Framing the Problem of Flood Risk and Flood Management in Metropolitan Los Angeles

American Meteorological Society (AMS) | January 1st, 2023

This paper develops the concept of flood problem framing to understand decision-makers’ priorities in flood risk management in the Los Angeles Metropolitan Region in Ca

This paper develops the concept of flood problem framing to understand decision-makers’ priorities in flood risk management in the Los Angeles Metropolitan Region in California (LA Metro). Problem frames shape an individual’s preferences for particular management strategies and their future behaviors. While flooding is a complex, multifaceted problem, with multiple causes and multiple impacts, a decision-maker is most likely to manage only those dimensions of flooding about which they are aware or concerned. To evaluate flood decision-makers’ primary concerns related to flood exposure, vulnerability, and management in the LA Metro, we draw on focus groups with flood control districts, city planners, nonprofit organizations, and other flood-related decision-makers. We identify numerous concerns, including concerns about specific types of floods (e.g., fluvial vs pluvial) and impacts to diverse infrastructure and communities. Our analyses demonstrate that flood concerns aggregate into three problem frames: one concerned with large fluvial floods exacerbated by climate change and their housing, economic, and infrastructure impacts; one concerned with pluvial nuisance flooding, pollution, and historic underinvestment in communities; and one concerned with coastal and fluvial flooding’s ecosystem impacts. While each individual typically articulated concerns that overlapped with only one problem frame, each problem frame was discussed by numerous organization types, suggesting low barriers to cross-organizational coordination in flood planning and response. This paper also advances our understanding of flood risk perception in a region that does not face frequent large floods.

From the extreme to the mean: Acceleration and tipping points of coastal inundation from sea level rise

American Geophysical Union (AGU) | December 18th, 2014

Relative sea level rise (RSLR) has driven large increases in annual water level exceedances (duration and frequency) above minor (nuisance level) coastal flooding elevat

Relative sea level rise (RSLR) has driven large increases in annual water level exceedances (duration and frequency) above minor (nuisance level) coastal flooding elevation thresholds established by the National Weather Service (NWS) at U.S. tide gauges over the last half-century. For threshold levels below 0.5 m above high tide, the rates of annual exceedances are accelerating along the U.S. East and Gulf Coasts, primarily from evolution of tidal water level distributions to higher elevations impinging on the flood threshold. These accelerations are quantified in terms of the local RSLR rate and tidal range through multiple regression analysis. Along the U.S. West Coast, annual exceedance rates are linearly increasing, complicated by sharp punctuations in RSLR anomalies during El Niño Southern Oscillation (ENSO) phases, and we account for annual exceedance variability along the U.S. West and East Coasts from ENSO forcing. Projections of annual exceedances above local NWS nuisance levels at U.S. tide gauges are estimated by shifting probability estimates of daily maximum water levels over a contemporary 5-year period following probabilistic RSLR projections of Koppet al.(2014) for representative concentration pathways(RCP) 2.6, 4.5, and 8.5. We suggest a tipping point for coastal inundation (30 days/per year with a threshold exceedance) based on the evolution of exceedance probabilities. Under forcing associated with the local-median projections of RSLR, the majority of locations surpass the tipping point over the next several decades regardless of specific RCP.

Future Atmospheric Rivers and Impacts on Precipitation: Overview of the ARTMIP Tier 2 High-Resolution Global Warming Experiment

American Geophysical Union (AGU) | March 14th, 2023

Atmospheric rivers (ARs) are long and narrow weather features often referred to as “rivers in the sky.” They often transport water from lower latitudes to higher lati

Atmospheric rivers (ARs) are long and narrow weather features often referred to as “rivers in the sky.” They often transport water from lower latitudes to higher latitudes typically across climate zones and produce precipitation necessary for local climates. Understanding ARs in a warming climate is challenging because of the variety of ways an AR can be defined on gridded data sets. Unlike weather features such as tropical cyclones where identification methodologies are similar, algorithms that determine the characteristics of ARs vary depending on the science question posed. Because there is no real consensus on AR identification methodology, we aim to quantify the algorithmic uncertainty in AR metrics and precipitation. We compare 16 different ways of defining an AR on gridded data sets and present the range of possibilities in which an AR could change under global warming. Generally, ARs are projected to increase but the amount of that increase is a function of the algorithm. Across all algorithms and focus regions, AR precipitation is projected to become more extreme.

Future Changes in Global Atmospheric Rivers Projected by CMIP6 Models

American Geophysical Union (AGU) | January 31st, 2024

Future extreme precipitation amplified by intensified mesoscale moisture convergence

Nature Portfolio (Springer Nature) | November 18th, 2025

Extreme precipitation events are driven by complex multiscale atmospheric dynamic interactions, fuelled by available moisture. They are expected to intensify with

Extreme precipitation events are driven by complex multiscale atmospheric dynamic interactions, fuelled by available moisture. They are expected to intensify with climate change, posing increasing risks to human communities and ecosystems. However, current low-resolution climate models struggle to accurately represent key extreme precipitation-generating phenomena, limiting our ability to generate robust and reliable future projections. Here we present an ensemble of climate simulations with a 10-to-25-km resolution and an improved representation of mesoscale convective systems to assess future changes in daily extreme precipitation and its drivers. Our high-resolution simulations more realistically capture the observed spatial distribution and intensity of daily extreme precipitation over the historical period than the 100-km resolution counterparts. In a future scenario with high carbon dioxide emissions, daily extreme precipitation over land could increase by about 41% by 2100, mainly as a result of increased mesoscale moisture convergence. The impact of this dynamical contribution to extreme precipitation is underestimated by a factor of three in the low-resolution model. These results highlight the crucial role of high-resolution climate modelling in constraining future extremes and informing more effective climate risk assessments and adaptation strategies. Click here for a Science news article on the model and its evolution.  

Future increase in extreme El Niño supported by past glacial changes

Nature Portfolio (Springer Nature) | September 25th, 2024

Future precipitation increase from very high resolution ensemble downscaling of extreme atmospheric river storms in California

American Association for the Advancement of Science (AAAS) | July 15th, 2020

Precipitation extremes will likely intensify under climate change. However, much uncertainty surrounds intensification of high-magnitude events that are often i

Precipitation extremes will likely intensify under climate change. However, much uncertainty surrounds intensification of high-magnitude events that are often inadequately resolved by global climate models. In this analysis, we develop a framework involving targeted dynamical downscaling of historical and future extreme precipitation events produced by a large ensemble of a global climate model. This framework is applied to extreme “atmospheric river” storms in California. We find a substantial (10 to 40%) increase in total accumulated precipitation, with the largest relative increases in valleys and mountain lee-side areas. We also report even higher and more spatially uniform increases in hourly maximum precipitation intensity, which exceed Clausius- Clapeyron expectations. Up to 85% of this increase arises from thermodynamically driven increases in water vapor, with a smaller contribution by increased zonal wind strength. These findings imply substantial challenges for water and flood management in California, given future increases in intense atmospheric river-induced precipitation extremes.

Gage Gap

Nature Conservancy | October 31st, 2018

Following a difficult five-year drought, California just had one of the wettest winters on record. The state is famous for its highly engineered water system, whi

Following a difficult five-year drought, California just had one of the wettest winters on record. The state is famous for its highly engineered water system, which moves millions of gallons of water from north to south and east to west. Thirty-nine million people and our $47 billion agricultural industry rely on that water, not to mention wildlife from the Sierra to the sea. And yet we have surprisingly little data about how much water is moving through our streams at any given time. There are over 3,600 locations in California where stream gages have been active at some point, but only 54% have been active recently. And even fewer of those provide the kind of rich, real-time reporting needed to manage what is arguably our state’s most precious and contentious resource.

Glenn County | Hamilton City Flood Damage Reduction and Ecosystem Restoration Project

Nature Conservancy | January 14th, 2016

Although not designed explicitly as a groundwater recharge project, the Hamilton City Project is expected to benefit local groundwater supplies. Having an additio

Although not designed explicitly as a groundwater recharge project, the Hamilton City Project is expected to benefit local groundwater supplies. Having an additional 1,450 acres of floodplain reconnected to the Sacramento River during high flow events will allow transitory storage of flood flows and help recharge the groundwater basin.

Global and Regional Sea Level Rise Scenarios for the United States

National Oceanic and Atmospheric Administration (NOAA) | January 15th, 2017

The Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task Force, jointly convened by the U.S. Global Change Research Program (USGCRP) and the Natio

The Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task Force, jointly convened by the U.S. Global Change Research Program (USGCRP) and the National Ocean Council (NOC), began its work in August 2015. The Task Force has focused its efforts on three primary tasks: 1) updating scenarios of global mean sea level (GMSL) rise, 2) integrating the global scenarios with regional factors contributing to sea level change for the entire U.S. coastline, and 3) incorporating these regionally appropriate scenarios within coastal risk management tools and capabilities deployed by individual agencies in support of the needs of specific stakeholder groups and user communities. This technical report focuses on the first two of these tasks and reports on the production of gridded relative sea level (RSL, which includes both ocean-level change and vertical land motion) projections for the United States associated with an updated set of GMSL scenarios. In addition to supporting the longer-term Task Force effort, this new product will be an important input into the USGCRP Sustained Assessment process and upcoming Fourth National Climate Assessment (NCA4) due in 2018. This report also serves as a key technical input into the in-progress USGCRP Climate Science Special Report (CSSR).