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Changes in Winter Atmospheric Rivers along the North American West Coast in CMIP5 Climate Models

American Meteorological Society (AMS) | April 1st, 2014

Summary

Most extreme precipitation events that occur along the North American west coast are associated with winter atmospheric river (AR) events. Global climate models h

Most extreme precipitation events that occur along the North American west coast are associated with winter atmospheric river (AR) events. Global climate models have sufficient resolution to simulate synoptic features associated with AR events, such as high values of vertically integrated water vapor transport (IVT) approaching the coast. From phase 5 of the Coupled Model Intercomparison Project (CMIP5), 10 simulations are used to identify changes in ARs impacting the west coast of North America between historical (1970–99) and end-of-century (2070–99) runs, using representative concentration pathway (RCP) 8.5. 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% [from 4% to 6% (8C)21], while precipitation on extreme IVT days increases by 15%–39% [from 5% to 19% (8C)21]. The frequency of IVT days above the historical 99th percentile threshold increases as much as 290% by the end of this century.

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Characterizing the spatial scales of extreme daily precipitation in the United States

American Meteorological Society (AMS) | October 1st, 2018

Summary

The spatial extent of an extreme precipitation event can be important for a basin’s hydrologic response and subsequent flood risk, and may yield insights into underlyi

The spatial extent of an extreme precipitation event can be important for a basin’s hydrologic response and subsequent flood risk, and may yield insights into underlying atmospheric processes. Using a relaxed movingneighborhood approach, we develop indicator semivariograms based on precipitation records from the Global Historical Climatology Network–Daily (GHCN-D) station network to directly quantify the climatological length scales of extreme daily precipitation over the United States during 1965–2014. We find that the length scales of extreme (90th percentile) daily precipitation events vary both regionally and seasonally. Over the eastern half of the United States, daily extreme precipitation length scales reach 400 km during the winter months, but are approximately half as large during the summer months. The Northwest region, on the other hand, exhibits little seasonal variation, with extreme precipitation length scales of approximately 150 km throughout the year. By leveraging in situ station measurements, our study avoids some of the uncertainties associated with satellite or interpolated precipitation data, and provides the longest climatological assessment of length scales of extreme daily precipitation over the United States to date. Although the length scales that we calculate can be sensitive to station density, neighborhood size, and neighborhood relaxation, we find that the interregional and inter-seasonal differences in length scales are relatively robust. Our method could be extended to quantify changes in the spatial extent of extreme daily precipitation in the recent past, and to investigate the underlying causes of any changes that are detected.

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Climate change contributions to future atmospheric river flood damages in the western United States

Nature Portfolio (Springer Nature) | August 12th, 2022

Summary

Atmospheric rivers (ARs) generate most of the economic losses associated with flooding in the western United States and are projected to increase in intensity with climat

Atmospheric rivers (ARs) generate most of the economic losses associated with flooding in the western United States and are projected to increase in intensity with climate change. This is of concern as flood damages have been shown to increase exponentially with AR intensity. To assess how AR-related flood damages are likely to respond to climate change, we constructed county-level damage models for the western 11 conterminous states using 40 years of flood insurance data linked to characteristics of ARs at landfall. Damage functions were applied to 14 CMIP5 global climate models under the RCP4.5 “intermediate emissions” and RCP8.5 “high emissions” scenarios, under the assumption that spatial patterns of exposure, vulnerability, and flood protection remain constant at present day levels. The models predict that annual expected AR-related flood damages in the western United States could increase from $1 billion in the historical period to $2.3 billion in the 2090s under the RCP4.5 scenario or to $3.2 billion under the RCP8.5 scenario. County-level projections were developed to identify counties at greatest risk, allowing policymakers to target efforts to increase resilience to climate change.

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Climate change intensification of horizontal water vapor transport in CMIP5

American Geophysical Union (AGU) | June 8th, 2015

Summary

Global warming of the Earth’s atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most pr

Global warming of the Earth’s atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most previous research has used precipitation. This study, however, investigates projected changes to global atmospheric water vapor transport (integrated vapor transport (IVT)), the key link between water source and sink regions. Using 22 global circulation models from the Climate Model Intercomparison Project Phase 5, we evaluate, globally, the mean, standard deviation, and the 95th percentiles of IVT from the historical simulations (1979–2005) and two emissions scenarios (2073–2099). Considering the more extreme emissions, multimodel mean IVT increases by 30–40% in the North Pacific and North Atlantic storm tracks and in the equatorial Pacific Ocean trade winds. An acceleration of the high-latitude IVT is also shown. Analysis of low-altitude moisture and winds suggests that these changes are mainly due to higher atmospheric water vapor content.

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Climate change intensification of horizontal water vapor transport in CMIP5

American Geophysical Union (AGU) | July 8th, 2015

Summary

Global warming of the Earth’s atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most pr

Global warming of the Earth’s atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most previous research has used precipitation. This study, however, investigates projected changes to global atmospheric water vapor transport (integrated vapor transport (IVT)), the key link between water source and sink regions. Using22 global circulation models from the Climate Model Intercomparison Project Phase 5, we evaluate, globally, the mean, standard deviation, and the 95th percentiles of IVT from the historical simulations(1979–2005) and two emissions scenarios (2073–2099). Considering the more extreme emissions, multimodel mean IVT increases by 30–40% in the North Pacific and North Atlantic storm tracks and in the equatorial Pacific Ocean trade winds. An acceleration of the high-latitude IVT is also shown. Analysis of low-altitude moisture and winds suggests that these changes are mainly due to higher atmospheric water vapor content.

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Climate change, atmospheric rivers, and floods in California - a multimodel analysis of storm frequency and magnitude changes

American Water Resources Association (AWRA) | June 1st, 2011

Summary

Recent studies have documented the important role that “atmospheric rivers” (ARs) of concentrated near‐surface water vapor above the Pacific Ocean play in the storm

Recent studies have documented the important role that “atmospheric rivers” (ARs) of concentrated near‐surface water vapor above the Pacific Ocean play in the storms and floods in California, Oregon, and Washington. By delivering large masses of warm, moist air (sometimes directly from the Tropics), ARs establish conditions for the kinds of high snowlines and copious orographic rainfall that have caused the largest historical storms. In many California rivers, essentially all major historical floods have been associated with AR storms. As an example of the kinds of storm changes that may influence future flood frequencies, the occurrence of such storms in historical observations and in a 7‐model ensemble of historical‐climate and projected future climate simulations is evaluated. Under an A2 greenhouse‐gas emissions scenario (with emissions accelerating throughout the 21st Century), average AR statistics do not change much in most climate models; however, extremes change notably. Years with many AR episodes increase, ARs with higher‐than‐historical water‐vapor transport rates increase, and AR storm‐temperatures increase. Furthermore, the peak season within which most ARs occur is commonly projected to lengthen, extending the flood‐hazard season. All of these tendencies could increase opportunities for both more frequent and more severe floods in California under projected climate changes.

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Climate Change, Atmospheric Rivers, and Floods in California – A Multimodel Analysis of Storm Frequency and Magnitude Changes

American Water Resources Association (AWRA) | June 1st, 2011

Summary

Recent studies have documented the important role that “atmospheric rivers” (ARs) of concentrated near‐surface water vapor above the Pacific Ocean play in the storm

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Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States

American Meteorological Society (AMS) | January 24th, 2014

Summary

Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, bu

Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, but knowledge of their influence over the interior is limited. Here, the authors use Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) observations to describe the characteristics of cool-season (November–April) ARs over the western United States. It is shown that AR frequency and duration exhibit a maximum along the Oregon–Washington coast, a strong transition zone upwind (west) of and over the Cascade–Sierra ranges, and a broad minimum that extends from the “high” Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade–Sierra ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade–Sierra ranges. Farther east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.

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Debris flow occurrence under changing climate and wildfire regimes: A southern California perspective

Geomorphology (Elsevier) | December 2nd, 2022

Summary

In southern California, wildfire incidents are increasing in frequency and magnitude. There is a great deal of research predicting the impact of climate change on large s

In southern California, wildfire incidents are increasing in frequency and magnitude. There is a great deal of research predicting the impact of climate change on large scale fire regimes. These impacts, now the new normal, include increases in future fire incidents and burn severities. Yet predictive studies on how ecosystems respond to changing fire regimes are lacking. Identifying trends in post-fire erosion patterns, such as post-fire debris flows, is critical for increasing human resiliency and adaptation to changing weather patterns and eventual long-term climate change. The relative historical abundance of post-fire debris flows in southern California presents this concern: will worsening wildfire regimes result in more post-fire debris flows? Our research addresses this concern by utilizing a descriptive approach to analyze post-fire debris flows in southern California from 2001 to 2018 to elucidate possible trends. We found that 84 % of debris flows originate from watersheds with soils burnt at moderate soil burn severity. We found that debris flow occurrence is more reliant on the percent coverage of a soil burn severity classification than its areal extent. Further, larger fires also tended to produce more debris flows and most (73 %) debris flow watersheds occurred in predominantly bedrock areas. Our research suggests that the number of post-fire runoff-generated debris flows in the region will increase if fires continue to increase in size, occur on slopes of at least 20°, burn south-facing watersheds with bedrock of igneous or sedimentary origin to at least a moderate severity, and peak rainfall intensity return intervals do not change. These findings highlight the interconnectivity between natural erosive processes, wildfires, humans, and climate change.

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Distribution of Landfalling Atmospheric Rivers on the U.S. West Coast During Water Year 2018 Through April

Scripps Institution of Oceanography at U.C. San Diego | March 1st, 2018

Summary

Forty-four atmospheric rivers (ARs) made landfall on the West Coast during the 2018 water year through April compared to 68 the year before, report meteorologists Chad H

Forty-four atmospheric rivers (ARs) made landfall on the West Coast during the 2018 water year through April compared to 68 the year before, report meteorologists Chad Hecht and Marty Ralph from the Center for Western Weather and Water Extremes, Scripps Institution of Oceanography at U.C. San Diego. This notes the ARs' landfall patterns, strength, geographic distribution, and timing.

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