How urban growth changes the heat island effect and human thermal sensations over the last 100 years and towards the future in a European city?

• Published in: Meteorological applications Vol. 28; no. 4
• Main Authors: Vitanova, Lidia Lazarova, Kusaka, Hiroyuki, Doan, Quang‐Van, Subasinghe, Shyamantha
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01-07-2021; John Wiley & Sons, Inc; Wiley
• Subjects: Air temperature; Bulgaria; Climate models; Climatic conditions; Control simulation; Diurnal variations; human thermal comfort; Humidity indexes; Island effects; Meteorological research; Numerical weather forecasting; Sofia; surface air temperature; Surface temperature; Surface-air temperature relationships; Temperature; Thermal comfort; Thermal environments; Urban climatology; Urban development; urban heat island; Urban heat islands; Urban sprawl; Urbanization; Weather forecasting; WRF model
• ISSN: 1350-4827; 1469-8080
• DOI: 10.1002/met.2019

Urban development exacerbates urban heat island effects of cities and thermal discomfort of the residents worldwide. However, such urban effects vary with the geographical and climatic conditions of the respective cities. This study is the first to quantitatively estimate the impact of past (1878), present (2012), and future (2050) urbanization on surface air temperatures and human thermal comfort in Sofia, the largest city of Bulgaria. We used the state‐of‐the‐art Weather Research and Forecasting (WRF) climate model, with a 1‐km horizontal resolution for three cases: LU1878, LU2012, and LU2050. We first validated the results of the control simulation (case LU2012) against actual observations. The model satisfactorily reproduced the diurnal variation of the observed monthly mean surface air temperatures for July (2011–2013) with a mean bias of ±0.6°C. We then estimated the potential impacts of past and future urbanization on surface temperature. Our results suggest that because of urbanization of Sofia, the surface air temperature at 0600 Local Standard Time (LST) has increased approximately 4.0°C in the past (1878–2012) and approximately 0.4°C in future (2012–2050). We also evaluated the impact of the urban thermal environment on human comfort levels. Considering the LU2050 case in the central part of Sofia, the temperature–humidity index during mid‐afternoon (1400–1700 LST) is estimated to be 24–26, indicating that approximately 50% of the residents could feel perceptibly uncomfortable.
Urbanisation in Sofia, the largest Bulgarian city, caused the air temperature (estimated at 0600 LST) to have risen by ∼4.0°C in the past (1878–2012) and by ∼0.4°C in future (2012–2050). Considering the LU2050 case for the central part of Sofia, the mid‐afternoon (1400–1700 LST) temperature‐humidity index is 24–26, which could be significantly uncomfortable for around 50% of the residents.