Numerical Study on Regional Climate Change due to the Past and Future Urbanization over Greater Ho Chi Minh City, Vietnam

  • Doan Quang Van, 2016: Numerical Study on Regional Climate Change due to the Past and Future Urbanization over Greater Ho Chi Minh City, Vietnam .

The Graduate School Doctoral Best Dissertation Award
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Abstract

Firstly, this study examines climatic impact of urbanization on the variability of the urban heat island (UHI) effect over Greater Ho Chi Minh City metropolits (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 (circa 1989, 1999, and 2009). The main findings are as follows: First, agreement between simulated results (for urban condition in 2009) 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 a conversion of agriculture or grassland into the urban structure which results in an 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. Secondary, the climatic responses to the coupled effects of greenhouse gas-induced global warming and the future urbanization over up to the mid of 21th century over GHCM were also examined. A dynamical downscaling approach with a high-resolution regional climate model coupled to an urban canopy model was adopted. The boundary condition for future climate were generated by pseudo-global-warming method using the results of GCMs provided by CMIP5 for two scenarios RCP4.5 and RCP8.5. The simulated results show that, the future urbanization, alone and separate from greenhouse gas-induced forcing, can be expected to raise near surface air temperature about 0.5 ° C in new urban areas, with the warming rate can reach peak of 0.8 0.9 ° C in the nighttime. On the other hand, greenhouse gas-induced global warming is estimated to raise the surface air temperature on the average about 1.2 ° C according to CMIP5 RCP4.5 and 1.8 ° C according to RCP8.5. This study shows that the future urbanization over the HMA can contribute 20 - 30% to the total warming rate of the city up to the mid of this century. This implied that there is a room for an appropriate adaptive urban design in order to reduce the urban warming effect. Thirdly, the multi-layer urban canopy model (MUCM) coupled with ray-tracing scheme was developed. Moreover, the one-way nesting system between the WRF model with MUCM was also designed. The performance of the MUCM was examined (off-line) using measured atmospheric data at Kugahara, Tokyo, on September 01 of 2005. The model was also climatologically run in one-way nesting mode using the WRF output data of Ho Chi Minh simulation. The results show that the air temperature within the urban canopy was well performed in both offline run and one-way nesting run. For the one-way nesting run, the diurnal range of surface air temperature appeared to be exactly estimated by the MUCM rather than that by the WRF coupled with a single layer canopy model. Furthermore, the one-way nesting system was applied to examine UHI mitigation strategies provided for Ho Chi Minh City. The simulated results showed that the applying of a green roof is the most effective way to mitigate UHI effect, by decreasing air temperature about -0.25 ° C in the nighttime. On the other hand, thermal-insulation plan can be expected to reduce UHI in the night time about -0.21 ° C, however, this plan appears to be useless in the daytime when increasing air temperature about 0.7 ° C

The article from a part of the doctoral thesis was published on International Journal of Climate. (2015) Details
The article from a part of the doctoral thesis was published on JUrban Climate. (2016) Details