6-research

Research

Physical Landscape and
Environmental Change
physical environment

The state of our physical environment affects our livelihood and well-being. Our common sustainable future depends on how we understand and manage environmental changes that occur in all spatial and temporal time scales. In this research cluster, we engage multidisciplinary initiatives and collaborate with emerging countries to study physical landscape and environmental changes. Using our cutting-edge research outcomes, we explore and provide sustainable solutions to our changing environments. Our research projects aim to:

 

(1) understand local and global changes in geological past and in human history;

(2) apply the latest field monitoring techniques to evaluate ecological well-being and health risk; and

(3) employ high-performance computing to investigate complex hydroclimatic changes.

flash floods hk

GRF Project (PI: Dr Jianfeng Li) Temporal evolution of sub-daily precipitation extremes in Hong Kong: Dependency on temperature and implications to flash floods

時間分辨率對極端降水特徵推算有重要影響。氣候暖化下,日尺度極端降水的增加率通常被認為遵循Clausius-Clapeyron (CC)關係,也就是當氣溫上升1 oC,大氣持水能力上升約6.5%。而小時尺度極端降水強度增加得更快,也被稱為超CC(super-CC)關係。然而,小時尺度仍難以準確反映歷時小於1小時的暴雨特徵。在全球暖化下,精細時間尺度(如分鐘到小時)極端降水及山洪的演化特徵(如長期趨勢和變化)是防洪設施設計和城市規劃(例如水塘和公路)的重要參數,尤其是對於香港等山多及人口密集地區。因為缺乏足夠時間長度和空間範圍的高質量日內尺度降水資料,人們對日內尺度極端降水(強度、歷時和總降水量等)和山洪對氣溫上升響應的認識仍較有限。另外,具有高度空間差異性的地形以及複雜的地表水文過程使得山洪的發生和過程具有不確定性。過往研究通常只關注日內尺度極端降水強度與山洪的關係,而較少考慮陸面水文特徵及流域過程的影響。 香港是世界人口第四密集的城市,其已發展地區毗鄰陡峭與崎嶇的地形。山多的地形以及濕潤的亞熱帶季風氣候使得該國際金融中心對山洪非常脆弱。本課題將基於香港超過130年的小時尺度降水記錄及超過30年的5分鐘尺度降水觀測數據,研究日內尺度極端降水事件強度、歷時和總量的時間演化及其與氣溫的關係,並使用耦合的水文大氣模式系統WRF/WRF-Hydro考慮陸面水文過程並模擬日內尺度極端降水變化對山洪的影響。本課題有助於提升人們對氣候暖化下日內尺度極端降水變化以及其對山洪潛在影響的認識。本課題研究結果將對香港及其他城市提升氣候變化應變能力,特別是防洪管理,具有實際意義。

pm 2.5 china

Early Career Scheme Project (PI: Dr Meng Gao) Assimilating surface PM2.5 and ozone measurements to improve health exposure assessment and air quality forecasting in South China

本研究擬發展同時同化地表PM2.5,臭氧和二氧化氮觀測數據的技術,以獲得更好的PM2.5和臭氧濃度時空分佈特徵,以減小污染健康暴露評估和空氣質量預測的不確定度。近年來有一些基於氣象-化學耦合模型的氣溶膠資料同化研究,而針對臭氧污染的同化研究仍然較少。然而,臭氧是華南區域非常嚴峻的污染問題,對於人類健康和作物生長都有不利影響。評價空氣污染的健康、氣候和生態效應也需要準確的污染物時空分佈特徵。本研究基於之前氣溶膠資料同化基礎,旨在開發同時同化PM2.5,臭氧和二氧化氮觀測數據的技術。開發的系統將應用於華南區域獲得PM2.5和臭氧時空分佈特徵,以及評估其對華南區域空氣污染暴露風險和空氣質量預測的改善效果。

kenya

GRF Project (PI: Prof Bernie Owen) Temporal Variations in the Controls of Lacustrine Sedimentation During Continental Rift Evolution: Evidence from the Northern Kenya Rift Valley

This study examines rock and sediment outcrops ranging in age from the Miocene to the present (~15–0 million years) in two areas of the northern Kenya Rift Valley.

 

This study will examine rock and sediment outcrops ranging in age from the Miocene to the present (~15–0 million years) in two areas of the northern Kenya Rift Valley. These include the arid Suguta Valley and, to the southwest, the eroded Tugen Hills. Both areas contain well-exposed deposits laid down in a variety of terrestrial and aquatic environments. This investigation will focus on those deposits formed in spring systems and fresh to saline lakes. The research will pursue four major lines of investigation: 1) a study of modern hot and cold springs and their deposits in the Suguta Valley; 2) characterising modern sedimentation in permanent and ephemeral lakes in the Suguta Valley, as well as changes in water chemistry between spring and river sources and modern saline lakes of the Suguta; 3) An investigation of past environments and sediments formed in ancient fresh to saline lakes that date back several million years in both the Suguta Valley and the Tugen Hills; and 4) integrating information on ancient and modern lakes in the northern Kenya Rift in order to determine if their are any systematic changes that can be related to rifting and volcanism in an evolving rift valley. The project will explore these major aims through field studies and systematic analyses of water (modern lake waters, rivers, springs and groundwater inflows) and sediment (outcrops, short cores) samples. We will provide field descriptions of sediment sequences and return samples for geochemical, sedimentological and diatom analyses. The results of this work will allow us to characterise the modern environments and to decipher the past depositional settings in which ancient sediments accumulated. In previous studies of the southern Kenya Rift, two members of the research team have detected many changes in the types of sediment that have accumulated at different times. These changes in deposition partly reflect varying past climates, but also mirror changes in the stage of development of the rift valley. This study will allow the research team to explore if their are similar, or different, long-term variations in deposition that may reflect changes in the evolving northern Kenya Rift and its tectonic setting and volcanic environments. We will then combine models from our earlier studies in the southern Kenya Rift with new models from this investigation in order to develop a broader-based understanding of how sedimentation changes with time in an evolving rift system.

saline lakes

Renaut, R.W., Owen, R.B., Lowenstein, T., De Cort, G., McNulty, E., Scott, J.J., Mbuthia, A. 2020. The role of hydrothermal fluids in sedimentation in saline alkaline lakes: evidence from Nasikie Engida, Kenya Rift Valley. Sedimentology 68(1), 1-27

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (‘Little Magadi’) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO3) forms bladed crystals that nucleate on the lake floor. Trona (Na2CO3·NaHCO3·2H2O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where p CO2 is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 ka, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.

grassland productivity

Hossain, M.K. & J. Li, 2020. Effects of long-term climatic variability and harvest frequency on grassland productivity across five ecoregions, Global Ecology and Conservation, https://doi.org/10.1016/j.gecco.2020.e01154

The degree to which grassland aboveground biomass responds to climatic variability (e.g. annual and growing season precipitation and temperature) as well as management practices (e.g. harvest frequency) has attracted considerable interest in ecological studies. This understanding is important for maintaining ecosystem stability and sustainable delivery of ecosystem services under climate change. Here, we analyzed grassland biomass observations in 31 study sites in 5 ecoregions (i.e. cold steppe, humid savanna, humid temperate, savanna, and temperate dry steppe) to examine the effects of growing season and annual climatic variability and harvest frequency on aboveground biomass productivity. Annual aboveground biomass productivity showed significant increasing trends in humid temperate and savanna, but the changes of annual biomass in cold steppe, humid savanna, and temperate dry steppe ecoregions were insignificant. Single harvest aboveground biomass in cold steppe, humid savanna and humid temperate ecoregions increased with higher growing season precipitation and temperature. Although annual precipitation had positive effects on annual biomass, we found growing season precipitation sum was a more important determinant in all ecoregions. Impacts of mean annual and growing season temperature on annual biomass in humid temperate were significantly positive, while significant adverse impacts of mean growing season temperature and mean annual temperature were found in savanna and temperate dry steppe ecoregions, respectively. Irrespective of climatic variability, annual biomass consistently increased with increasing harvest frequency across ecoregions. Our study found significant gains in grassland aboveground biomass across ecoregions with increased precipitation and harvest frequency, and significant loses of biomass in savanna and temperate ecoregions with increased temperature. Our results help improve the understanding of the differences in the responses of grassland productivity to climate variability and harvest frequency across various ecoregions, which is of importance to achieve sustainable grassland management in different geographical regions.