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Erin Dougherty, Erin Sherman, and Kristen L. Rasmussen

Abstract

California receives much of its precipitation from cool-season atmospheric rivers, which contribute to water resources and flooding. In winter 2017, a large number of atmospheric rivers caused anomalous winter precipitation, near-saturated soils, and a partial melting of snowpack, which led to excessive runoff that damaged the emergency spillway of the Oroville Dam. Given the positive and negative impacts ARs have in California, it is necessary to understand how they will change in a future climate. While prior studies have examined future changes in the frequency of atmospheric rivers impacting the West Coast of the United States, these studies primarily use coarse global climate models that are unable to resolve the complex terrain of this region. Such a limitation is overcome by using a high-resolution convection-permitting regional climate model, which resolves complex topography and orographic rainfall processes that are the main drivers of heavy precipitation in landfalling atmospheric rivers. This high-resolution model is used to examine changes to precipitation and runoff in California’s cool season from 2002 to 2013, particularly in flood-producing storms associated with atmospheric rivers, in a future, warmer climate using a pseudo–global warming approach. In 45 flood-producing storms, precipitation and runoff increase by 21%–26% and 15%–34%, respectively, while SWE decreases by 32%–90%, with the greatest changes at mid-elevations. These trends are consistent with future precipitation changes during the entire cool season. Results suggest more intense floods and less snowpack available for water resources in the future, which should be carefully considered in California’s future water management plans.

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Erin Dougherty and Kristen L. Rasmussen

Abstract

The Mississippi River Basin (MRB) is a flash flood hotspot receiving the most frequent flash floods and highest average rainfall accumulation of any region in the United States. Given the destruction flash floods cause in the current climate in the MRB, it is critical to understand how they will change in a future, warmer climate in order to prepare for these impacts. Recent work utilizing convection-permitting climate simulations to analyze future precipitation changes in flash flood-producing storms in the United States shows that the MRB experiences the greatest future increase in flash flood rainfall. This result motivates the goal of the present study to better understand the changes to precipitation characteristics and vertical velocity in flash flood-producing storms in the MRB. Specifically, the variations in flash flood-producing storm characteristics related to changes in vertical velocity in the MRB are examined by identifying 484 historical flash flood-producing storms from 2002–2013 and studying how they change in a future climate using 4-km convection-permitting simulations under a pseudo-global warming framework. In a future climate, precipitation and runoff increase by 17% and 32%, respectively, in flash flood-producing storms in the MRB. While rainfall increases in all flash flood-producing storms due to similar increases in moisture, it increases the most in storms with the strongest vertical velocity, suggesting that storm dynamics might modulate future changes in rainfall. These results are necessary to predict and prepare for the multifaceted impacts of climate change on flash flood-producing storms in order to create more resilient communities.

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Erin Dougherty and Kristen L. Rasmussen

Abstract

Floods are one of the deadliest weather-related natural disasters in the continental United States (CONUS). Given that rainfall intensity and the amount of CONUS population exposed to floods is expected to increase in the future, it is critical to understand flood characteristics across the CONUS. Therefore, the purpose of this study is to develop a flood-producing storm climatology over the CONUS from 2002 to 2013 to better understand rainfall characteristics of these storms and spatiotemporal differences across the country. Flood reports from the NCEI Storm Events Database are grouped by causative meteorological event and are merged with a database of stream-gauge-indicated floods to provide a robust indication of significant hydrologic events with a meteorological linkage. High-resolution Stage IV rainfall data were matched to 5559 flood episodes across the CONUS to identify rainfall characteristics of flood-producing storms in a variety of environments. This storm climatology indicates that flash flood–producing storms frequently occur with high rainfall accumulations in the summer east of the Rockies. Slow-rise flood-producing storms frequently occur in the spring–early summer (winter), with high rainfall accumulations over the northern and central CONUS (Pacific Northwest) due to rain-on-snowmelt, synoptic systems, and mesoscale convective systems (atmospheric rivers). Hybrid flood-producing storms, sharing characteristics of flash and slow-rise floods, frequently occur in spring–summer and have high rainfall accumulations in the central CONUS, Northeast, and mid-Atlantic. Results from this climatology may provide useful for emergency managers, city planners, and policy makers seeking efforts to protect their communities against risks associated with flood-producing storms.

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Erin Dougherty and Kristen L. Rasmussen

Abstract

Flash floods are high-impact events that can result in massive destruction, such as the May 2010 flash floods in the south-central United States that resulted in over $2 billion of damage. While floods in the current climate are already destructive, future flood risk is projected to increase based on work using global climate models. However, global climate models struggle to resolve precipitation structure, intensity, and duration, which motivated the use of convection-permitting climate models that more accurately depict these precipitation processes on a regional scale due to explicit representation of convection. These high-resolution convection-permitting simulations have been used to examine future changes to rainfall, but not explicitly floods. This study aims to fill this gap by examining future changes to rainfall characteristics and runoff in flash flood–producing storms over the United States using convection-permitting models under a pseudo–global warming framework. Flash flood accumulated rainfall increases on average by 21% over the United States in a future climate. Storm-generated runoff increases by 50% on average, suggesting increased runoff efficiency in future flash flood–producing storms. In addition to changes in nonmeteorological factors, which were not explored in this study, increased future runoff is possible due to the 7.5% K−1 increase in future hourly maximum rain rates. Though this median change in rain rates is consistent with Clausius–Clapeyron theory, some storms exhibit increased future rain rates well above this, likely associated with storm dynamics. Overall, results suggest that U.S. cities might need to prepare for more intense flash flood–producing storms in a future climate.

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Erin M. Dougherty, John Molinari, Robert F. Rogers, Jun A. Zhang, and James P. Kossin

Abstract

Hurricane Bonnie (1998) was an unusually resilient hurricane that maintained a steady-state intensity while experiencing strong (12–16 m s−1) vertical wind shear and an eyewall replacement cycle. This remarkable behavior was examined using observations from flight-level data, microwave imagery, radar, and dropsondes over the 2-day period encompassing these events. Similar to other observed eyewall replacement cycles, Bonnie exhibited the development, strengthening, and dominance of a secondary eyewall while a primary eyewall decayed. However, Bonnie’s structure was highly asymmetric because of the large vertical wind shear, in contrast to the more symmetric structures observed in other hurricanes undergoing eyewall replacement cycles. It is hypothesized that the unusual nature of Bonnie’s evolution arose as a result of an increase in vertical wind shear from 2 to 12 m s−1 even as the storm intensified to a major hurricane in the presence of high ambient sea surface temperatures. These circumstances allowed for the development of outer rainbands with intense convection downshear, where the formation of the outer eyewall commenced. In addition, the circulation broadened considerably during this time. The secondary eyewall developed within a well-defined beta skirt in the radial velocity profile, consistent with an earlier theory. Despite the large ambient vertical wind shear, the outer eyewall steadily extended upshear, supported by 35% larger surface wind speed upshear than downshear. The larger radius of maximum winds during and after the eyewall replacement cycle might have aided Bonnie’s resiliency directly, but also increased the likelihood that diabatic heating would fall inside the radius of maximum winds.

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