Towards an improved understanding of precipitation variations over the Tibetan Plateau

Sammanfattning: The Tibetan Plateau (TP) and surrounding regions are known as the ``Third Pole'' because of their polar-like environment and large reservoirs of fresh water. Due to the latter aspect, the TP is also considered the ``Water Tower of Asia'', providing essential water resources to the surrounding regions. The most important supply to this water tower is atmospheric precipitation, which is affected by both local processes, such as convection and orographic lifting, and remote mechanisms including atmospheric circulation patterns, such as dynamics of monsoon systems and the westerly jets. However, how precipitation over the TP has changed spatially and temporally and the key mechanisms behind these changes remain to be fully understood. This dissertation addresses the following research questions related to TP precipitation: 1) How does the seasonality of TP precipitation vary spatially, and what roles do large-scale atmospheric circulation patterns over and around the TP play in determining precipitation seasonality? 2) Can fine-resolution regional atmospheric models simulate precipitation over the TP more realistically than state-of-the-art global reanalysis datasets? The dissertation aims to advance our understanding of precipitation and related large-scale to mesoscale processes over the TP using ground-based and remote sensing observations, reanalyses, and high-resolution atmospheric modeling. Specifically, the dissertation focuses on the seasonality and interannual variability of regional precipitation, as well as long-term changes in atmospheric large-scale circulations. The work also evaluates the capability of long-integration high-resolution atmospheric modeling and explores the model uncertainties from different models, domain sizes, and physical parameterizations of subscale processes in the model. The first part of the key findings is the spatial variations in the seasonality of precipitation over the TP. Three distinct precipitation regimes were found: winter peak in the western TP, early summer peak in the eastern TP, and late summer peak mainly in the southwestern TP. The winter peak regime is the most robust region with a relatively constant extent in time, while the boundary between the other two regimes varies on an interannual time scale, especially in the central TP. The variations in the boundary are associated with combined changes in summer monsoons, westerly jets, and other large-scale systems. In terms of changes in large-scale circulations, there is a positive trend in the occurrence of summer-type wind patterns. Additionally, while westerly jets were previously thought to mainly influence the winter precipitation, we found that variations in the jets can have a dominant role in changing the precipitation patterns during the transitions seasons as well. Another finding is that a long-term integration of a high-resolution meteorological model improves upon the state-of-the-art global ERA5 reanalysis, with significantly reduced wet bias over the TP by simulating weaker low-level southerly winds. The findings in this dissertation enhance our understanding of the TP precipitation patterns and associated atmospheric processes, particularly large-scale circulations. Moreover, the positive evaluation demonstrates the potential of high-resolution modeling in simulating precipitation and related atmospheric processes, thereby providing valuable insights into what control the precipitation changes over the TP in the past and potentially in the future.