fnctId=thesis,fnctNo=367
Dynamics of Land-Atmosphere Interaction over Snow/Ice-covered Greenland and High-Mountain Regions
- 작성자
- 기후시스템전공
- 저자
- Manuel Tobias Blau
- 발행사항
- 발행일
- 2025-02
- 저널명
- 국문초록
- 영문초록
- Snow cover is a crucial component of the integrated climate system. Its potential to alter surface conditions significantly impacts climate change by modifying the energy budget. Furthermore, it is part of the Earth’s hydrological cycle. A substantial amount of freshwater is stored in glaciers and snow cover. Upon release during warm seasons, this water can supply vast areas in the catchment of its outlet rivers for irrigation. Ablation due to melting or sublimation relates to rising temperatures; therefore, with respect to global climate change, snow cover has undergone and may experience a continuous decline. This poses risks to many settlements in the catchments of the runoff rivers or coastal areas due to sea level rise. The apriori hypothesis that snow cover has declined proportionally with temperature warming motivated the present study. In particular, the thesis sheds light on the complex relationship between temperature and snow cover trends to derive the underlying physical mechanisms of recent snow cover trends. Moreover, the results provide a deeper understanding of the processes determining extreme temperatures in the summer season over Greenland, which are related to the most recent ablation events of the Greenland ice sheet. For this, the reanalysis product from the fifth generation European Centre for MediumRange Weather Forecasts (ERA5) provided high-quality data of the recent past from 1979 onwards. Finally, the thesis discusses snow cover on the Tibetan Plateau using output from general circulation models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The first part focuses on the persistent mountain snow cover. Here, snow cover in regions above 1 km, with a snow persistence of at least 190 days in one year and significant trends, is defined as persistent mountain snow cover. From ERA5-Land reanalysis data, global mountain regions warmed by about 1.19 °C over 44 years starting from 1979. Simultaneously, persistent snow cover exhibited a significant decline of 7.79 %. However, the response of snow cover to temperature trends was non-linearly distributed over different mountain regions and depended on the regional mean surface temperature. The results indicate that reduced snow accumulation and a decline in precipitation in warmer mountain regions, which led to snowfall trends, were the leading processes explaining the decline in snow cover. In cold mountain regions, the decline in snowfall exceeded precipitation trends, leading to snow cover depletion due to more precipitation falling as rain. The heterogeneous snow cover trends attributed a considerable change in the surface energy budget and led to regionally amplified or dampened warming trends. The second part sheds light on summer temperature extremes over Greenland. The mass balance of the Greenland ice sheet has undergone considerable interannual variability, with extreme ablation coinciding with above-normal temperatures. This analysis considered four warm and four cold years between 1979 and 2021, which mainly attributed to the surface albedo feedback and perturbations of downwelling longwave radiation, excluding the cloud effect. Anomalies in air temperature due to anomalous diabatic and adiabatic heating, and anomalous temperature advection resulting from changes in circulation explained the distinct characteristics of the surface energy budget. Furthermore, an anomalous anticyclone with high-pressure blocking features prevailed over Greenland in warm years, enhancing adiabatic warming on leeward-facing slopes. To the southwest of Greenland, anomalous dynamic warm air advection led to intense snow and ice melting alongside diabatic heating. A reinforcement of the blocking anti-cyclone resulted in the buildup of a heat dome over Greenland. The thesis includes a detailed evaluation of the modeled snow cover. State-of-the-art general circulation models from the Coupled Model Intercomparison Project Phase 6 had difficulty capturing the persistent mountain snow cover. However, historical simulations of some models can accurately simulate the spatial distribution of snow cover. Focusing on the Tibetan Plateau revealed a mismatch in snow cover trends related to temperature-related variables, including the snowfall fraction and snow melt, which are associated with temperature biases regarding mean state and trends. There was no robustness in the surface energy budget trends in the historical experiment. This considerably offsets changes in snow cover seasonality. In conclusion, the presented elaboration aims to enhance the physical understanding of the intrinsic relationship between snow cover and temperature trends. The findings are essential for various disciplines in climate science, such as the study of the energy and hydrological cycles. The results also raise awareness and improve preparedness within society, as changes in snow cover pose significant threats to populations worldwide.
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