Idealized Simulations of City-Storm Interactions in a Two-Dimensional Framework

Jason Naylor

doi:10.3390/atmos11070707

Abstract

Numerous studies have identified spatial variability in convective parameters such as rainfall totals and lightning flashes in the vicinity of large urban areas, yet many questions remain regarding the storm-scale processes that are altered during interaction with a city as well as which urban features are most responsible for storm modification. This study uses an idealized, two-dimensional cloud model to investigate structural and evolutionary changes in a squall line as it passes over a simplified representation of a large city. A parameter space exploration is done in which the parameters of the city—surface temperature and surface roughness length—are systemically increased relative to the region surrounding the idealized city. The resultant suite of simulations demonstrates that storm parameters such as vertical velocity, hydrometeor mass, upward mass flux, and buoyant accelerations are enhanced when the storm passes over the idealized city. No such enhancement occurs in the control simulation without an idealized city.

Highlights

Comprising only a small percentage of the world’s land use, urban areas contain the majority of the world’s population
The depth of this dome is confined below z = 1 km in all simulations, in agreement with observations of real urban heat islands [36]
The surface temperature and/or surface roughness length within the idealized city were systematically increased relative to the surrounding area by some defined value

Summary

Introduction

Comprising only a small percentage of the world’s land use, urban areas contain the majority of the world’s population. Convective events that occur over dense, populous urban areas may expose a large number of people to hazards such as flash flooding, damaging wind, large hail, and tornadoes. For this reason, timely and accurate threat information must be communicated to residents of large cities. Large urban areas have been shown to alter the spatial distribution and intensity of convective rainfall around them [3,4,5,6,7,8,9,10,11], with the most commonly observed signal being enhanced rainfall downwind of the urban core. There is increasing evidence that urban areas can impact organized storm systems and modify the distribution of convective hazards associated with those storms. Huff and Changnon [4] noted that, beginning in the late

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