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Anthropogenic influences on extreme precipitation events over the Pearl River Delta (PRD) region in South China with the use of a convection-resolving model
Anthropogenic influences on extreme precipitation events over the Pearl River Delta (PRD) region in South China with the use of a convection-resolving model
The CMIP5 global models show that the PRD region has warmed by around 0.9 ℃ since the pre-industrial era, resulted from the increasing emission of anthropogenic greenhouse gases. As the climate warms, the atmosphere is expected to hold more moisture in line with the Clausius-Clapeyron (CC) relation, which increases the potential risks of precipitation extremes as a result. Indeed, observational results have corroborated the increasing trend of extreme rain intensity and the enhanced likelihood of such events over the PRD, which threaten our environment and society.
My research aims to quantitatively investigate the extent to which PRD extreme rainfall in different seasons can be attributed to human influences and the underlying physical mechanisms. Attribution results suggest that human activities could have contributed at least a 10% increase in daily extreme precipitation over the inland PRD. In particular, changes of rainfall in MJJAS (i.e. from May to September) are nearly CC scaling of 7% K⁻¹, whereas a super-CC increase of 12.4% K⁻¹ is found for non-MJJAS rainfall. The greater increment of non-MJJAS rainfall highlights the important role of dynamic effect on heavy precipitation, in addition to the moisture-driven thermodynamic effect.
The importance of future urban development in hourly extreme rainfall projections- comparing global warming and urbanization forcing over the Pearl River Delta region
The importance of future urban development in hourly extreme rainfall projections- comparing global warming and urbanization forcing over the Pearl River Delta region
The impacts of future urban development and global warming forcing on hourly extreme rainfall over the Pearl River Delta (PRD) area have been investigated, by dynamically downscaling General Circulation Model (GCM) outputs using the Weather Research and Forecasting Model (WRF) at convection-permitting resolution, coupled with an Urban Canopy Model (UCM). Three downscaling experiments corresponding to different urban land cover (1999 and projected 2030) and climate (1951-to-2000 and 2001-to-2050 GCM simulations) were designed. Near-future climate change (up to 2050) and 1999-to-2030 urban development effects on PRD extreme precipitation were then examined. Results show that climate change and rapid urban development forcing have comparable positive effects on the intensity as well as heavy hourly rainfall probability over the PRD megacity. Global warming tends to increase heavy rainfall probability (from 40 to 60mm/hr) by about 1.3 to 1.8 times, but suppresses the frequency of light rainfall. Urban development increases urban rainfall probability within the whole range of intensity, with frequency for very heavy rainfall (> 90mm/hr) almost doubled. Overall, forcing due to rapid urban development plays an important role for projecting rainfall characteristic over the highly urbanized coastal PRD megacity, with impacts that can be comparable to global warming in the near future.
Impacts of decaying EI Niño on East Asian (EA) extreme precipitation and its diversity
Impacts of decaying EI Niño on East Asian (EA) extreme precipitation and its diversity
The spatial distribution of extremes shows the opposite spatial patterns during Eastern-Pacific (EP) and central-Pacific (CP) El Niño. More intense spring extremes over the southern China (SC) are observed during EP El Niño, while the situation is revise during CP El Niño.
In decay summer, intense extreme events over the south of Yangtze River (SYR) during EP El Niño and less extremes over the Mei-Yu rain band in China, Baiu in Japan, Changma in South Korea (MBC) are observed. During CP El Niño, weaker (stronger) extremes over SYR (MBC) are found. The displacement of Western Pacific subtropical high (WPSH) is the main reason for the opposite distribution of extremes during EP and CP El Niño. In decay spring, the anticyclonic anomaly is found over WNP basin during EP El Niño. Southwest winds contribute to form the moisture source of extremes over SC (figure 1a). During CP El Niño, the anticyclonic anomaly extends westward to the east Bay of Bengal (EBOB), which impedes the moisture transported from the EBOB (figure 1b). In decay summer (Figure 1c and 1d), when EP (CP) El Niño occurs, the westerly jet (WJ) tends to be displaced southward (northward) in relation to the positioning of WPSH, contributing to stronger vertically integrated moisture flux convergence over SYR (MBC), while there is moisture flux divergence over MBC (SYR).
The different flow patterns associated with El Niño diversity appear to be forced by different SST warming signals in either tropical Indian Ocean or Maritime Continent regions, which can induce anomalous regional atmospheric circulation that affects the behaviors of WPSH and WJ.
Figure 1. Mechanisms leading to extreme precipitation changes over SC in March-April-May (MAM), and SYR and MBC in June-July-August (JJA) during (a, c) EP and (b, d) CP EI Niño.
Future changes in the ENSO, IOD, and their joint effects on East Asian monsoon
Future changes in the ENSO, IOD, and their joint effects on East Asian monsoon
The climate of East Asians is deeply modulating by the Indo-Pacific climate modes, especially the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events. However, the future changes in the ENSO, IOD, and their joint effects on East Asian monsoon are not yet entirely understood. It is necessary to explore these changes and find out the dominant processes connecting the Indo-Pacific climate modes with the East Asian monsoon in future projected climate using CMIP6 models.
Due to the strong interactions between ENSO and IOD, their joint effects on the East Asian monsoon may be more complex under global warming. This study provides some necessary references for the relevant government policies and even the preparations for the predictable climate disasters.
Influence of anthropogenic warming on tropical cyclones activities
Influence of anthropogenic warming on tropical cyclones activities
Anthropogenic warming will alter the atmospheric and ocean environment, hence influencing tropical cyclone activities in the future. To a medium-to-high confidence that global tropical cyclone mean intensity in terms of lifetime maximum surface wind speed will have an increase by 5%, and differs among different ocean basins. Using high-resolution WRF model with a 15km and 3km nesting grid, future tropical cyclones intensity and structure due to thermodynamical changes in RCP8.5 and RCP4.5 emission scenario in the near and far future were simulated by pseudo-global warming technique. In the South China Sea Region, the intensities of the tropical cyclones in terms of maximum wind speed (Vmax) were simulated to have an increase under a warming environment. Moreover, the Radius of Maximum wind (RMW) of the simulated tropical cyclones was expected to have a decrease. Mean tropical cyclones’ size, defined as the radius of the gale-force wind (r17), were simulated to have minimal changes. However, the changes of Vmax, RMW and R17 among different tropical cyclones have a large variability. Relations of the changing sea surface temperature, vertical temperature profile, relative humidity and the vertical wind shear to the intensities and structures of the tropical cyclones were examined using multiple linear regression analysis.
Azimuthally averaged surface wind profile fitted by Holland Model in near and far future under RCP8.5 and RCP4.5 emission scenario of Tropical Cyclone Rammasun (2014) during its peak intensity in the South China Sea region, indicating an increase in surface maximum wind speed and a decrease in the Radius of Maximum wind.
Numerical Investigation of the Eyewall Evolution in Landfalling Tropical Cyclones (TCs)
Numerical Investigation of the Eyewall Evolution in Landfalling Tropical Cyclones (TCs)
This project is supervised by Prof. Chun-Chieh Wu at the National Taiwan University Typhoon Dynamics Research Center as part of a (remote) summer research visit.
The study aims to investigate the eyewall evolution of Typhoon Mangkhut through numerical simulations using the Weather Research and Forecasting Model (WRF). Mangkhut’s eyewall contracted before landfall over Luzon, broke down after landfall, and a much larger eyewall formed as Mangkhut re-entered the ocean (Fig. 1). Similar phenomena have already been observed in historical TCs making landfall over Luzon. Examples include TC Zeb (1998) [1] and TC Megi (2010) [2].
The role of terrain, land surfaces, and ocean on the observed eyewall evolution in Mangkhut will be investigated through carefully designed sensitivity experiments. It is hoped that research results will lead to better understanding of the effects of land surfaces on the eyewall evolution of TCs, and hopefully improve future intensity forecasts for landfalling TCs.
Fig 1. Radar reflectivity (unit: dBZ) at the lowest model level from 4 September 10:00 UTC to 15 September 08:00 UTC 2018 in two-hourly intervals. The contraction and breakdown of the original eyewall and the formation of a new, much larger eyewall is observed.
[1] Wu, C. C., Cheng, H. J., Wang, Y., & Chou, K. H. (2009). A numerical investigation of the eyewall evolution in a landfalling typhoon. Monthly weather review, 137(1), 21-40.
[2] Wang, H., & Wang, Y. (2021). A Numerical Study of Typhoon Megi (2010). Part II: Eyewall Evolution Crossing the Luzon Island. Monthly Weather Review, 149(2), 375-394.
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