• New study improves understanding of Sout

    From ScienceDaily@1:317/3 to All on Thu Jan 27 21:30:50 2022
    New study improves understanding of Southern California's intense winter
    rains
    Accurately predicting localized, intense rainfall is critical for keeping communities safe from landslides and flash floods

    Date:
    January 27, 2022
    Source:
    American Geophysical Union
    Summary:
    New research looks to improve prediction of brief but intense
    rainstorms that can cause devastating flash floods and
    landslides. Intense rain associated with narrow cold-frontal
    rainbands may last only a few minutes at a particular location,
    yet the rain can cause catastrophic flash flooding, debris flows
    and landslides, and can occur along with tornadoes and severe
    thunderstorms.



    FULL STORY ==========================================================================
    New research looks to improve prediction of brief but intense rainstorms
    that can cause devastating flash floods and landslides.


    ==========================================================================
    The storms, called narrow cold-frontal rainbands, are long strips of rain
    that can stretch for tens to hundreds of kilometers in length but are
    only a few kilometers wide. They form along many of the world's coasts, including both coasts of the U.S.

    Intense rain associated with narrow cold-frontal rainbands may last only a
    few minutes at a particular location, yet the rain can cause catastrophic
    flash flooding, debris flows and landslides, and can occur along with
    tornadoes and severe thunderstorms. With all these potential hazards,
    the storms can prompt broad evacuation alerts. But predicting when and
    where the intense rain will both hit and cause hazards is difficult, and evacuations can be issued over too large an area. Improving predictions
    of these atmospheric phenomena is important for avoiding evacuation
    fatigue and maintaining public trust in warning systems.

    A new study analyzes three decades of radar and hazard-warning data from southern California, building a catalogue of rainbands that will help
    improve predictions of when and where these events will be dangerous. The
    study was published in the AGU journal Geophysical Research Letters, which publishes high-impact, short-format reports with immediate implications spanning all Earth and space sciences.

    Researchers found that from 1995 to 2020, 60% of these events resulted in
    at least one National Weather Service warning, and 15 of the 94 events led
    to 10 or more warnings as the bands swept across southern California. Some
    of the most intense events were associated with atmospheric rivers. And
    most hit in the region's already-wet winter months of January through
    March.

    The new catalogue of rainbands will help scientists create storm
    prediction and tracking algorithms, with the eventual goal of "nowcasting"
    the storms: predicting their patterns and hazards just a few hours
    in advance.



    ========================================================================== Improving predictions is necessary to help people who live in potentially affected areas, says study co-author Nina Oakley, an atmospheric scientist
    at the Scripps Institution of Oceanography.

    "Our current predictive models struggle with location, timing and
    intensity of these events, and we need to improve that for evacuation
    warnings. ... If we issue evacuation warnings for what turns out not
    to be a high-impact event, you start to run into evacuation fatigue,
    where people might not trust the warnings in the future," says Oakley.

    Searching the clouds To discern how frequently, when and where the
    rainbands occur, the researchers manually examined years of radar data, searching for telltale "gap and core" patterns of these rainbands,
    and paired their results with hazard warnings from the National Weather
    Service over the same time period.

    They found the number of narrow cold-frontal rainband events varied
    widely, from a total absence of these features to about a dozen per year,
    with most occurring between January and March. Approximately 60% of the
    events between 1995 and 2020 resulted in at least one warning from the
    National Weather Service and 15 exceptionally intense events resulted
    in 10 or more warnings of flash floods, tornadoes or debris flow and
    landslide risk as they swept across Southern California.



    ==========================================================================
    "All but two of these intense rainband events were associated with
    atmospheric rivers, and two-thirds of our entire catalogue were as well,"
    says Marian de Orla-Barile, lead study author and geography Ph.D. student
    at the University of California, Santa Barbara. High water vapor content
    in the atmosphere and intensifying low-pressure systems were also commonly associated with the rainbands.

    "Narrow cold-frontal rainbands produce some of the highest rainfalls
    of any extra-tropical cyclone precipitation band," says Brian Collins,
    a geomorphologist at the USGS who was not involved in the study. "The
    hazards vary depending on where you are, but they key part of the
    equation is a lot of rain in a really short time." Predicting disaster
    The short timeframe Collins mentions highlights one of the difficulties in forecasting damage. The study's catalogue of narrow cold-frontal rainbands
    is an early, critical step in building an algorithm that can automatically identify and track areas of intense, potentially damaging rain. Such an algorithm is the heart of "nowcasting," which aims to forecast rainfall
    within hours. Ultimately, as the algorithm and its database are improved, predicting storm tracks and impacts will be within reach.

    Because the rainbands can strengthen or weaken over time, especially as
    they encounter mountainous topography, pinpointing when and where they
    will produce the most intense rainfall and pose hazards is difficult. Not
    all narrow cold- frontal rainbands produce disastrous rainfall, nor
    do all pass over a populated area below a recent wildfire burn scar or landslide hazard area that would need to evacuate.

    "A lot of times, warnings are issued for large areas because of the
    uncertainty in where the rain band will have that high-intensity
    precipitation. So you end up warning a larger area than needs to be
    warned," says Collins. Taking local conditions into account -- such
    as recent wildfires or heavy rains that may have saturated the soil,
    making slopes prone to landslides -- is important for predicting narrow cold-frontal rainband hazard risks, Collins stresses.

    "With projected increases in wildfires as well as the projected
    intensification of short-duration rainfall, it's likely that in a
    warming climate, there will be increased post-fire debris flow events,"
    some of which could be associated with narrow cold-front rainbands,
    says de Orla-Barile.

    ========================================================================== Story Source: Materials provided by American_Geophysical_Union. Note:
    Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Marian Orla‐Barile, Forest Cannon, Nina S. Oakley, F. Martin
    Ralph.

    A Climatology of Narrow Cold‐Frontal Rainbands in Southern
    California. Geophysical Research Letters, 2022; 49 (2) DOI:
    10.1029/ 2021GL095362 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/01/220127172619.htm

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