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Funding information SICATFuture urban climates will be influenced by both global climate change and localized urbanization, especially in fast-growing cities. This study provides regional climate projections for the 2050s for greater Ho Chi Minh (HCM) City, a fast-growing megacity in Southeast Asia. These projections are generated through dynamical downscaling of three different Coupled Model Intercomparison Project Phase 5 (CMIP5) global climate models driven with two different representative concentration pathway (RCP) emission scenarios. Furthermore, this study numerically evaluates the impacts of future urbanization and global climate change on the thermal environment of this city. The Weather Research and Forecasting (WRF) model is used to produce these projections, having first been updated with current and future (master plan-based) land use data with a horizontal resolution of 1 km. The results show that, in rural areas, the spatially averaged monthly mean air temperature in April is projected to increase by 1.2 and 1.7 C by the 2050s under the RCP4.5 and RCP8.5 scenarios, respectively. In newly urbanized areas, an additional warming of 0.5 C is expected under both scenarios, which corresponds to 20-30% of the global warming. In particular, the additional warming due to urbanization can exceed 0.8 C at night. The impact of future urbanization (0.5 C) is comparable to the difference in the temperature increases achieved under the different RCP scenarios. Thus, this impact should be considered in studies of the future urban climates of fastgrowing cities in developing countries. K E Y W O R D Sdeveloping countries, dynamical downscaling, mesoscale, Ho Chi Minh City, urban climate projection, Vietnam, WRF

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As an effect of climate change, cities need detailed information on urban climates at decision scale that cannot be easily delivered using current observation networks, nor global and even regional climate models. A review is presented of the recent literature and recommendations are formulated for future work. In most cities, historical observational records are too short, discontinuous, or of too poor quality to support trend analysis and climate change attribution. For climate modeling, on the other hand, specific dynamical and thermal parameterization dedicated to the exchange of water and energy between the atmosphere and the urban surfaces have to be implemented. Therefore, to fully understand how cities are impacted by climate change, it is important to have (1) simulations of the urban climate at fine spatial scales (including coastal hazards for coastal cities) integrating global climate scenarios with urban expansion and population growth scenarios and their associated uncertainty estimates, (2) urban climate observations, especially in Global South cities, and (3) spatial data of high resolution on urban structure and form, human behavior, and energy consumption.

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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.

There is still no consensus on the mechanisms that modify precipitation over and around cities, especially for those located in the tropics where convective processes primarily drive rainfall. Here we contribute to the ongoing discussion about the urban-associated precipitation by investigating the urban effect on the diurnal cycle of rainfall over Singapore. We use the urban version of the numerical weather prediction system of the Meteorological Service Singapore (hereafter called uSINGV) at a 300 m horizontal resolution to simulate the rainfall conditions over Singapore and its surroundings during the inter-monsoon period between 2010 and 2014. Two simulations with different land surface conditions are conducted: one with urban areas (i.e. present conditions) and one without urban areas. uSINGV is shown to perform well for rainfall when compared to observations. Comparison between simulations reveals that the urban area is responsible for the formation of a rainfall "hot spot" over Singapore and Johor Bahru, located at the southern tip of the Malay Peninsula, and the urban effect is accountable for 20-30% of total rainfall during late afternoons and evenings, highlighting a strong urban effect on localized rainfall over a tropical city. Enhancement of convection due to the urban heat island effect, increased frictional convergence due to buildings' drag, the seaward shift of the sea-breeze front, and the increased inflow of boundary-layer moisture by the stronger sea breeze are suggested as most probable reasons for the increased rainfall in the urban area.