Publications

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2019
Gershunov, A, Shulgina T, Clemesha RES, Guirguis K, Pierce DW, Dettinger MD, Lavers DA, Cayan DR, Polade SD, Kalansky J, Ralph FM.  2019.  Precipitation regime change in Western North America: The role of atmospheric rivers. Scientific Reports. 9   10.1038/s41598-019-46169-w   AbstractWebsite

Daily precipitation in California has been projected to become less frequent even as precipitation extremes intensify, leading to uncertainty in the overall response to climate warming. Precipitation extremes are historically associated with Atmospheric Rivers (ARs). Sixteen global climate models are evaluated for realism in modeled historical AR behavior and contribution of the resulting daily precipitation to annual total precipitation over Western North America. The five most realistic models display consistent changes in future AR behavior, constraining the spread of the full ensemble. They, moreover, project increasing year-to-year variability of total annual precipitation, particularly over California, where change in total annual precipitation is not projected with confidence. Focusing on three representative river basins along the West Coast, we show that, while the decrease in precipitation frequency is mostly due to non-AR events, the increase in heavy and extreme precipitation is almost entirely due to ARs. This research demonstrates that examining meteorological causes of precipitation regime change can lead to better and more nuanced understanding of climate projections. It highlights the critical role of future changes in ARs to Western water resources, especially over California.

2018
Nash, D, Waliser D, Guan B, Ye HC, Ralph FM.  2018.  The role of atmospheric rivers in extratropical and polar hydroclimate. Journal of Geophysical Research-Atmospheres. 123:6804-6821.   10.1029/2017jd028130   AbstractWebsite

Atmospheric rivers (ARs) are narrow, long, transient, water vapor-rich corridors of the atmosphere that are responsible for over 90% of the poleward water vapor transport in and across midlatitudes. However, the role of ARs in modulating extratropical and polar hydroclimate features (e.g., water vapor content and precipitation) has not been fully studied, even though moistening of the polar atmosphere is both a key result and amplifier of Arctic warming and sea ice melt, and precipitation is key to the surface mass balance of polar sea ice and ice sheets. This study uses the Modern-Era Retrospective analysis for Research and Applications, Version 2 reanalysis to characterize the roles of AR water vapor transport on the column-integrated atmospheric water vapor budget in the extratropical and polar regions of both hemispheres. Meridional water vapor transport by ARs across a given latitude (examined for 40 degrees, 50 degrees, 60 degrees, and 70 degrees) is strongly related to variations in area-averaged (i.e., over the cap poleward of the given latitude) total water vapor storage and precipitation poleward of that latitude. For the climatological annual cycle, both AR transport (i.e., nonlocal sources) and total evaporation (i.e., local sources) are most correlated with total precipitation, although with slightly different phases. However, for monthly anomalies, the water budget at higher latitudes is largely dominated by the relationship between AR transport and precipitation. For pentad and daily anomalies, AR transport is related to both precipitation and water vapor storage variations. These results demonstrate the important role of episodic, extreme water vapor transports by ARs in modulating extratropical and polar hydroclimate. Plain Language Summary The term atmospheric river (AR) was coined by scientists Zhu and Newell in the early 1990s with the main result highlighting the importance of relatively infrequent, long conduits of strong moisture transport being responsible for most of the poleward transport of moisture across the midlatitudes and into the polar regions. While it is generally understood that this moisture is critical to the water and energy budgets of high latitudes, there have been no studies that have ever quantified the relationship between AR poleward moisture transports and the hydroclimate features of high latitudes. After a long hiatus in the consideration of the role of ARs on global climate since those of Zhu and Newell, this study quantifies the connections between water vapor transport by ARs across specific latitudes (e.g., 40 degrees) and the hydroclimate poleward of this latitude. The findings show there are strong, time scale-dependent (e.g., daily and monthly) connections between ARs and high-latitude hydroclimate features. For example, the findings show a strong relationship between AR water vapor transport at a given latitude and the area-averaged total precipitation of the region poleward. This and other results in this study indicate the importance of ARs in shaping our global weather and climate.

DeFlorio, MJ, Waliser DE, Guan B, Lavers DA, Ralph FM, Vitart F.  2018.  Global assessment of atmospheric river prediction skill. Journal of Hydrometeorology. 19:409-426.   10.1175/jhm-d-17-0135.1   AbstractWebsite

Atmospheric rivers (ARs) are global phenomena that transport water vapor horizontally and are associated with hydrological extremes. In this study, the Atmospheric River Skill (ATRISK) algorithm is introduced, which quantifies AR prediction skill in an object-based framework using Subseasonal to Seasonal (S2S) Project global hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The dependence of AR forecast skill is globally characterized by season, lead time, and distance between observed and forecasted ARs. Mean values of daily AR prediction skill saturate around 7-10 days, and seasonal variations are highest over the Northern Hemispheric ocean basins, where AR prediction skill increases by 15%-20% at a 7-day lead during boreal winter relative to boreal summer. AR hit and false alarm rates are explicitly considered using relative operating characteristic (ROC) curves. This analysis reveals that AR forecast utility increases at 10-day lead over the North Pacific/western U.S. region during positive El Nino-Southern Oscillation (ENSO) conditions and at 7-and 10-day leads over the North Atlantic/U.K. region during negative Arctic Oscillation (AO) conditions and decreases at a 10-day lead over the North Pacific/western U.S. region during negative Pacific-North America (PNA) teleconnection conditions. Exceptionally large increases in AR forecast utility are found over the North Pacific/western United States at a 10-day lead during El Nino + positive PNA conditions and over the North Atlantic/United Kingdom at a 7-day lead during La Nina + negative PNA conditions. These results represent the first global assessment of AR prediction skill and highlight climate variability conditions that modulate regional AR forecast skill.

2014
Neiman, PJ, Ralph FM, Moore BJ, Zamora RJ.  2014.  The regional influence of an intense Sierra Barrier jet and landfalling atmospheric river on orographic precipitation in Northern California: A case study. Journal of Hydrometeorology. 15:1419-1439.   10.1175/jhm-d-13-0183.1   AbstractWebsite

A 915-MHz wind profiler, a GPS receiver, and surface meteorological sites in and near California's northern Central Valley (CV) provide the observational anchor for a case study on 23-25 October 2010. The study highlights key orographic influences on precipitation distributions and intensities across northern California during a landfalling atmospheric river (AR) and an associated Sierra barrier jet (SBJ). A detailed wind profiler/GPS analysis documents an intense AR overriding a shallow SBJ at similar to 750 m MSL, resulting in record early season precipitation. The SBJ diverts shallow, pre-cold-frontal, incoming water vapor within the AR poleward from the San Francisco Bay gap to the northern CV. The SBJ ultimately decays following the passage of the AR and trailing polar cold front aloft. A statistical analysis of orographic forcing reveals that both the AR and SBJ are crucial factors in determining the amount and spatial distribution of precipitation in the northern Sierra Nevada and in the Shasta-Trinity region at the northern terminus of the CV. As the AR and SBJ flow ascends the steep and tall terrain of the northern Sierra and Shasta-Trinity region, respectively, the precipitation becomes enhanced. Vertical profiles of the linear correlation coefficient quantify the orographic linkage between hourly upslope water vapor flux profiles and hourly rain rate. The altitude of maximum correlation (i.e., orographic controlling layer) is lower for the shallow SBJ than for the deeper AR (i.e., 0.90 versus 1.15 km MSL, respectively). This case study expands the understanding of orographic precipitation enhancement from coastal California to its interior. It also quantifies the connection between dry antecedent soils and reduced flood potential.

2013
White, AB, Anderson ML, Dettinger MD, Ralph FM, Hinojosa A, Cayan DR, Hartman RK, Reynolds DW, Johnson LE, Schneider TL, Cifelli R, Toth Z, Gutman SI, King CW, Gehrke F, Johnston PE, Walls C, Mann D, Gottas DJ, Coleman T.  2013.  A twenty-first-century California observing network for monitoring extreme weather events. Journal of Atmospheric and Oceanic Technology. 30:1585-1603.   10.1175/jtech-d-12-00217.1   AbstractWebsite

During Northern Hemisphere winters, the West Coast of North America is battered by extratropical storms. The impact of these storms is of paramount concern to California, where aging water supply and flood protection infrastructures are challenged by increased standards for urban flood protection, an unusually variable weather regime, and projections of climate change. Additionally, there are inherent conflicts between releasing water to provide flood protection and storing water to meet requirements for the water supply, water quality, hydropower generation, water temperature and flow for at-risk species, and recreation. To improve reservoir management and meet the increasing demands on water, improved forecasts of precipitation, especially during extreme events, are required. Here, the authors describe how California is addressing their most important and costliest environmental issue-water management-in part, by installing a state-of-the-art observing system to better track the area's most severe wintertime storms.

2012