Quantifying the Impacts of an Urban Area on Clouds and Precipitation Patterns in Downwind Regions: A Modeling Perspective

Over the past century, rapid urbanization has greatly altered landscapes across the globe. This transformation has significantly impacted precipitation patterns and affected a wide range of sectors including human welfare, infrastructure, and ecosystems. As a result, there is a growing imperative to improve understanding of urbanization impacts on local and regional weather events and hydroclimates.

This study investigates the influence of urbanization on precipitation amounts and cloud cover at different heights around the metropolitan areas of Indianapolis. While prior literature has mostly focused on analysis of either in situ, RADAR or satellite observations in quantifying long-term effects from urban areas [Liu et al. – 2019], we employ a modeling approach based on simulations with the Weather Research and Forecasting (WRF) model to directly incorporate and mechanistically analyze the urban signal. An ensemble of WRF simulations (three-nested domains with resolution respectively: 12km, 4km, 1.3km) is performed to decouple the role of the background climate from the urban effect on precipitation and cloud properties. Specifically, we employ a climatologically representative year-long run to (i) evaluate WRF performance in capturing spatial-temporal patterns of key meteorological variables and precipitation properties against a suite of ground-based and remotely-sensed observations, (ii) identify primary wind patterns across the area, and (iii) to quantify the urban signal by comparing the upwind and downwind cloud fractions and precipitation amounts for flow crossing the urban area. Two perturbed simulations, where the urban land cover is either replaced by croplands or is increased in size, are also generated for the two rainiest months, aiming to separate the regional climatology from the urban rainfall effect signal.

Comparisons of the control simulation against the perturbed runs indicate statistically significant impacts from urbanization in enhancing precipitation and cloud fraction, with the peak observed within a 75 to 125 km radius downwind of the city. The changes are a non-linear function of urban area. We identify that key drivers of cloud cover, including temperature at 2m height, planetary boundary layer height, turbulent kinetic energy, and convective available potential energy (CAPE), are altered in the perturbed runs compared to the control run. These modifications subsequently impact precipitation patterns and cloud behavior. The modest surplus of CAPE and the associated enhanced turbulence indicate a possible role of the urban area in affecting low energy systems and nimbostratus formation.

Bibliography

[Liu et al. – 2019] - Liu, Jie & Niyogi, Dev. (2019) - Meta-analysis of urbanization impact on rainfall modification - Scientific Reports. 9. 10.1038/s41598-019-42494-2