Changes in Seasonal Precipitation and its Impacts on the Terrestrial Hydrological Cycle

• Main Authors: Chia-Wei Lan, 藍嘉偉
• Other Authors: Min-Hui Lo
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/t3pqea

博士 === 國立臺灣大學 === 大氣科學研究所 === 107 === This dissertation investigated the mechanisms of seasonal precipitation changes between Coupled Model Intercomparison Project Phase 5 (CMIP5) AMIP-type outputs and reanalysis datasets. We also suggested that the possible impacts of frequent extreme precipitation on the terrestrial hydrological cycle under global warming. Finally, we investigate the responses of tropical and monsoon circulations to the different soil water conditions resulting from different land-atmospheric interactions. Previous studies demonstrated the increased annual range of precipitation as the climate warms. However, these documents only used the model outputs to discuss the mechanism of seasonal precipitation changes and found that the increased water vapor plays a curial role on such precipitation changes. When using the reanalysis datasets, we revealed the changes in the dynamic component in the water budget analysis are more important for the observed precipitation changes. Such discrepancy might be due to the tendency toward stability in CMIP5_AMIP owing to more tropical warming rate in the mid-upper troposphere compared to that of reanalysis datasets. Such a tendency also leads to weakening tropical vertical motion in CMIP5_AMIP. The increased annual range of precipitation is indicative of higher frequency of extreme precipitation, which can affect the terrestrial hydrological cycle. We also investigated net water flux into the soil over the rainforest areas, including Amazon, Congo, and the Maritime Continent. Nonlinear responses to extreme precipitation lead to a reduction of infiltration and a proportionately higher amount of direct runoff, particularly for the Maritime Continent, where both the amount and intensity of precipitation increase under global warming, and such precipitation changes are related to increased water vapor under global warming. In addition, the near-surface soil moisture is obviously increased over the Maritime Continent. Finally, a pair of idealized experiments corresponding to contrast fixed groundwater table depths over the Earth’s continents by AMIP-type simulations in the Community Earth System Model (CESM) was conducted. In the wet (shallow water table) experiment, both land evapotranspiration and soil moisture tend to increase, leading to an increased meridional surface temperature gradient, which also causes the tropical circulation stronger than that of the dry (deep water table) experiment. Relative to the dry experiment, the wet experiment exhibited the enhancement of southward (northward) latent energy (dry static energy) transport coincide with the stronger tropical circulation. Despite larger surface latent heat fluxes to the atmosphere in the wet soil case, the monthly mean of stationary eddy demonstrated the reductions of northward latent energy transports due to compensation by a notably weakening South-Asia monsoon circulation associated with weaker land-sea thermal contrast. This study indicates the importance of groundwater variations and land surface conditions in global energy transport and has further implications for earth system model development. Under global warming, land use and climate changes have profound impacts on the terrestrial hydrological cycle, which further cause the difficulty of water resource management, the higher risk of flooding and drought, and even the food shortage. Consequently, how the atmosphere affects the terrestrial hydrological cycle and its feedbacks to the atmosphere through land-atmospheric interactions will be a critical issue in the warming future.