Multi-scale features of atmospheric rivers and the linkages with local-scale hydrological impacts on the U.S. West Coast

Atmospheric Rivers;Multi-scale features;Hydrologic impacts

Hu, Huancui

关键词: Atmospheric Rivers;    Multi-scale features;    Hydrologic impacts;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101507/HU-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Atmospheric rivers (ARs), defined as filamentary structures of strong water vapor transport in the atmosphere, are an important component of the hydrologic cycle and are characterized by a multi-scale nature. On the one hand, ARs are embedded in the planetary-scale Rossby waves and account for the majority of poleward moisture transport in the midlatitudes. On the other hand, ARs are the fundamental reason for generating extreme basin-scale precipitation and floodings over many regions of the world including the U.S. West Coast in the winter season. The goal of this dissertation is to examine the multi-scale features associated with ARs. The links to local-scale hydrological impacts are of particular interest because they are the key in estimating local hydrological consequences associated with climate variability and projecting flood-risk changes with global warming. The first part of this dissertation presents the connection between the characteristics of large-scale Rossby wave breaking (RWB) over the eastern North Pacific and the regional-scale hydrological impacts associated with landfalling ARs on the U.S. West Coast. ARs associated with RWB account for 2/3 of the landfalling AR events and >70% of total AR-precipitation in the winter season. The two regimes of RWB – anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB) – are associated with different directions of the vertically integrated water vapor transport (IVT). AWB-ARs impinge in a more westerly direction on the coast while CWB-ARs impinge in a more southwesterly direction. Most of the landfalling ARs along the northwestern coast of the U.S. (states of Washington and Oregon) are AWB-ARs, due to their westerly impinging angles arriving more orthogonally to the western Cascades and more efficiently transforming water vapor into precipitation through orographic lift. Consequently, AWB-ARs account for the most extreme streamflows in the region. Along the southwest coast of the U.S. (California), the southwesterly impinging angles of CWB-ARs are more orthogonal to the local topography, and are also characterized by more intense IVT. Consequently, CWB-ARs are associated with the most intense precipitation and account most of the extreme streamflows in southwest coastal basins. The second part of this dissertation investigates the contribution of tropical moisture on AR strengths and the associated meso-scale mechanisms. We used the water vapor tracer tool embedded in WRF (WRF-WVT) to “tag” moisture from the tropics in the simulations of the top 15% of ARs affecting the Northwest (based on their IVT). This tool helps to provide a joint view of tracer moisture (that from the tropics) and total moisture in ARs’ lifecycles. We first analyze an AR event in detail and find that tropical moisture affects AR development through direct and indirect mechanisms in the following ways: 1) narrow line-convection over the cold front is mostly from tropical moisture; 2) latent heat release associated with cold-frontal precipitation induces positive potential vorticity that helps enhance the pre-cold-frontal low-level jet (LLJ); 3) the enhanced LLJ in turn enhances entrainment of tropical moisture along this AR. We also found significant “mutual amplification” near the warm front where most landfall is generated. It is achieved through strong warm air advection ahead of the tropical moisture arrival that helps enhance low-level moisture convergence and vertical ascent (in other words, an enhanced ageostropic circulation). These processes associated with tropical moisture are confirmed by other simulated cases. We found increasing trends of cold-frontal precipitation and latent heat generated potential vorticity in line with enhanced LLJ. However, ARs with stronger tropical moisture contribution are not characterized by significantly stronger ageostrophic circulation. This indicates the importance of other dynamical factors in modulating AR precipitation. The third part of this dissertation examines the interactions of local land surface conditions with ARs. We develop and implement a novel numerical water tracer model within the Noah-Multiparameterizations (WT-Noah-MP) land surface model to investigate the regional responses to ARs arriving in the U.S. Pacific Northwest. This approach is specifically designed to track individual hydrometeorological events (such as ARs) and it provides a more comprehensive representation of the physical processes beyond the standard land surface model output. In the WT-Noah-MP simulations, we “tag” the precipitation from individual AR events using this tool, and it provides stores, fluxes and transit time estimates of the “tagged” water in the surface-subsurface system. Consequently, we are able to test the tracer mixing assumptions by comparing transit times obtained from isotope observations in the WS10 watershed (Oregon). In addition, from a regional simulation of an extreme AR, the model helps to differentiate the flood response due to direct precipitation from indirect thermal effects and showed that a large portion of this event water was retained in the soil after 6 months. The water tracer addition in Noah-MP can help us quantify the long-term memory in the hydrologic system that can impact seasonal hydroclimate variability through evapotranspiration and groundwater recharge.