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Storms, Floods, and the Science of Atmospheric Rivers

American Geophysical Union (AGU) | August 9th, 2011

Summary

Imagine a stream of water thousands of kilometers long and as wide as the distance between New York City and Washington, D. C., flowing toward you at 30 miles perhour.

Imagine a stream of water thousands of kilometers long and as wide as the distance between New York City and Washington, D. C., flowing toward you at 30 miles perhour. No, this is not some hypothetical physics problem—it is a real river, carrying more water than 7–15 Mississippi Rivers combined. But it is not on land. It’s a river of water vapor in the atmosphere.

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Strum Und Drand – California’s Remarkable Storm-Drought Connection

International Association for Hydro-Environment Engineering and Research | May 31st, 2019

Summary

Storm and drought are essentially the whole story of water and life in California in ways that have always made hydro-environmental engineering a unique proposition th

Storm and drought are essentially the whole story of water and life in California in ways that have always made hydro-environmental engineering a unique proposition there. To begin with, California experiences larger year-to-year variations in precipitation than elsewhere in the US, with standard deviations of annual precipitation between 30-50% of long-term averages, compared to 10-30% nearly everywhere else. California’s annual precipitation totals routinely vary from as little as 50% to more than 200% of long-term averages, with those dry excursions forming our droughts. This extreme variability arises because California’s Mediterranean climate only provides a limited seasonal window of precipitation events, and within that period a small number of storms deliver most of the State’s precipitation each year. If a few extra large storms reach California in a given winter, we can have a very wet year indeed; if some are lacking, we face drought. But the storm-drought connection is deeper and more pervasive in California than anywhere else in the US.

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Summarizing Relationships among Landfalling Atmospheric Rivers, Integrated Water Vapor Transport, and California Watershed Precipitation 1982–2019

American Meteorological Society (AMS) | September 16th, 2022

Summary

Atmospheric rivers (ARs) are defined as corridors of enhanced integrated water vapor transport (IVT) and produce large fractions of annual precipitation in regions with c

Atmospheric rivers (ARs) are defined as corridors of enhanced integrated water vapor transport (IVT) and produce large fractions of annual precipitation in regions with complex terrain along the western coastlines of midlatitude continents (e.g., 30%–50% along the U.S. West Coast in California). This study investigates this relationship among landfalling ARs, IVT, and watershed mean areal precipitation (MAP) for a 38-yr period over California. On average, the daily average IVT magnitude at different coastal locations explains ∼34% of the variance in annual watershed MAP across 140 Hydrologic Unit Code 8 (HUC-8) watersheds with large spatial variability across California. Further investigation of the IVT magnitude and direction at coastal locations illustrated that accounting for water vapor transport direction increases the explained variance in annual MAP to an average of 45%, with highest values (∼65%) occurring in watersheds over Northern and coastal California. Similar investigation of the lower-tropospheric water vapor flux vector at 850 and 925 hPa revealed further increases in the explained variance in annual MAP to an average of >50%. The results of this study 1) emphasize the importance of both IVT direction and water vapor flux altitude to watershed MAP, 2) align well with previous studies for select locations that highlight the importance of upslope (i.e., lower tropospheric) water vapor flux during landfalling ARs and precipitation, and 3) motivate the development of AR-related and watershed-centric forecast tools that incorporate IVT direction and water vapor flux altitude parameters in addition to IVT magnitude.

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Synergistic Effect of El Niño and Negative Phase of North Atlantic Oscillation on Winter Precipitation in the Southeastern United States

American Meteorological Society (AMS) | February 20th, 2023

Summary

This study reveals that the significant increase of winter precipitation over the southeastern United States (SEUS) is associated with El Niño and the negative phase of

This study reveals that the significant increase of winter precipitation over the southeastern United States (SEUS) is associated with El Niño and the negative phase of North Atlantic Oscillation (NAO−). Diagnosis of large-scale dynamics shows that El Niño and NAO− have a synergistic effect on the enhancement of transient eddies and stationary waves in the eastern Pacific, the southern United States, and the North Atlantic. These enhanced transient eddies are associated with subtropical jet stream acceleration and maintenance of subtropical low from the eastern Pacific to the Atlantic, influencing stronger stationary waves propagation from the tropical Pacific to the SEUS and North Atlantic, and from the northwestern Atlantic to the SEUS and Pacific. This favors a positive phase of the meridional dipole in geopotential height anomalies between the tropics and subtropics in the Western Hemisphere (WTSD) during the co-occurrence of El Niño and NAO−. The strong positive WTSD-like pattern, accompanied by a zonally extended and southerly shifted subtropical westerly jet, induces a northward-tilted secondary circulation with ascending motion over the SEUS and Gulf of Mexico. Simultaneously, an intensified trough over the SEUS promotes moisture and warm advection from the subtropical and tropical Pacific converging with cold advection from the northwestern North Atlantic in the adjacent Gulf of Mexico, which is beneficial for front and cyclone generation, and induces precipitation in the SEUS. This study suggests that the synergistic effects of El Niño and NAO− help to understand the variability of winter SEUS precipitation.

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System Reoperation Study, Phase 2: Appendix A: Forecast-Based Operations Analysis

California Department of Water Resources (DWR) | January 29th, 2014

Summary

The System Reoperation Study (“Reoperation Study”) is investigating changes in operations and other strategies that may result in improved system performance in terms

The System Reoperation Study (“Reoperation Study”) is investigating changes in operations and other strategies that may result in improved system performance in terms of additional water supply, flood hazard reduction, and ecosystem protection and restoration. It is recognized that existing reservoir operations are typically based on meeting all or some of these three objectives. As such, the Reoperation Study focuses on modifications to existing operations and expansion of these benefits. The Reoperation Study team has developed several reoperation components to evaluate at select river/reservoir systems to quantify benefits and improve understanding of the trade-offs involved. MBK Engineers is providing technical, strategic, and coordination support for the Reoperation Study. An analysis of Forecast-Based Operation (F-BO) is part of that support. The following report summarizes this effort. F-BO can be applied to any reservoir with a traditional water supply and flood control space allocation paradigm. The incorporation of weather forecasts allows for greater flexibility in the management of the reservoir’s space (flood control versus conservation) and inherently increases the opportunities for gaining additional water supply and flood control benefits through its implementation. System reoperation is informed by historical flood patterns and utilizes records of inflow, outflow, and storage in the reservoirs to develop modified rules for flood releases. Changes to historical operations of the reservoirs is expected to reveal potential for additional water supply storage. With improvements to many of the reservoir facilities, such as increased release capacity, operations are no longer required to be as conservative as they once were. By reducing outflow and allowing an increase in water storage, significant improvements in water supply can be achieved. However, care must be taken so that sufficient water can still be evacuated in the event of a significant forecasted flood. RELATED DOCUMENTS: System Reoperation Study, Phase 1: Plan of Study System Reoperation Study, Phase 2: Strategy Formulation and Refinement System Reoperation Study, Phase 2: Appendix A: Forecast?Based Operations Analysis System Reoperation Study, Phase 2: Appendix B: Tradeoff Analysis System Reoperation Study, Phase 3 Report: Assessment of Reoperation Strategies System Reoperation, Phase 3: Appendix C: State Water Project and Central Valley Project Integrated Operations Analysis System Reoperation, Phase 3 Appendix D: Ecosystem Evaluation

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The 2010/2011 snow season in California’s Sierra Nevada: Role of atmospheric rivers and modes of large-scale variability

American Geophysical Union (AGU) | October 17th, 2013

Summary

The anomalously snowy winter season of 2010/2011 in the Sierra Nevada is analyzed interms of snow water equivalent (SWE) anomalies and the role of atmospheric rivers (AR

The anomalously snowy winter season of 2010/2011 in the Sierra Nevada is analyzed interms of snow water equivalent (SWE) anomalies and the role of atmospheric rivers (ARs)—narrow channels of enhanced meridional water vapor transport between the tropics and extratropics. Mean April 1 SWE was 0.44 m (56%) above normal averaged over 100 snow sensors. AR occurrence was anomalously high during the period, with 20 AR dates during the season and 14 in the month of December 2010, compared to the mean occurrence of nine dates per season. Fifteen out of the 20 AR dates were associated with the negative phases of the Arctic Oscillation (AO) and the Pacific-North American (PNA) teleconnection pattern. Analysis of all winter ARs in California during water years 1998–2011 indicates more ARs occur during the negative phase of AO and PNA, with the increase between positive and negative phases being90% for AO, and50% for PNA. The circulation pattern associated with concurrent negative phases of AO and PNA, characterized by cyclonic anomalies centered northwest of California, provides a favorable dynamical condition for ARs. The analysis suggests that the massive Sierra Nevada snowpack during the 2010/2011 winter season is primarily related to anomalously high frequency of ARs favored by the joint phasing of AO and PNA, and that a secondary contribution is from increased snow accumulation during these ARs favored by colder air temperatures associated with AO, PNA, and La Nina.

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The Evaporative Demand Drought Index. Part I: Linking Drought Evolution to Variations in Evaporative Demand

American Meteorological Society (AMS) | March 1st, 2016

Summary

Many operational drought indices focus primarily on precipitation and temperature when depicting hydroclimatic anomalies, and this perspective can be augmented by analys

Many operational drought indices focus primarily on precipitation and temperature when depicting hydroclimatic anomalies, and this perspective can be augmented by analyses and products that reflect the evaporative dynamics of drought. The linkage between atmospheric evaporative demand E0 and actual evapotranspiration (ET) is leveraged in a new drought index based solely on E0—the Evaporative Demand Drought Index (EDDI). EDDI measures the signal of drought through the response of E0 to surface drying anomalies that result from two distinct land surface–atmosphere interactions: 1) a complementary relationship between E0 and ET that develops under moisture limitations at the land surface, leading to ET declining and increasing E0, as in sustained droughts, and 2) parallel ET and E0 increases arising from increased energy availability that lead to surface moisture limitations, as in flash droughts. To calculate EDDI from E0, a longterm, daily reanalysis of reference ET estimated from the American Society of Civil Engineers (ASCE) standardized reference ET equation using radiation and meteorological variables from the North American Land Data Assimilation System phase 2 (NLDAS-2) is used. EDDI is obtained by deriving empirical probabilities of aggregated E0 depths relative to their climatologic means across a user-specific time period and normalizing these probabilities. Positive EDDI values then indicate drier-than-normal conditions and the potential for drought. EDDI is a physically based, multiscalar drought index that that can serve as an indicator of both flash and sustained droughts, in some hydroclimates offering early warning relative to current operational drought indices. The performance of EDDI is assessed against other commonly used drought metrics across CONUS in Part II.

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The Future Hydrology of the Colorado River Basin

Utah State University | August 31st, 2020

Summary

Long-range planning of the water supply provided by the Colorado River requires realistic assessments of the impact of a continuation of the current drought that began in

Long-range planning of the water supply provided by the Colorado River requires realistic assessments of the impact of a continuation of the current drought that began in 2000, the impact of potentially extreme future droughts, and the long-term and progressive decline in watershed runoff that is caused by a warming climate. Water-supply managers want to know the maximum plausible stresses to water users so that plans for conservation, reservoir operations, and/or construction of new infrastructure can be properly developed. River managers want to know the implications of various water-supply plans on the flow-regime and water-quality characteristics of the Colorado River and its headwater branches in order to develop natural resource management plans that maintain desired attributes of river ecosystems. Although it is relatively easy to qualitatively describe scenarios of drought or water abundance, it is much harder to quantitatively estimate likely future conditions. In the white paper, we developed methods to make such quantitative estimates, thereby providing an approximate answer to the question, “How dry might future conditions in the Colorado River watershed become?” It is difficult to assign a probability to this assessment, and our analysis is guided by the principle that what has happened in the past might happen again in the future.

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The growing importance of oceanic moisture sources for continental precipitation

Nature Portfolio (Springer Nature) | July 24th, 2020

Summary

The precipitation that falls on the continents defines the extent and nature of terrestrial ecosystems and human activity in them, all of which are adapted to a

The precipitation that falls on the continents defines the extent and nature of terrestrial ecosystems and human activity in them, all of which are adapted to and maintained by present-day precipitation. In essence, precipitation is supplied by moisture that either comes directly from the ocean, or is subsequently recycled from the continents themselves. Both the processes that control evaporation and the main mechanisms of moisture transport clearly differ between the ocean and the continent, thus within the context of a changing climate, it may be expected that the relationship between precipitation of oceanic and terrestrial origin varies globally and regionally, as will the influence of these two basic components of total precipitation on global and regional precipitation trends, especially in tropical regions. We describe an approach based on a Lagrangian technique for estimating the precipitation in a target region given the proportions of moisture transported from the two sources (ocean and continent) to reveal that the percentage of precipitation of oceanic origin has increased globally in the current climate (1980–2016). The greatest observed rate of increase is in the tropical regions; furthermore, the trends of precipitation in these regions are controlled by trends in precipitation for which the source of moisture is the ocean.

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The key role of dry days in changing regional climate and precipitation regimes

Nature Portfolio (Springer Nature) | March 13th, 2014

Summary

Future changes in the number of dry days per year can either reinforce or counteract projected increases in daily precipitation intensity as the climate warms. We

Future changes in the number of dry days per year can either reinforce or counteract projected increases in daily precipitation intensity as the climate warms. We analyze climate model projected changes in the number of dry days using 28 coupled global climate models from the Coupled Model Intercomparison Project, version 5 (CMIP5). We find that the Mediterranean Sea region, parts of Central and South America, and western Indonesia could experience up to 30 more dry days per year by the end of this century. We illustrate how changes in the number of dry days and the precipitation intensity on precipitating days combine to produce changes in annual precipitation, and show that over much of the subtropics the change in number of dry days dominates the annual changes in precipitation and accounts for a large part of the change in interannual precipitation variability.

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