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Simulations of five November months (2010–2014) using the urban version of the numerical weather prediction system of the Meteorological Service Singapore (uSINGV) are used to analyse the urban effect on convective precipitation over Singapore and Johor Bahru (Malaysia). The model is able to closely predict locations where rainfall peaks occur, but rainfall totals are overestimated compared to radar data. The temporal variability of rainfall in the region shows that urban areas increase the frequency and severity of rainfall events and that such impact increases with the rainfall intensity. Results show that low-level moisture advection is enhanced in this coastal conurbation as a result of the strengthening of wind convergence. The latter is likely caused by increasing sea-breeze strength due to lower surface pressure over the urban area, and higher urban surface roughness, respectively. As a consequence, more precipitable water is available in the region, enhancing convection and increasing the probability of heavy rainfall over the centre and north of Singapore island and Johor Bahru. Stronger convection further increases moisture advection from the vicinity. By studying the temporal variability and the spatial distribution of rainfall events, the present study provides new insights on the urban impact on heavy rainfall in tropical areas. The conclusions are only valid for the November inter-monsoon period, when local forcing, rather than large-scale influences, dominates rainfall generation. National Environment Agency - Singapore, R-109-000-338 236-490

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.

Short-term extreme rainfall events (EREs) often produce urban flooding and result in the loss of life and property. In this study, three high spatial-temporal resolution precipitation products were used to analyze hourly EREs during May-October from 2001 to 2018 over the Yangtze River Delta (YRD). To quantify the possible link between EREs and environmental factors (such as urbanization, climate change, and orography), the Hovmoller diagrams and Generalized additive model for location, scale, and shape (GAMLSS) nonstationary model were used. The results showed that the eastward propagating precipitation systems created a heavy rainfall center in the southwest mountainous of YRD due to orographic lifting, and enhanced in the plains over urbanized areas, especially in the vicinity of Nanjing, Changzhou, and Nantong metropolitan regions. The variability of hourly rainfall extreme in the urbanized regions is high correlated to the multivariate ENSO index (MEI), and shows a greater magnitude in the urban than rural regions. This phenomenon was intensified by large-scale circulation patterns due to strong-ENSO over the Nanjing-Changzhou-Nantong (NCN) urban regions, but weakened over the Shanghai-Hangzhou-Ningbo (SHN) urban regions. An analysis of nonstationary GAMLSS also suggests the large-scale circulation presents a greater impact on the hourly extreme rainfall than the urbanization, while the hourly rainfall extremes in the urban areas are still higher than that in rural regions under the nonstationary scenario. Those results suggest intense rainfall occurred more frequently in the urban region under observed climate change and urbanization in YRD.

This study examines the climate impact of future urbanization on the urban heat island effect over one of the developing megacities in tropical Southeast Asia, the greater Ho Chi Minh City area, using a dynamical downscaling approach with a high-resolution regional climate model coupled to urban canopy model. Simulations incorporate different land use/cover and anthropogenic heat release data sets for the current and future periods, while the boundary condition was fixed in the current time to detect only the impact of urbanization. Furthermore, human thermal comfort due to changes in the thermal environment was examined by analyzing the temperature-humidity index. Simulated results show that the increase in the surface air temperature is approximately 0.22 °C in the preexisting urbanized area and approximately 0.41 °C in new highly urbanized areas. The main factor in these changes is conversion from agricultural or grassland to urban structures, which results in increased sensible heating and decreased latent heating. Simulated results also show that despite the increase in air temperature, there is no significant change in thermal comfort. This is because human comfort is not only determined by temperature alone, but also by relative humidity. Urbanization increases temperatures, but decreases evaporation, which then decreases the relative humidity.

This study examines climatic impact of urbanization on the variability of the urban heat island (UHI) effect over Greater Ho Chi Minh City metropolitan area (GHCM), since the late 1980s, using the dynamical downscaling with very high-resolution regional climate model coupled to an urban canopy model (RCM/UCM). This is the first application of RCM/UCM to a city in developing countries in Southeast Asia in assessing the impacts of the past land-use and anthropogenic heat release during the selected three periods (ca. 1989, 1999, and 2009). The main findings are as follows: First, agreement between simulated results (for case of ca. 2009 urban) and observation demonstrates that the RCM/UCM is able to reproduce the urban climate of GHCM. Second, the evolution of spatial distribution of UHI is closely associated with urban expansion. The increase in the surface air temperature was about 0.3 °C in the pre-existing urbanized area and about 0.6 °C in newly urbanized area in the last 20 years. Main factor of these changes is conversion of agriculture or grassland into urban structure, which results in increase in sensible heating and decrease in latent heating. In addition, in the central GHCM, the urbanization impact was estimated at 0.31 °C, while the temperature increase was observed at 0.64 °C in the last 20 years. This suggests that the urbanization may contribute about half to the increase of surface air temperature in the central GHCM.

Tropical cities are more susceptible to the suggested fall outs from projected global warming scenarios as they are located in the Torrid Zone and growing at rapid rates. Therefore, research on the mitigation of urban heat island (UHI) effects in tropical cities has attained much significance and increased immensely over recent years. The UHI mitigation strategies commonly used for temperate cities need to be examined in the tropical context since the mechanism of attaining a surface energy balance in the tropics is quite different from that in the mid-latitudes. The present paper evaluates the performance of four different mitigation strategies to counterbalance the impact of UHI phenomena for climate resilient adaptation in the Kolkata Metropolitan Area (KMA), India. This has been achieved by reproducing the study sites, selected from three different urban morphologies of open low-rise, compact low-rise and mid-rise residential areas, using ENVI-met V 4.0 and simulating the effects of different mitigation strategies- cool pavement, cool roof, added urban vegetation and cool city (a combination of the three former strategies), in reducing the UHI intensity. Simulation results show that at a diurnal scale during summer, the green city model performed best at neighborhood level to reduce air temperature (Ta) by 0.7 °C, 0.8 °C and 1.1 °C, whereas the cool city model was the most effective strategy to reduce physiologically equivalent temperature (PET) by 2.8° – 3.1 °C, 2.2° – 2.8 °C and 2.8° – 2.9 °C in the mid-rise, compact low-rise and open low-rise residential areas, respectively. It was observed that (for all the built environment types) vegetation played the most significant role in determining surface energy balance in the study area, compared to cool roofs and cool pavements. This study also finds that irrespective of building environments, tropical cities are less sensitive to the selected strategies of UHI mitigation than their temperate counter parts, which can be attributed to the difference in magnitude of urbanness.