Study of Urban Heat Islands Using Different Urban Canopy Models and Identification Methods

Rui Silva,David Carvalho,Alfredo Rocha,Ana Cristina Carvalho

doi:10.3390/atmos12040521

Rui Silva, David Carvalho + Show 2 more

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https://doi.org/10.3390/atmos12040521

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Abstract

This work aims to compare the performance of the single‑(SLUCM) and multilayer (BEP-Building effect parameterization) urban canopy models (UCMs) coupled with the Weather Research and Forecasting model (WRF), along with the application of two urban heat island (UHI) identification methods. The identification methods are: (1) the “classic method”, based on the temperature difference between urban and rural areas; (2) the “local method” based on the temperature difference at each urban location when the model land use is considered urban, and when it is replaced by the dominant rural land use category of the urban surroundings. The study is performed as a case study for the city of Lisbon, Portugal, during the record-breaking August 2003 heatwave event. Two main differences were found in the UHI intensity (UHII) and spatial distribution between the identification methods: a reduction by half in the UHII during nighttime when using the local method; and a dipole signal in the daytime and nighttime UHI spatial pattern when using the classic method, associated with the sheltering effect provided by the high topography in the northern part of the city, that reduces the advective cooling in the lower areas under prevalent northern wind conditions. These results highlight the importance of using the local method in UHI modeling studies to fully isolate urban canopy and regional geographic contributions to the UHII and distribution. Considerable improvements were obtained in the near‑surface temperature representation by coupling WRF with the UCMs but better with SLUCM. The nighttime UHII over the most densely urbanized areas is lower in BEP, which can be linked to its larger nocturnal turbulent kinetic energy (TKE) near the surface and negative sensible heat (SH) fluxes. The latter may be associated with the lower surface skin temperature found in BEP, possibly owing to larger turbulent SH fluxes near the surface. Due to its higher urban TKE, BEP significantly overestimates the planetary boundary layer height compared with SLUCM and observations from soundings. The comparison with a previous study for the city of Lisbon shows that BEP model simulation results heavily rely on the number and distribution of vertical levels within the urban canopy.

Highlights

The UHI intensity (UHII) tends to be greater at night, under calm synoptic and clear-sky conditions [1,2], in some cases, urban cold islands (UCIs) can occur during the day [3]
Europe experienced extreme hot conditions during August 2003 which resulted in a persistent heatwave over the European continent [32]
This heatwave was due to anticyclonic conditions with weak near-surface pressure gradients and a subtle low pressure southwest of the Iberian Peninsula which drove the weak, hot, and dry, near-surface southerly winds across the region (Figure 4)

Summary

Introduction

The urban materials, with lower reflectivity and high thermal inertia, tend to absorb large amounts of shortwave radiation energy during the day, which is later released as longwave radiation and sensible heat during the night. Characteristics such as buildings geometry and road width can influence and reduce the sky view factor in urban canyons, causing radiation trapping, while augmenting the exposed area for radiation absorption during the beginning and end of the day [6,7,8,9]. Urban environments are characterized by the lack of vegetation and the presence of large areas of impervious surfaces that can reduce soil evapotranspiration (i.e., evaporation from soil and vegetation) [10,11,12,13,14,15]

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