Evaluation of Non-hydrostatic Simulations of Northeast Pacific Atmospheric Rivers and Comparison to In-situ Observations

American Meteorological Society (AMS) | August 31st, 2015

Atmospheric rivers are long, narrow bands of concentrated atmospheric water vapor transport that provide an important atmospheric linkage between the subtropics and the m

Atmospheric rivers are long, narrow bands of concentrated atmospheric water vapor transport that provide an important atmospheric linkage between the subtropics and the midlatitudes, facilitating over 90% of meridional water vapor flux and often resulting in extreme precipitation events in regions of enhanced coastal orography. In this investigation, the authors conduct continuous (3 month), large-domain (3600 km × 3200 km), high-resolution (4 km), nonhydrostatic simulations using the Weather Research and Forecasting (WRF) Model and compare the observations to previously reported dropsonde observations from the California Land-Falling Jets Experiment (CALJET) and the Pacific Land-Falling Jets Experiment (PACJET) in order to address an existing gap in knowledge regarding the ability of atmospheric models to simulate the finescale vertical and horizontal structure of atmospheric rivers. The WRF simulations reproduce key structural and thermodynamic characteristics of atmospheric rivers—including well-defined corridors of strong water vapor transport, moist-neutral stability in the lower troposphere, and strong low-level jet/water vapor transport maxima near ~1 km MSL. While WRF does generally capture the extreme values of instantaneous vertically integrated water transport—a defining feature of real-world atmospheric rivers—constituent variables exhibit biases relative to observations, including −11.2% for integrated vapor transport, +5.9% for integrated water vapor, and −17.7% for 1 km MSL wind speed. Findings suggest that high-resolution nonhydrostatic atmospheric simulations are an appropriate tool for investigating atmospheric rivers in contexts where finescale spatial structure and realistic water vapor transport maxima are important.

Evaluation of the Subseasonal Forecast Skill of Floods Associated with Atmospheric Rivers in Coastal Western U.S. Watersheds

American Meteorological Society (AMS) | May 27th, 2021

Atmospheric rivers (ARs) are responsible for up to 90% of major flood events along the U.S. West Coast. The time scale of subseasonal forecasting (from 2 weeks to 1 month

Atmospheric rivers (ARs) are responsible for up to 90% of major flood events along the U.S. West Coast. The time scale of subseasonal forecasting (from 2 weeks to 1 month) is a critical lead time for proactive mitigation of flood disasters. The NOAA Climate Testbed Subseasonal Experiment (SubX) is a research-to-operations project with almost immediate availability of forecasts. It has produced a reforecast database that facilitates evaluation of flood forecasts at these subseasonal lead times. Here, we examine the SubX-driven forecast skill of AR-related flooding out to 4-week lead using the Distributed Hydrology Soil Vegetation Model (DHSVM), with particular attention to the role of antecedent soil moisture (ASM), which modulates the relationship between meteorological and hydrological forecast skill. We study three watersheds along a transect of the U.S. West Coast: the Chehalis River basin in Washington, the Russian River basin in Northern California, and the Santa Margarita River basin in Southern California. We find that the SubX-driven flood forecast skill drops quickly after week 1, during which there is relatively high deterministic forecast skill. We find some probabilistic forecast skill relative to climatology as well as ensemble streamflow prediction (ESP) in week 2, but minimal skill in weeks 3–4, especially for annual maximum floods, notwithstanding some probabilistic skill for smaller floods in week 3. Using ESP and reverse-ESP experiments to consider the relative influence of ASM and SubX reforecast skill, we find that ASM dominates probabilistic forecast skill only for small flood events at week 1, while SubX reforecast skill dominates for large flood events at all lead times.

Executive Order N-4-23

State of California | March 10th, 2023

Executive Order N-7-23

State of California | May 17th, 2023

Extreme California Rains During Winter 2015/16: A Change in El Niño Teleconnection?

American Meteorological Society (AMS) | January 25th, 2018

This is a story of two extreme events—one that was expected but failed to occur and the other that actually did occur but was not anticipated. The one that failed was

This is a story of two extreme events—one that was expected but failed to occur and the other that actually did occur but was not anticipated. The one that failed was extreme wetness over Southern California (SCAL) during winter 2015/16, which was predicted by seasonal forecasts. The extreme event that did occur was dryness whose considerable magnitude exacerbated one of the worst droughts on record over SCAL. *Chapter 10,  Explaining Extreme Events of 2016 from a Climate Perspective, Herring, S. C., N. Christidis, A. Hoell, J. P. Kossin, C. J. Schreck III, and P. A. Stott, Eds., 2018: Bull. Amer. Meteor. Soc., 99 (1), S1–S157.

Extreme hydrological changes in the southwestern US drive reductions in water supply to Southern California by mid century

Environmental Research Letters (IOP) | September 21st, 2016

The Southwestern United States has a greater vulnerability to climate change impacts on water security due to a reliance on snowmelt driven imported water. The State of C

The Southwestern United States has a greater vulnerability to climate change impacts on water security due to a reliance on snowmelt driven imported water. The State of California, which is the most populous and agriculturally productive in the United States, depends on an extensive arti cial water storage and conveyance system primarily for irrigated agriculture, municipal and industrial supply and hydropower generation. Here we take an integrative high-resolution ensemble modeling approach to examine near term climate change impacts on all imported and local sources of water supply to Southern California. While annual precipitation is projected to remain the same or slightly increase, rising temperatures result in a shift towards more rainfall, reduced cold season snowpack and earlier snowmelt. Associated with these hydrological changes are substantial increases in the frequency and the intensity of both drier conditions and ooding events. The 50 year extreme daily maximum precipitation and runoff events are 1.5–6 times more likely to occur depending on the water supply basin. Simultaneously, a clear de cit in total annual runoff over mountainous snow generating regions like the Sierra Nevada is projected. On one hand, the greater probability of drought decreases imported water supply availability. On the other hand, earlier snowmelt and signicantly stronger winter precipitation events pose increased flood risk requiring water releases from control reservoirs, which may potentially decrease water availability outside of the wet season. Lack of timely local water resource expansion coupled with projected climate changes and population increases may leave the area in extended periods of shortages.

Extreme Precipitation-Temperature Scaling in California: The Role of Atmospheric Rivers

American Geophysical Union (AGU) | July 25th, 2023

Factors and Processes Affecting Delta Levee System Vulnerability

University of California, Davis (UC Davis) | December 1st, 2016

The authors appraised factors and processes related to human activities and high water, subsidence, and seismicity. Farming and drainage of peat soils over time has cause

The authors appraised factors and processes related to human activities and high water, subsidence, and seismicity. Farming and drainage of peat soils over time has caused a gradual sinking which contributed to internal levee failures. Although these subsidence rates have decreased with time, they still contribute to levee instability. Additional data is needed to assess spatial and temporal effects of subsidence caused by peat thinning and deformation. Since the mid-1970s large-scale, State investments in levee upgrades have increased conformance, however accounts continue to conflict on how these investments correspond to the numbers of failures. Both modeling and history suggest that the projected increases of high-flow frequency associated with climate change will increase levee-failures rates. Quantification of this increased threat requires further research. A reappraisal of seismic threats resulted in updated ground motion estimates for multiple faults and earthquake occurrence frequencies. The immediate seismic threat, liquefaction, is the sudden loss of strength due to an increase in the pressure of the pore fluid and corresponding loss of contact forces. However, levees damaged during an earthquake that do not immediately fail, may eventually breach. Consequences for future levee failure are estimated to cost up to billions of dollars. The analysis of future risks will benefit from more detailed descriptions of levee strength and upgrades, consideration of subsidence, climate change, and earthquake threats. Ecosystem benefits of levee habitats in this highly altered system are thin. Better recognition and coordination is needed among the creation of high value habitats, levee needs, costs, benefits of levee improvements, and breaches.  

Federal Investments for the California Bay-Delta Region

U.S. Department of the Interior (DOI) | February 5th, 2014

The San Francisco Bay/Sacramento-San Joaquin Delta (Bay-Delta) region of California is an ecosystem of national significance. It is also the hub of the nation’s largest

The San Francisco Bay/Sacramento-San Joaquin Delta (Bay-Delta) region of California is an ecosystem of national significance. It is also the hub of the nation’s largest water delivery system—infrastructure that is the foundation for economic activity in the agricultural, industrial, and recreational sectors of the economy, and provides drinking water to a population of over 25 million Californians. This document is intended to provide an overview of the federal land and water programs in and around the Bay-Delta region, their purpose, options for future programs or actions, and an overview of the actions in which agencies are engaged already. Successful long term management of water resources and the environment to achieve the co-equal goals of the Bay Delta Conservation Plan (BDCP) and overall Delta sustainability requires continued improvement in managing California’s finite water supply. Contents I. Introduction II. Bay Delta-Big Picture: A Holistic Approach • State and Federal Integrated Water Management • Reducing Water Demand • Increasing Water Reliability and Supply • Improve Operational Efficiency and Transfers/Exchanges • Potential Federal Water and Ecosystem Management Actions III. Role of the Federal agencies • Department of the Interior Bureau of Reclamation U.S. Fish and Wildlife Service • Environmental Protection Agency • Department of Commerce National Oceanic and Atmospheric Administration • Army Corps of Engineers • Department of Agriculture Natural Resource Conservation Service U.S. Forest Service Rural Development Farm Services Agency Agriculture Research Service IV. Conclusion

Flood Control System Status Report

California Department of Water Resources (DWR) | December 12th, 2011

This Flood Control System Status Report (FCSSR) describes the current status (physical condition) of SPFC facilities at a systemwide level. DWR prepared the FCSSR to meet

This Flood Control System Status Report (FCSSR) describes the current status (physical condition) of SPFC facilities at a systemwide level. DWR prepared the FCSSR to meet the legislative requirements of California Water Code Section 9120, and to contribute to development of the Central Valley Flood Protection Plan (CVFPP). The CVFPP will guide future State investments through projects to address identified problems in the SPFC. The FCSSR is primarily intended to present information on the physical condition of SPFC facilities, and to help guide future inspection, evaluation, reconstruction, and improvement of the facilities. Information contained in the FCSSR should not be used to predict how a levee orassociated facilities may perform in a specific flood event. More detailed information (such as additional geotechnical explorations and analyses at a greater frequency) would be necessary to meet other purposes, such as assessing whether a levee could be certified under Federal Emergency Management Agency (FEMA) standards to provide base flood protection under the National Flood Insurance Program.