CalWater1

Precipitation, Aerosols, and Pacific Atmospheric Rivers Experiment
Daily Summary Plots
(Sugar Pine Dam)

Background

Our current understanding of heavy precipitation events and how precipitation will change in a changing climate is hampered by incomplete scientific knowledge. In an effort to identify the key gaps in our knowledge and develop the most effective strategies to address them, science leaders from the California Energy Commission, NOAA, and Scripps Institution of Oceanography at the University of California, San Diego came together in 2008 to explore how to proceed, to take advantage of the infrastructure and knowledge gained through NOAA's Hydrometeorology Testbed (HMT) efforts in California (hmt.noaa.gov), and aid the CEC planning for a possible follow-up Public Interest Energy Research (PIER) field study of the role of aerosols on orographic precipitation. [Prior PIER studies suggested aerosols may negatively affect precipitation amounts in the Sierra Nevada.] Inputs from scientists as well as key state and federal agencies were collected in a science workshop held in September 2008. Two key scientific challenges emerged, and have become the focus of a multi-year field experiment - "CalWater."

  1. How do aerosols (small solid or liquid particles in the air) affect the formation of clouds and precipitation, and what are the sources of the aerosols that act as seeds for the formation of droplets and/or ice. Determination of how these might affect a changing climate represents a longer term goal, since the treatment of aerosols in global and regional climate models is one of the main sources of uncertainty in climate science. It is intended that results obtained here will eventually be used to develop future regional model experiments.
  2. How well are atmospheric rivers (ARs), and the major precipitation events associated with them, represented in global and regional simulation and forecast models. How well are ARs represented in climate models, and how might AR amplitudes, frequencies and locations vary in a changing climate.

Desired Outcomes

CalWater's advances in scientific understanding, numerical modeling and measurements of critical physical processes underlying future changes in water supply and flood risks are likely to have impacts across science and applications. Results from CalWater may have important implications for California's water supply systems, because the effects of aerosols on clouds over California have been hypothesized to influence precipitation amounts in the Sierra Nevada and thus inflows to California's major reservoirs. Also, just a few atmospheric river events each wet season in California provide up to 50% of the water supply, and the strongest events can cause flooding. Uncertainty in regional precipitation projections in a changing climate is one the greatest uncertainties in climate change impacts. Finally, results from studies in this important region can provide lessons for elsewhere in the nation, and globally, as scientists, leaders and decision makers at all levels plan for how to handle risks of either too much or too little water in the future.


Past Events

Event Dates Location Additional Information
CalWater Science Workshop 08-10 Jun 2011 Scripps, La Jolla, CA Agenda
CalWater Science Workshop 04-05 Oct 2010 Scripps, La Jolla, CA Agenda
CEC-NOAA-Scripps CalWater Science Planning Workshop 15-17 Sep 2008 Scripps, La Jolla, CA Agenda | Science Team Memo | Participant List

Field Seasons

Jan–Feb 2009: CalWater Early Start brought together ground based aerosol and hydrometeorological sensors at a new field site in the Sierra Nevada mountains. During the two weeks of sampling, 10-20 inches of rain or liquid equivalent snow was observed. The precipitation came mostly during two similar AR events, one of which showed the presence of Asian dust as the primary aerosol content within the precipitation.

Jan-Mar 2010: CalWater 2010 added another combined aerosol/hydrometeorology field site in the Southern Sierra, a new AR observatory to document the western edge of the Sierra barrier jet, and enhancements to other HMT field sites. Several major storms were documented, including some with well-defined AR and Sierra Barrier Jet conditions.

Dec 2010 - Mar 2011: CalWater 2011 added to the earlier deployments by fielding the PNNL G-1 research aircraft (Feb-Mar 2010 from Sacramento) to document cloud microphysics and aerosol conditions aloft, in coordination with ground-based observations in the Sierra Nevada and Central Valley. Also, a high-powered scanning C-band Doppler radar plus a balloon sounding system from NOAA's Physical Science Division were deployed in the Central Valley (north of Sacramento) to monitor the Sierra Barrier Jet and possible interactions with land-falling ARs. Several ARs and SBJs were documented.


CalWater "Aerosol—Precipitation" Research Theme Hypotheses


  1. Aerosols will impact precipitation derived from shallow cloud events or shallow portions of mixed events but will have little impact on AR events with deep vertical structure.
  2. Anthropogenic aerosols suppress orographic precipitation by slowing the autoconversion rate, and hence decrease the amount of precipitation from the shallow and short living orographic clouds.
  3. Uncertainty regarding the magnitude of potential anthropogenic impacts of aerosols on precipitation in California results from a lack of understanding of the fundamental physics of precipitation generation in ARs.
  4. Advancements of numerical model representations of precipitation founded on observations are needed to properly represent key precipitation mechanisms associated with aerosols and black carbon.
  5. Measuring the chemical composition of the aerosols that act as cloud condensation nuclei (CCN) or ice nuclei (IC) will allow for the determination of the sources that emit these aerosols
    1. different aerosol sources can impact cloud microphysics in different ways, or to different extents.
    2. changes in microphysics change the precipitation type and amount.
  6. Transport of dust aloft from Asia may have a substantial impact on precipitation levels and type (rain versus snow)
  7. Collaboration between CalWater meteorologists and aerosol experts are required to ensure that interpretations of aerosol impacts on precipitation do not overlook alternative hypotheses that equally explain the observed behavior without invoking aerosol impacts.
  8. The role of black/brown carbon on snow albedo can have an important impact on snow melt.

CalWater "Atmospheric Rivers—Precipitation" Research Theme Hypotheses


  1. The representation of cloud formation in AR conditions in current versions of global and regional weather models includes significant errors that can be diagnosed through specialized observations, the results of which can then inform enhancements of existing models.
  2. Uncertainty in IPCC projections of annual precipitation and extreme precipitation events in California results partly from uncertainty in the representation of ARs offshore in the IPCC models.
  3. Significant errors in projections of precipitation and associated flood risks exist in IPCC Global models and attendant regional models because they do not adequately represent the effects of the coast range and Sierras on land-falling atmospheric rivers.
  4. Other sources of uncertainty in California precipitation projections are related to how the IPCC models represent shallow rainfall processes, and "cut-off lows" and associated large scale circulations, which may or may not involve ARs.
  5. Downscaling of global models is needed to properly represent the key water vapor transport and precipitation processes, and yet it remains unclear what models, resolutions or other model parameters are needed to do so.
  6. If a model does not represent the coastal barrier jet or the Sierra barrier jet, it is unable to represent the potential impacts of aerosols on orographic precipitation.

The Relationship Between CalWater and HMT

CalWater, with its emphasis on orographic precipitation and atmospheric water vapor budget, has been planned and conducted on the backbone of field observations and scientific understanding that has been developed in California largely as part of the Hydrometeorology Testbed (HMT) and prior PIER studies by CEC. A number of field observations collected solely for HMT, are also valuable for CalWater. However, additional specialized observation sites have been and will be needed to meet CalWater science objectives, especially those at Sugar Pine, Mariposa, Lincoln and Concord, CA. Experience gained in developing the integrated observing methods used by HMT have been incorporated in CalWater, especially the development of S-Prof radar, disdrometer, and surface met sites, and invention of atmospheric river observatories (AROs). The scientific problems being addressed have been shaped and built upon physical process understanding that has been a key focus of HMT, especially atmospheric rivers, the role of barrier jets in modulating precipitation, and the existence and characteristics of shallow "non-bright-band" rain. Programmatically and practically, CalWater leverages staff, equipment and funding that have been focused on creating and executing HMT-West, particularly NOAA/ESRL/Physical Sciences Laboratory/Water Cycle Branch. Finally, the success of CalWater depends on people whose experience in observing systems, scientific understanding and numerical modeling has been greatly influenced by lessons learned through HMT-West.