CW3E Publication Notice: Global Analysis of Climate Change Projection Effects on Atmospheric Rivers

CW3E Publication Notice

Global Analysis of Climate Change Projection Effects on Atmospheric Rivers

May 24, 2018

Vicky Espinoza (UC Merced) and CW3E collaborators Bin Guan (UCLA), Duane Waliser (NASA/JPL), along with CW3E director Marty Ralph and David Lavers European Centre for Medium‐Range Weather Forecast, recently published a paper in Geophysical Research Letters, titled Global Analysis of Climate Change Projection Effects on Atmospheric Rivers.

Atmospheric rivers (ARs) are elongated strands of horizontal water vapor transport, accounting for over 90% of the poleward water vapor transport across midlatitudes. ARs have important implications for extreme precipitation when they make landfall, particularly along the west coasts of many midlatitude continents (e.g., North America, South America, and West Europe) due to orographic lifting. ARs are important contributors to extreme weather and precipitation events, and while their presence can contribute to beneficial rainfall and snowfall, which can mitigate droughts, they can also lead to flooding and extreme winds. This study takes a uniform, global approach that is used to quantify how ARs change between Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations and future projections under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios globally. The projections indicate that while there will be ~10% fewer ARs in the future, the ARs will be ~25% longer, ~25% wider, and exhibit stronger integrated water vapor transports under RCP8.5 (Figure 1). These changes result in pronounced increases in the frequency (integrated water vapor transport strength) of AR conditions under RCP8.5: ~50% (25%) globally, ~50% (20%) in the northern midlatitudes, and ~60% (20%) in the southern midlatitudes (Figure 2).

Figure 2 from Espinoza et al., 2018. AR frequency (shading; percent of time steps) and IVT (vectors; kg · m−1 · s−1) for (a) ERA‐Interim reanalysis for the historical period (1979–2002) with six green boxes depicting regions analyzed in Figures S2 and S3, (b) the MMM for the 21 CMIP5 models analyzed in this study for the historical period (1979–2002), (c) RCP4.5 warming scenario (2073–2096), and (d) RCP8.5 warming scenario (2073–2096).

This research was supported by the NASA Energy and Water cycle Study (NEWS) program. Vicky Espinoza’s contribution to this study was made possible by NASA Jet Propulsion Laboratory’s Year-Round Internship Program during her graduate studies at the University of Southern California. Please contact Duane Waliser at duane.waliser@jpl.nasa.gov with inquiries. More information can be found from the NASA website https://www.jpl.nasa.gov/news/news.php?feature=7141.

Espinoza, V., Waliser, D. E., Guan, B., Lavers, D. A., & Ralph, F. M. 2018: Global Analysis of Climate Change Projection Effects on Atmospheric Rivers. Geophysical Research Letters. 45. https://doi.org/10.1029/2017GL076968

Odds of Reaching 100% Water Year Precipitation – May Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2018 (May Update)

May 2, 2018

Contribution from Dr. M.D. Dettinger, USGS

Here is how we usually tend to see the water-year precip-drought to-date or last month’s contributions represented:

Figure 1: Total precipitation anomaly (large map) and total precipitation (smaller map) during water 2018 (September 2017-April 2018). Images courtesy PRISM Climate Group.

A somewhat different viewpoint on the development of drought considers how much precipitation has fallen (or not) AND how much is likely to fall in coming months, based on climatology. April 2018 produced precipitation over much of northern California and improved odds of reaching normal in some locales, but overall did little to undo the deficits of the previous months in a majority of the state. The following are maps of this year’s drought development that explicitly takes both of these aspects into account.

Here is how the drought has evolved so far this water year in terms of the odds of reaching 100% of normal precipitation by end of water-year 2018.

Figure 2: Odds of reaching 100% of water-year normal precipitation totals throughout water-year 2018.

  • Drought conditions have continued to develop across the Southwest, as odds of reaching normal have progressively dwindled month by month. Although April was wet over parts of northern California, it was—arguably—too little too late to set us up for reaching 100% of normal this year, in all but a few locales.

Figure 3 shows the current odds of reaching various fractions (including but not limited to 100%) of water-year-total this year (top row), as well as the corresponding odds prior to April (bottom row).
This approach offers a far different view than the precipitation anomalies of figure 1, emphasizing different “hot spots” of hope & despair.

Figure 3: Odds of water-year 2018 reaching various fractions of water year normal precipitation totals based on water year precipitation through April (top row) and prior to April (bottom row).

Finally, figure 4 is the “flipped” version of the analysis, asking-at each pixel-how large a water-year total precipitation has a 50% (and other exceedances) chance of being equaled or exceeded this year, as of May 1, 2018.

Figure 4: Chance of water-year total precipitation being equaled or exceeded this year.

  • A different color bar is used here to emphasize that the shades now are illustrating something quite different from the previous maps

How the probabilities above were estimated:
At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record (WY1948-2017 in these figures) and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, etc., for these figures.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested (across all climate divisions in California, so far) for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero.]

Contact: Michael Dettinger (USGS)

CW3E Director Featured in the Water Zone Podcast on KCAA San Bernardino

CW3E Director Featured in the Water Zone Podcast on KCAA San Bernardino

April 25, 2018

The Water Zone is a KCAA (Loma Linda, CA) radio show, hosted by Paul McFadden, that explores water issues in agriculture and farming from various perspectives to advance water conservation. The April 19, 2018 episode featured two notable guests: Dr. F. Martin Ralph, CW3E Director, and Dr. Thomas Philp, a Pulitzer Prize winning journalist who is the executive strategist for the Metropolitan Water District of Southern California.

Follow the link to listen: http://podcasts.kcaastreaming.com/water/20180419.html. Information related to western weather and water can be heard during minutes 11-44 of the episode.

CW3E Hosts 3rd Annual Meeting

CW3E Hosts Third Annual Meeting

April 20, 2018

CW3E hosted its third annual meeting this week, April 16-19, 2018. Part of the vision of CW3E is to create a community of collaboratively-oriented scientists, students, engineers and staff, and to generate the support to enable these collaborations. This was the first annual meeting of this scope. Collaborators from all over the U.S., and National Weather Service forecasters from Reno, Monterey/San Francisco, and San Diego attended. The objectives of the meeting were to bring together scientists and engineers to address major science challenges, share ideas, develop collaborative projects, and to provide early career scientists and students opportunities to present their work.

CW3E Postdoctoral Scholar, Forest Cannon, presents on airborne radar observations of oceanic precipitation in atmospheric rivers.

The meeting began on the afternoon of Monday, April 16th and went through Thursday, April 19th. There was a poster session on the afternoon of April 17th, to give more post-docs and graduate students a chance to present their work. Session topics included extreme precipitation climatology and future projections, mesoscale and physical processes, forecasting, subseasonal to seasonal forecasting, AR reconnaissance and data assimilation, and hydrology and snow. There was a strategic planning session in which all attendees brainstormed and shared their thoughts on where CW3E will be in the next 5 years.

Throughout the meeting there were several chances to discuss future scientific collaborations. Attendees reported how valuable the face-to-face meetings between collaborators were, and how productive it was to discuss potential research avenues in person. Although the exact form of the meeting will evolve in response to feedback, the annual in-person scientific meeting will continue to be central to CW3E’s mission to build a community of scientists and engineers working together to solve challenging problems relating to extreme weather and water events in the west.

CW3E Publication Notice: Evaluation of Atmospheric River Predictions by the WRF Model Using Aircraft and Regional Mesonet Observations of Orographic Precipitation and Its Forcing

CW3E Publication Notice

Evaluation of Atmospheric River Predictions by the WRF Model Using Aircraft and Regional Mesonet Observations of Orographic Precipitation and Its Forcing

April 16, 2018

CW3E project scientist Andrew Martin and co-authors have published a study characterizing predictability limits in Atmospheric River (AR) forecasts and apportioning Russian River precipitation forecast errors among vapor transport and orographic precipitation components. The article, titled Evaluation of Atmospheric River Predictions by the WRF Model Using Aircraft and Regional Mesonet Observations of Orographic Precipitation and its Forcing, is now in early online release at the Journal of Hydrometeorology.

This study leveraged airborne dropsonde observations of offshore Atmospheric Rivers completed during the CalWater experiment and the Atmospheric River Observatory at Bodega Bay and Cazadero, CA to verify forecasts of AR properties and their resulting precipitation. Forecasts were created by CW3E’s numerical weather prediction model, West-WRF, and compared to Global Forecast System reforecasts (GFSRe) valid for the same events. Forecast skill in AR properties and precipitation was evaluated at lead times up to 7 days ahead. Notably, the study found that deterministic skill in integrated vapor transport and other related fields degrades (meaning that forecasts created from climatology perform just as well or better) more than 4 days ahead for both models. However, West-WRF improves upon GFSRe skill in IVT at days 1, 2 and 3 ahead (see Fig. 1c).

Figure 1. a) Value added by GFSRe over GFSRe climatology validated against 145 CalWater dropsondes for the variables z500 (blue), IVT (black), IWV (green) and e925 (red). b) as in a, except for West-WRF value added over GFSRe climatology. c) as in b, except reference forecast is GFSRe.

The study also employed a novel forecast error separation technique to apportion precipitation forecast errors among the component caused by vapor transport simulation and orographic precipitation process simulation. Data from the Atmospheric River Observatory was used to demonstrate that West-WRF forecasts of orographic precipitation during landfalling AR are more accurate in simulating both components; but also that West-WRF forecasts of precipitation can be improved by improving the vapor transport component because its orographic precipitation process is accurate. This lends confidence that CW3E’s effort to improve west coast precipitation forecasts by assimilating offshore observations into West-WRF analyses can yield successful results.

Co-authors include Dr. F Martin Ralph, Reuben Demirdjian, Laurel DeHaan, and Dr. Rachel Weihs of CW3E with Dr. David Reynolds of the Cooperative Institute for Research in Environmental Sciences and Dr. Sam Iacobellis of Scripps Institution of Oceanography. The study was funded by the US Army Corps of Engineers, the California Department of Water Resources, and the National Science Foundation XSEDE program.

Odds of Reaching 100% Water Year Precipitation – April Update

Odds of Reaching 100% of Normal Precipitation for Water Year 2018 (April Update)

April 10, 2018

Contribution from Dr. M.D. Dettinger, USGS

Here is how we usually tend to see the water-year precip-drought to-date or last month’s contributions represented:

Figure 1: Total precipitation anomaly (large map) and total precipitation (smaller map) during water 2018 (September 2017-March 2018). Images courtesy PRISM Climate Group.

A somewhat different viewpoint on the development of precipitation drought considers that development to be a matter of both how much precipitation has fallen (or not) already AND how much more is realistically likely to fall in coming months. E.g., March 2018 simultaneously produced helpful additions to this year’s precipitation totals in California AND was disappointingly far from completely undoing the deficits of the preceding months in most of the State. The following are maps of this year’s drought development that explicitly take both of these aspects into account.

Here is how the drought has evolved so far this water year in terms of the odds of reaching 100% of normal precipitation by end of water-year 2018.

Figure 2: Odds of reaching 100% of water-year normal precipitation totals throughout water-year 2018.

  • Notice how drought conditions have developed across the Southwest, as odds of reaching normal have progressively dwindled month by month. Also notice that, although March was wet in California/Nevada, it was—arguably—too little too late to set us up well for reaching 100% of normal this year, in all but a few locales.

The top row in figure 3 shows the current odds of reaching various fractions (including but not limited to 100%) of water-year-total this year.
Also shown are the corresponding odds prior to March (middle row), and the amount that March precip changed the odds (bottom). This approach offers a far different view than the precipitation anomalies of figure 1, emphasizing different “hot spots” of hope & despair.

Figure 3: Odds of water-year 2018 reaching various fractions of water year normal precipitation totals and the change in these odds during March 2018.

Finally, figure 4 is the “flipped” version of the analysis, asking-at each pixel-how large a water-year total precipitation has a 50% (and other exceedances) chance of being equaled or exceeded this year, as of April 1, 2018.

Figure 4: Chance of water-year total precipitation being equaled or exceeded this year.

  • A different color bar is used here to emphasize that the shades now are illustrating something quite different from the previous maps

How the probabilities above were estimated:
At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record (WY1948-2017 in these figures) and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, etc., for these figures.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested (across all climate divisions in California, so far) for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero.]

Contact: Michael Dettinger (USGS)

CW3E AR Update: 02 April 2018 Outlook

CW3E AR Update: 02 April Outlook

April 02, 2018

Click here for a pdf of this information.

Atmospheric river forecast to impact Northern California later this week

  • GFS Ensemble members are currently forecasting a potentially strong to extreme AR over Northern California later this week
  • There is currently large uncertainty in the onset, duration, and magnitude of AR conditions, creating uncertainties in the potential impacts of this event
  • >5 inches of precipitation could fall during this event over the high elevations of the Coastal and Sierra Nevada Mountains in Northern California
  • The GFS is currently suggesting freezing levels >8,000 feet for most of this event, which may lead to most precipitation over the high Sierra falling as rain

Click IVT or IWV image to see loop of 0-141 hour GFS forecast

Valid 1200 UTC 02 April – 0900 UTC 08 April 2018

 

 

 

 

 

Summary provided by C. Hecht, F.M. Ralph, and B. Kawzenuk; 1 PM PT Monday 02 April 2018

*Outlook products are considered experimental

CW3E AR Update: 21 March 2018 Outlook

CW3E AR Update: 21 March Outlook

March 21, 2018

Click here for a pdf of this information.

Update on Atmospheric River Currently Impacting California

  • Light to moderate precipitation has begun falling over portions of California
  • GFS Ensemble members have continued to converge on coastal IVT forecast for the remainder of the AR
  • Portions of Southern California are forecast to experience Strong AR conditions between 2 and 5 AM on Thursday 22 March
  • ~1–2 inches of precipitation has fallen over the Santa Ynez Mountains over the past 24 hours
  • An additional 10+ inches of precipitation are forecast to fall over the higher elevations of Santa Barbara and Ventura Counties during the remainder of this AR

National Weather Service Composite Radar Reflectivity

Valid 1748-1858 UTC 21 March 2018

Images from weather.gov

Click IVT or IWV image to see loop of 0-72 hour GFS forecast

Valid 1200 UTC 21 March – 1200 UTC 24 March 2018

 

 

 

 

 

 

 

 

Summary provided by C. Hecht, F.M. Ralph, and B. Kawzenuk; 1 PM PT Wednesday 21 March 2018

*Outlook products are considered experimental

CW3E AR Update: 20 March 2018 Outlook

CW3E AR Update: 20 March Outlook

March 20, 2018

Click here for a pdf of this information.

Update on Atmospheric River Forecast to Impact California This Week

  • The terminus of the atmospheric river plume is approaching coastal CA and precipitation will begin today
  • Models are suggesting potentially strong (IVT >750 kg m-1 s-1) AR conditions over San Luis Obispo and Santa Barbara Counties
  • Locations further south may experience moderate strength AR conditions (IVT >500 kg m-1 s-1)
  • AR conditions are forecast to peak over portions of SoCal between Midnight and 11 AM PDT on Thursday, 22 March 2018
  • As much as 10 inches of precipitation may fall over the higher elevations of Santa Barbara and Ventura Counties
  • The National Weather Service has issued numerous Flash Flood Watches and Winter Weather Warnings in California

SSMI/SSMIS/AMSR2-derived Integrated Water Vapor (IWV)

Valid 0000 UTC 18 March – 1600 UTC 20 March 2018

Images from CIMSS/Univ. of Wisconsin

Click IVT or IWV image to see loop of 0-72 hour GFS forecast

Valid 1200 UTC 20 March – 1200 UTC 23 March 2018

 

 

 

 

 

 

 

 

 

 

Summary provided by C. Hecht, F.M. Ralph, J. Rutz, and B. Kawzenuk; 1 PM PT Tuesday 20 March 2018

*Outlook products are considered experimental

CW3E AR Update: 19 March 2018 Outlook

CW3E AR Update: 19 March Outlook

March 19, 2018

Click here for a pdf of this information.

Update on Atmospheric River Forecast to Impact California Next Week

  • Several changes have occurred in the forecast for the AR that may impact CA this week
  • GFS Ensemble members have continued to converge on Coastal AR conditions
  • While there is more agreement between GFS Ensemble members there are still numerous changes from model run to model run, introducing several uncertainties in the impacts associated with this event

Click IVT or IWV image to see loop of 0-102 hour GFS forecast

Valid 1200 UTC 18 March – 1800 UTC 23 March 2018

 

 

 

 

 

 

 

 

Summary provided by C. Hecht, F.M. Ralph, and B. Kawzenuk; 1 PM PT Monday 19 March 2018

*Outlook products are considered experimental