Winter extreme precipitation along the North American west coast
Warner, Michael D.
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Most extreme precipitation events that occur along the North American west coast are associated with winter atmospheric river (AR) events, causing flooding, landslides, extensive property damage, and loss of life. The studies contained within this dissertation use a combination of NCDC precipitation observations, NCEP-NCAR reanalysis, a 10-model ensemble of historical and future CMIP5 climate model simulations, and an NCEP-NCAR reanalysis driven regionally downscaled WRF model simulation to characterize the synoptic evolution of AR events along the North American west coast, the spatial variability of precipitation along the coast and inland, and changes in AR intensity and frequency that are expected by the end of the 21st century. Most regional flooding events are associated with precipitation periods of 24 hours or less, and two-day precipitation totals identify nearly all major events. Precipitation areas of major events are generally narrow, roughly 200 km in width, and most are associated with ARs. Composite evolutions indicate negative anomalies in sea-level pressure and upper-level height in the central Pacific, high-pressure anomalies over the southwest U.S., large positive 850-hPa temperature anomalies along the coast and offshore, and enhanced precipitable water and integrated water vapor fluxes in southwest- to northeast-oriented swaths. A small subset of extreme precipitation events over the southern portion of the domain is associated with a very different synoptic evolution: a sharp trough in northwesterly flow and post-cold-frontal convection. High precipitable water values are more frequent during the summer but are not associated with heavy precipitation because of upper-level ridging over the eastern Pacific and weak onshore flow that limits upward vertical velocities. Global climate models have sufficient resolution to simulate synoptic features associated with AR events, such as high values of vertically integrated vapor transport (IVT) approaching the coast. Ten CMIP5 simulations are used to identify changes in ARs impacting the west coast of North America between historical (1970-1999) and end-of-century (2070-2099) representative concentration pathway (RCP) 8.5 runs. The most extreme ARs are identified in both time periods by the 99th percentile of IVT days along a north-south transect offshore of the coast. Integrated water vapor (IWV) and IVT are predicted to increase, while lower-tropospheric winds change little. Winter-mean precipitation along the West Coast increases by 11-18% (4-6% °C<super>-1</super>) while precipitation on extreme IVT days increases by 15-39% (5-19% °C<super>-1</super>). The frequency of IVT days above the historical 99th percentile threshold increases as much as 290% by the end of this century. There appear to be only very slight changes in annual AR climatology from historical to future time periods when considering the most extreme events (99th percentile). However, when evaluating by the number of future days exceeding the historical threshold, there are significant increases in extreme IVT events in all months, especially when the majority of events take place. The peaks in historical and future frequency occur in similar months given the amount of model variability. Extreme IVT events appear to be occurring slightly earlier in the season, particularly in the northern part of the domain, and these results are similar to other studies. Spatially, 10-model mean historical composites of IVT reveal canonical AR conditions. At locations farther south, there is less model agreement on what AR events should look like, both in spatial extent and intensity; whereas farther north, the various models agree more. The future composites indicate very little spatial change. The models behave similarly in both the historical and future runs, suggesting little change in dynamics. The future-historical difference plots highlight the largest changes expected in the future, namely increases in IVT intensity which are primarily associated with thermodynamic changes related to future IWV increases due to warming. The dynamically downscaled NCEP-NCAR reanalysis-driven WRF model, run with a 36-km resolution outer domain and a 12-km nest, contains more realistic terrain than most GCMs and highlights the spatial precipitation distribution over the Pacific Northwest. Winter precipitation in the Pacific Northwest correlates well with offshore daily IVT (as high as &sim0.8) with spatial signatures indicative of frequent coastal mid-latitude cyclones impacting the coast. However, the most extreme AR events did not correlate as highly as expected with daily precipitation (as high as &sim0.4), despite ARs accounting for 8% or more of the total winter precipitation. When wind direction was taken into account, the correlations were much higher (&sim0.7-0.8), indicating wind direction is an important factor when extreme precipitation occurs along the coast.
- Atmospheric sciences