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Tag Archive: precipitation

  1. Increasing precipitation extremes in California; likelihood of 40 day flood event will increase significantly over decades ahead

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    By Daniel Swain April 22 2018 Read full CA Weather Blog article here

    Previous studies have found that future changes in California’s overall average annual precipitation are likely to be fairly modest, even under rather extreme global warming scenarios. Most climate models suggest that the boundary between mean wetting (in the already moist mid-latitude regions to the north) and mean drying (in the already arid subtropics to the south) in a warming world will likely fall somewhere over California—which increases uncertainty regarding whether the region will become slightly wetter or slightly drier on average. The notion that California’s average precipitation might not change much in the future is actually somewhat surprising, as there is high confidence that other “mediterranean” climate regions on Earth will experience progressively less precipitation as the world warms and the region of stable subtropical influence expands. As we demonstrate in our new research, however, these small shifts in average precipitation mask profound changes in the character of California precipitation. We find that the occurrence of both extreme wet and extreme dry events in California—and of rapid transitions between the two—will likely increase with atmospheric greenhouse gas concentrations. The rising risk of historically unprecedented precipitation extremes will seriously test California’s existing water storage, distribution, and flood protection infrastructure….

    ….As most of us already know, global climate is presently changing at a rate faster than has occurred in thousands of years, almost exclusively due to the emission of greenhouse gases (like carbon dioxide) into the Earth’s atmosphere. But the Earth is not warming at the same rate everywhere, and regional differences are subject to considerably more scientific uncertainty than the overall global warming trend. That’s especially true for many of the complex meteorological phenomena that we care most about: the dramatic storms, floods, heatwaves, and droughts that tend to have the largest impacts upon human lives, economies, physical infrastructure, and the environment….

    ….Our new analysis suggests that the risk of an extreme “sub-seasonal” 40-day precipitation event similar in magnitude to that which caused the 1862 flood will rise substantially as the climate warms. By the end of the 21st century, we find a 300 – 400+ % increase in the relative risk of such an event across the entire state. One specific statistic that my colleagues and I found particularly eyebrow-raising: on our current emissions trajectory, at least one occurrence of an 1862-level precipitation event is more likely than not over the next 40 years (between 2018 and 2060), with multiple occurrences plausible between now and the end of the century. In practical terms, this means that what is today considered to be the “200-year flood”—an event that would overwhelm the vast majority of California’s flood defenses and water infrastructure—will become the “40-50 year flood” in the coming decades….

    ….Our research suggests that the frequency of such “precipitation whiplash” events—in which California experiences a very dry year followed immediately by a very wet year—will increase considerably as the climate warms. We find anywhere from a 25% increase in far northern California to over a 100% increase over far southern California in the frequency of these dry-to-wet whiplash events (of a magnitude that has historically occurred about four times per century). …

    Swain, D. L., B. Langenbrunner, J. D. Neelin, and A. Hall, “Increasing precipitation volatility in 21st-century-California,” Nature Climate Change, doi:10.1038/s41558-018-0140-y, 2018.

  2. World’s largest cities depend on evaporated water from surrounding lands

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    • A study found that 19 of the 29 largest cities in the world depend on evaporation from surrounding lands for more than one-third of their water supplies.
    • The team previously coined the term “precipitationsheds,” a watershed of the sky that identifies the origin of precipitation falling in a given region.

    March 13, 2018 Colorado State University read full ScienceDaily article here

    Urbanization has taken billions of people from the rural countryside to urban centers, adding pressure to existing water resources. Many cities rely on renewable freshwater regularly refilled by precipitation, rather than groundwater or desalinated water.

    A study led by Colorado State University found that 19 of the 29 largest cities in the world depend on evaporation from surrounding lands for more than one-third of their water supplies. Researchers also found that the dependence on this water supply is higher in dry years. The findings have implications for land managers and policymakers who oversee urban water security…

    …Moisture recycling occurs when water evaporates from the land and rises up into the atmosphere. This moisture then flows along prevailing wind currents through the atmosphere, falling out as precipitation elsewhere.

    …Said Keys: “With climate change, and demographic and land use fluctuations, it is important to understand where vulnerabilities exist and have a full picture.”

    Patrick W. Keys, Lan Wang-Erlandsson, Line J. Gordon. Megacity precipitationsheds reveal tele-connected water security challenges. PLOS ONE, 2018; 13 (3): e0194311 DOI: 10.1371/journal.pone.0194311

  3. Strikingly dry conditions persist; Thomas Fire now largest California wildfire

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    by Daniel Swain December 24, 2017 read full CA Weather Blog post here

    Bone dry in Southern California, and below average precip throughout CA

    All of California is now experiencing well-below-average precipitation for the season to date. Southern California has seen almost no precipitation at all…

    Why has California been so dry? (Regular blog visitors already know where I’m going with this.) Well, a remarkably persistent zone of atmospheric pressure has been present more often than not across the region for the past few months. ….More recently, the bigger & stronger West Coast ridge has pushed the Pacific storm track even further north. Remarkably, this powerful ridge has forced several very moist atmospheric river storms over the mid-Pacific to make a hard “left turn” over the open ocean–veering directly northward and bringing almost inconceivably heavy snowfall to the coastal mountains of southern Alaska.

    Thomas Fire becomes largest wildfire in modern California history–in December

    The Thomas Fire has become the largest wildfire in modern California history–in December. (Via NASA)

    ….While December wildfires are not unheard of in this part of the world, the extent and severity of the December 2017 fires in SoCal really is unprecedented in California history. The Thomas Fire–which has now burned nearly 275,000 acres and over a thousand structures in Ventura and Santa Barbara counties–yesterday became the single largest wildfire in modern California history. That this dubious milestone was reached in December, which is typically the midst of the California rainy season, is truly extraordinary. Indeed, recent months have brought not only near-record low precipitation, but also record-high temperatures across a wide swath of SoCal….

    …..While long-term humidity records are hard to come by in most spots, all signs suggest that these were at or near record-low humidity values for many of these recording stations (and certainly for the time of year).

    Is this (another) return of the Ridiculously Resilient Ridge?

    Persistent high pressure ridging over the American West has kept conditions unusually warm and dry so far this autumn and early winter, especially across Southern California and Arizona. (NOAA via ESRL)……we’re not in Triple R [ridiculously resilient ridge] territory quite yet, but we’re getting close. We have certainly witnessed the return of resilient ridging near California, but I don’t think we’ve yet reached the “ridiculous” level of multi-month persistence that occurred during the height of the recent California drought. Should present conditions persist through January, and if seasonal precipitation has not started to recover from its early deficit by that time, I may have to revise that answer.


    …The not-so-good news: parts of Southern California that depend exclusively on local water supplies (such as much of Santa Barbara and Ventura counties) never really recovered from the last drought, and these regions remain quite susceptible to the impacts of drought re-intensification. And even further north, forested regions remain quite stressed as a result from the previous multi-year drought, and tree mortality in the Sierra Nevada remains far above historically observed levels.

    Unfortunately, 2+ weeks of unusually dry conditions still probable….The “Warm West/Cool East” pattern discussed in the previous post is still quite prominent over North America. The present West Coast dryness is largely consistent with seasonal model predictions for this winter, and those same models presently suggest that the present pattern is likely to persist for much of the California rainy season.All of this is to say: it’s still too early to say whether we’re headed into a new drought, though there are some compelling signs that we may be (especially in Southern California). And even in a dry year, California can still experience big storms and very wet months. But at this point, it probably makes sense to start thinking about the possibility of yet another big swing in California–from drought, to flood, and then (perhaps) back again.

  4. Droughts and wildfires in southwest: Global warming is drying up the North American monsoon

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    • North American monsoon, the summer rains that sweep across the southweset US and northwestern Mexico, is not simply delayed; the Southwest faces a dramatic reduction in rainfall

    October 9, 2017 Princeton University read full ScienceDaily article here

    Previous researchers had concluded that global warming was simply delaying the North American monsoon, which brings summer rains to the southwestern US and northwestern Mexico. But a new, high-resolution climate model that corrects for persistent sea surface temperature (SST) biases now accurately reflects current rainfall conditions and demonstrates that the monsoon is not simply delayed, but that the region’s total rainfall is facing a dramatic reduction

    ..most of those droughts are attributed to the change in winter storms, said Pascale. “The storm track is projected to shift northward, so these regions might get less rain in winter, but it was very uncertain what happens to the monsoon, which is the other contributor to the rains of the region. We didn’t know, and it’s crucial to know,” he said…[they quantified] the monsoon response to the doubling of atmospheric carbon dioxide, increased temperatures and other individual changes…

    Salvatore Pascale, William R. Boos, Simona Bordoni, Thomas L. Delworth, Sarah B. Kapnick, Hiroyuki Murakami, Gabriel A. Vecchi, Wei Zhang. Weakening of the North American monsoon with global warming. Nature Climate Change, 2017; DOI: 10.1038/nclimate3412

  5. Extreme precipitation frequency in CA expected to increase but decrease in other Mediterranean climate regions

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    • Frequencies of extreme precipitation are projected to increase over Northern Hemisphere- especially Califormia- where projected warming is strongest.
    • California’s more nuanced hydrological future reflects a precarious balance between the expanding subtropical high from the south and the south-eastward extending Aleutian low from the north-west– bolstering more extreme precipitation events.
    • More drought expected over Mediterranean basin due to decreased winter precipitation.

    Suraj D. Polade, Alexander Gershunov, Daniel R. Cayan, Michael D. Dettinger & David W. Pierce. Precipitation in a warming world: Assessing projected hydro-climate changes in California and other Mediterranean climate regions. Scientific Reports 7, Article number: 10783 (2017) doi:10.1038/s41598-017-11285-y

    ABSTRACT: In most Mediterranean climate (MedClim) regions around the world, global climate models (GCMs) consistently project drier futures. In California, however, projections of changes in annual precipitation are inconsistent. Analysis of daily precipitation in 30 GCMs reveals patterns in projected hydrometeorology over each of the five MedClm regions globally and helps disentangle their causes. MedClim regions, except California, are expected to dry via decreased frequency of winter precipitation. Frequencies of extreme precipitation, however, are projected to increase over the two MedClim regions of the Northern Hemisphere where projected warming is strongest. The increase in heavy and extreme precipitation is particularly robust over California, where it is only partially offset by projected decreases in low-medium intensity precipitation. Over the Mediterranean Basin, however, losses from decreasing frequency of low-medium-intensity precipitation are projected to dominate gains from intensifying projected extreme precipitation. MedClim regions are projected to become more sub-tropical, i.e. made dryer via pole-ward expanding subtropical subsidence. California’s more nuanced hydrological future reflects a precarious balance between the expanding subtropical high from the south and the south-eastward extending Aleutian low from the north-west. These dynamical mechanisms and thermodynamic moistening of the warming atmosphere result in increased horizontal water vapor transport, bolstering extreme precipitation events.

  6. Urban climate change: urban runoff strongly controlled by proportion of built up vs vegetated surfaces

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    • Model of impact of changing precipitation patterns in northern European, North American cities
    September 11, 2017 University of California – Santa Barbara
    Southern cities such as Houston and Tampa — which faced the wrath of hurricanes Harvey and Irma, respectively — may not be the only urban environments vulnerable to extreme weather. Northern cities also face the potential for flooding as global temperatures continue to warm…. In fact, higher temperatures have been found to disproportionately affect northern land areas, particularly the Arctic, which has already experienced fallout from climate change….
    A new study by a group of international researchers, including UC Santa Barbara’s Joe McFadden, combines observations and modeling to assess the impact of climate and urbanization on the hydrological cycle across the distinct seasons in four cold climate cities in Europe and North America….the amount of precipitation is increasing but also the kind of precipitation is changing,” said McFadden, an associate professor in UCSB’s Department of Geography. “While more precipitation may fall in a year, it arrives as rain rather than snow because temperatures are rising. A shorter period covered by snow, more spring rain and faster snow melt can combine to release large amounts of runoff that have the potential to stress urban hydrologic systems and cause flooding in urban areas.”…

    …The investigators found that after snow melt, urban runoff returns to being strongly controlled by the proportion of built-up versus vegetated surfaces, which can absorb water. However in winter, the presence of snow masks this influence….

    L. Järvi, C. S. B. Grimmond, J. P. McFadden, A. Christen, I. B. Strachan, M. Taka, L. Warsta, M. Heimann. Warming effects on the urban hydrology in cold climate regions. Scientific Reports, 2017; 7 (1) DOI: 10.1038/s41598-017-05733-y

  7. U.S. had 2nd warmest year through July 2017

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    • Northern Plains drought intensified, wildfires raged in the West, and rains flooded parts of the Midwest and Northeast.

    August 8, 2017  see full NOAA monthly update here

    This monthly summary from NOAA’s National Centers for Environmental Information is part of the suite of climate information services NOAA provides to government, business, academia and the public to support informed decision-making.

    The July average temperature for the contiguous U.S. was 75.7°F, 2.1°F above the 20th century average and was the 10th warmest July in 123 years of record-keeping. Much-above-average temperatures were observed across the West and parts of the Mid-Atlantic and Southeast. The year-to-date (January-July) average temperature for the contiguous U.S. was 54.5°F, 3.2°F above average and second warmest on record. This was slightly warmer than the same period in 2006 and 1.2°F cooler than the record set in 2012.

    …Above-average temperature spanned the nation for the first seven months of 2017 with only parts of the Northwest cooler than average. Much-above-average temperatures were observed for most locations in the Southwest and from the Rockies to the East Coast, mostly due to record and near-record warmth early in the year. Florida, North Carolina and South Carolina had their warmest January-July on record.

    …According to the August 1 U.S. Drought Monitor report, 11.8 percent of the contiguous U.S. was in drought, up about 3.7 percent compared to the end of June. Drought improved across parts of the Southwest, southern High Plains and in the Washington, DC, area. Drought intensified and expanded in the Northwest, Northern Rockies and Central to Northern Plains driven by below-average precipitation and above-average temperatures. The drought and heat decimated crops in the Northern Plains. Drought and abnormally dry conditions developed in parts of the Southeast and northern Maine. Drought continued to impact parts of Hawaii and western Alaska….

  8. Most of California projected to see more precipitation through this century due to warming tropical eastern Pacific ocean

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    • Researchers analyze 38 climate models and project California will get on average 12 percent more precipitation through 2100
    • warming in the tropical eastern Pacific Ocean sea surface temperatures, an area about 2,500 miles east of the international date line, is the main reason for the predicted increase in precipitation levels.
    • The warming sea surface temperatures encourage a southeastward shift of the jet stream, which helps steer more rain-producing mid-latitude cyclones toward California.
    • El-Nino-like years to become norm in California

    July 6, 2017 University of California – Riverside  see full ScienceDaily article here

    Under business-as-usual greenhouse gas emissions, climate models predict California will get warmer during the rest of the century and most also predict the state will get drier. But, new research published in Nature Communications predicts California will actually get wetter. The scientists from the University of California, Riverside predict the state will get an average of 12 percent more precipitation through the end of this century, compared to the last 20 years of last century.

    Scientists at the University of California, Riverside predict California will get an average of 12 percent more precipitation through the end of this century, compared to the last 20 years of last century. Credit: UC Riverside Strategic Communications

    ….They also found the winter months of December, January and February, when California traditionally gets the bulk of its precipitation, would account for much of the overall increase in precipitation. During those three months, precipitation levels would increase 31.6 percent in northern California, 39.2 percent in central California and 10.6 percent in southern California.  All these percentages are in comparison to data from the Global Precipitation Climatology Project observed between 1979 and 1999

    …They found that warming in the tropical eastern Pacific Ocean sea surface temperatures, an area about 2,500 miles east of the international date line, is the main reason for the predicted increase in precipitation levels.

    The warming sea surface temperatures encourage a southeastward shift of the jet stream, which helps steer more rain-producing mid-latitude cyclones toward California. “Essentially, this mechanism is similar to what we in California expect during an El Nino year,” Allen said. “Ultimately, what I am arguing is El Nino-like years are going to become more the norm in California.” But, Allen cautions that prediction of an El Nino-like year is no guarantee of a more wet winter in California. The 2015-16 winter was an example of that. Many other climatic factors must be considered….

    ….”Most previous research emphasized uncertainty with regards to future precipitation levels in California, but the overall thought was California would become drier with continued climate change,” said Robert Allen, an associate professor at UC Riverside and one of the authors of the paper. “We found the opposite, which is quite surprising.”

    Robert J. Allen, Rainer Luptowitz. El Niño-like teleconnection increases California precipitation in response to warming. Nature Communications, 2017; 8: 16055 DOI: 10.1038/ncomms16055

  9. Climate study: More intense and frequent severe rainstorms likely

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    Climate study: More intense and frequent severe rainstorms likely

    Posted: 07 Mar 2017 07:03 AM PST

    A climate scientist confirms that more intense and more frequent severe rainstorms will likely continue as temperatures rise due to global warming, despite some observations that seem to suggest otherwise…

    Guiling Wang, Dagang Wang, Kevin E. Trenberth, Amir Erfanian, Miao Yu, Michael G. Bosilovich, Dana T. Parr. The peak structure and future changes of the relationships between extreme precipitation and temperature. Nature Climate Change, 2017; DOI: 10.1038/nclimate3239

  10. Grasslands may be more sensitive to rising temperatures than precipitation

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    March 8, 2017 0 Comments Fangzhou Liu  Managing Editor of News  Stanford University full article here

    A team of Stanford and Columbia University researchers have found that U.S. grasslands may be more sensitive to atmospheric dryness than rainfall; their study suggests that scientists may have to look more to rising temperatures than precipitation in predicting plants’ response to global warming. Published on March 6 in Nature Geoscience, the researchers’ study examined 33 years of satellite data to understand grassland productivity in dry conditions. The timescale and quantity of data the team examined allows the study to inform predictive models of how environments will respond to droughts — which are likely to become more prevalent with rising temperatures around the globe.  …”U.S. grasslands are way more sensitive to vapor pressure deficit (VPD), which is important. Because VPD is so tightly linked to temperature, we can predict that it’s going to keep going up in the future.”…

    Sensitivity of grassland productivity to aridity controlled by stomatal and xylem regulation

    1. Konings, A. P. Williams & P. Gentine Nature Geoscience (2017) doi:10.1038/ngeo2903

    …We conclude that increases in vapour pressure deficit rather than changes in precipitation—both of which are expected impacts of climate change—will be a dominant influence on future grassland productivity.