study provides the first investigation of the impact of city shape on urban rainfall in inland and coastal environments. Under calm synoptic conditions, the city shape impact is much more evident in coastal environments. 

The team idealized large eddy simulations coupled with the Weather Research and Forecasting model. In the inland vs coastal environment, there are changes in the timing of urban-induced rainfall. This is linked to the land-sea breeze’s differing diurnal cycles of vertical velocity and cloud water mixing ratio. The effect of city shape on rainfall is especially visible at the coast, where buoyant flows from cities modify the interactions between urban-rural circulation and sea breeze.

Spatial distribution of mean daily rainfall amount (mm) over cities under the quasi-equilibrium state. (a) C-land, (b) S-land, (c) T-land, (d) C-coast, (e) S-coast, and (f) T-coast cases, respectively. The black line marks the location of the urban area.

The frictional convergence displaces the sea wind higher and increases afternoon rainfall over coastal towns, while rapid heating of the land surface provides severe morning rainfall inland. In coastal areas, the impact of city design is substantially more visible during calm synoptic conditions. The circular city receives the most daily rainfall, which is 22.0 percent more than the triangular city. Due to changes in city design, the variance in morning peak rainfall rate might exceed 78.6 percent. The rainfall anomaly is mostly due to city forms altering regional circulation, where differing low-level convergence intensities regulate convective energy and moisture vertical transit.

Given the likelihood that climate change would exacerbate future rainfall hazards, worldwide cities have invested significant resources in researching and implementing a variety of infrastructures as adaptation methods.

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Our results identify a hitherto poorly understood but an important role of urban layout especially in the coastal regions. Circular city shape shows potential risks of extreme rainfall and resultant flood risk. Moreover, such risks can not be assessed through climate models with coarse resolutions that cannot accurately represent the city shape. 

Impacts of City Shape on Rainfall in Inland and Coastal Environments, Wufan Zhang, Jiachuan Yang, Long Yang, Dev Niyogi

Published: April 2022
DOI: 10.1029/2022EF002654

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