Seeding Clouds to Enhance Precipitation: Methods and Effectiveness
Bart Geerts
Dept. of Atmospheric Science
University of Wyoming
cloud seeding artificial snow making
• clouds are seeded with nuclei so that cloud droplets become precipitation-size (rain or snow)
• air needs to be saturated • precip enhancement may occur
many miles downwind • advertent “weather modification”
• water is sprayed under high pressure in cold air
• the water contains ice nuclei (Pseudomonas syringae bacteria) so that tiny droplets freeze immediately upon exiting the nozzle
• air can be dry • snow falls close to source
• clouds consist of tiny droplets, ~100 cm-3, floating with the wind
• most droplets remain liquid below the freezing point (0°C)
• growth by condensation is very slow, usually just more droplets form
• numerous collisions are needed to make a rain drop
• homogeneous nucleation requires cooling to -39°C
• nature has few ice nuclei on which heterogeneous nucleation can occur (~10 m-3 at -10°C, 14°F)
cloud physics 101
sector plates
Rasmussen & Libbrecht, 2003
Rasmussen & Libbrecht, 2003
dendrites
Rasmussen & Libbrecht, 2003
rosettes
supercooled water is ubiquitous in clouds
the Bergeron process
• most precipitation on Earth derives from snow, via the Bergeron process
90% RH
90% RH
90% RH
vapor
100% RH
100% RH
100% RH
two cloud seeding methods
• large, hygroscopic nuclei (e.g. sea salt) act as condensation nuclei that jump-start the growth of drops by collision-coalescence
• commercial operations where clouds are too warm for glaciogenic seeding
• effectiveness not proven
• homogeneous nucleation – dry ice (CO2 at -80°C) – liquid propane
• ice nuclei (usually tiny AgI crystals) – applied pyrotechnically – AgI nucleation temp -6 to -9°C – most commonly used in cold clouds
hygroscopic glaciogenic
Remote-Controlled Ground Generator Demo Unit
Seeding Agent
Propane Burner
Inside the Chamber
Two target clouds for glaciogenic seeding
• thunderstorms: dynamic effect – latent heat release by glaciation
invigorates thunderstorm • also used to mitigate/prevent hail
damage
• orographic clouds: static effect – without ice crystals, clouds would
simply rise over the mountain and evaporate on the lee side
– Bergeron process is jump-started by injection of ice crystals
As air is forced up over a mountain barrier, it cools.
In the lee of the mountains, the air descends & warms.
When the air is moist, condensation occurs, and a cloud of ~20 micron water droplets forms. These droplets are far too small to precipitate.
In the lee, the droplets evaporate.
These droplets remain liquid well below 0°C. Eventually, nature usually introduces ice in these supercooled clouds, by tiny particles called ice nuclei.
Such ice nuclei are rare.
. First Ice
Once ice forms, the particles grow quickly through the Bergeron process, forming flakes that become large enough to precipitate. Snow begins to fall.
. First Snow
The key is to get the snow to fall out upwind of the crest. Snow falling downwind in the descending, drying air will evaporate.
Cloud seeding provides additional ice nuclei that function at warmer temperatures, allowing ice formation to begin sooner. This allows snow to grow sooner, and increase the amount of snow falling upwind of or around the crest.
. First Ice, seeding
. First Ice, not seeded
Most of the precipitation in the
Western USA is orographic.
Water demand is rising as population increases, yet supply may decrease
in a warming climate.
Wintertime precipitation trend over the next 100 years remains highly uncertain
Wintertime precipitation trend over the next 100 years remains highly uncertain
2050 – 2000 wintertime (NDJFMA) precipitation according to the CCSM A2 simulation (Rasmussen et al. 2012)
WY
CO UT
AZ
ID
NM
Reduce consumptive use Increase water production
New Water/Reuse from the System
• Use wasted brackish water • Recycle wastewater • Use desalted ocean or inland
water
Add to Rainfall • Weather modification,
esp. cloud seeding Reduce Evapotranspiration • Vegetation Management
Add to Inflow • Water from coal bed
methane production • Importation alternatives
Reduce Outflow from System
Reduce Loss to Groundwater
Add to Groundwater •Aquifer storage and recovery •Conjunctive use
Concept Examples Water shortage?
National Research Council Report on Weather Modification, 2003
• “Notwithstanding several decades of research, scientific proof of the effectiveness of cloud seeding remains lacking”
• catch-22 paradox: the federal government is not willing to fund research to understand the efficacy of weather modification technologies, yet private interests are willing to spend funds to apply these unproven techniques.
• NRC (2003) advocates a renewed commitment in federal funding to support weather modification research,
• and recommends that the opportunities offered by operational weather modification programs should be capitalized upon.
Weather modification
research
• Meteorology: Taming the sky. Nature, 453, 970-974 (June 2008) • Editorial: Change in the weather. Nature, 453, 957-958.
“Other countries, such as the United States, have simply given up;; the most
promising experiment in America is run not by the federal government but by the state of Wyoming, which is spending $9 million on a seven-year series of cloud-seeding experiments evaluated by experts from the National Center for Atmospheric Research. That's the type of targeted and rigorous study that needs to be done in weather modification, but it took Wyoming to do it.” … ”The stakes are high, as weather modification is one of those areas in which science can have an immediate and obvious benefit for society. It's long past time to invest modest funds in the basic understanding of it.”
Can we trust peer-reviewed publications?
• Science contributor John Bohannon sent a deliberately faked research article 305 times to online journals. More than half the journals that supposedly reviewed the fake paper accepted it. (NPR, 3 Oct 2013)
• Is the analysis objective? If an organization funds a research project that will benefit them financially, then we cannot accept the findings as "evidence" unless different researchers (from unrelated organizations) come to the same conclusions through their own independent research.
• Are the data representative? It is crucial for researchers to ensure that the data they collect is representative of the "true" situation.
• Is the peer review robust? Having your research findings published in a peer-reviewed journal means that other scientists who specialize in that kind of research have verified the quality and validity of the research. It is increasingly difficult for editors to find reviewers willing to make the effort.
Weather modification research
• Hundreds of peer-reviewed papers on weather modification research have been published.
• This all goes back to mid-1940s when scientists at General Electric Research Laboratory in New York demonstrated that dry ice and silver iodide (AgI), which have a similar crystal structure to ice, could initiate ice in a laboratory supercooled liquid cloud. When dry ice pellets or AgI nuclei were released from an aircraft into supercooled stratus clouds, the affected cloud rapidly cleared (Schaefer 1946; Vonnegut 1947).
http://www.gereports.com/thinking-outside-the-cold-box/
Irving Langmuir, Bernard Vonnegut,
and Vincent Schaefer
WWII B-17
The start of glaciogenic seeding
11/24/1948. B-17 flew ~100 m above cloud top. Cloud top temp ~-6°C.
various Intro Meteorology textbooks, e.g. Lutgens and Tarbuck
The start of glaciogenic seeding
http://www.gereports.com/thinking-outside-the-cold-box/
Clearing of this thin cloud layer is obvious, but no in situ or remote sensing probes were available to prove the interpretation that ice crystals formed and consumed the available liquid water.
The start of glaciogenic seeding
Chris Walcek, pers. comm.
The start of glaciogenic seeding
Was glaciogenic seeding responsible for the observed cloud clearing?
Dry ice was crunched into "cm" sized chunks, and dropped from a B-17 porthole, yielding significant "spacing" between individual falling pellets of dry ice Did the resulting ice crystals really grow into snowflakes consuming the SLW? Chris Walcek (Univ. of Albany) believes that the aircraft’‛s downwash could have triggered cloud-top entrainment instability (CTEI)
Was glaciogenic seeding responsible for the observed cloud clearing?
Randall 1980, JAS
cloud-top entrainment instability
Maybe Schaeffer and Vonnegut’‛s cloud clearings were the same as “cloud holes” that are often punched through
supercooled stratus clouds by aircraft?
• Propellers produce significant adiabatic cooling below -39°C, yielding a trail of ice crystals in a supercooled cloud.
Heymsfield et al. (2010 in BAMS; 2011 in Science; 2013)
Can we trust peer-reviewed papers on glaciogenic cloud seeding?
• example: “Radar detection of cloud seeding effects” by Hobbs et al. in Science, 1981.
• The reported seeding rate is just ~0.05 kg/km, implying a spacing between dry ice pellets of ~50 m!
• We have since learned that AgI generators do not produce plumes of enhanced radar reflectivity, at least not in orographic clouds.
• Questions must be asked about persistence, repeatability, and uniqueness.
• In a 2001 opinion letter in BAMS, Hobbs himself is critical about weather mod research and states that the public deserves a more straightforward statement on the uncertainties of cloud seeding to increase precipitation.
• The reality of the 1960s-1980s cloud seeding literature is a monument to subjectivity, lack of peer-review by outsiders, numerous bogus reports, and exaggerated findings.
• This is evidence of short-sightedness, in the long term it became a self-inflicted wound. Federal funding for weather mod research dried up in the late 1980s.
• The fact is the precipitation is an extremely noisy field, and therefore it is difficult to accomplish meaningful control vs. target experiments (Garstang 2005).
Can we trust peer-reviewed papers on glaciogenic cloud seeding?
Is there hope?
• The NRC 2003 reports encourages large randomized experiments with a strong statistical basis, as well as physically-based experiments, to improve our understanding of the changes in cloud microphysical and dynamical processes resulting from cloud seeding.
• Several rigorous statistical studies have come out recently (e.g. Morrison et al. 2009)
• NSF recently funded a study to examine cloud processes in seeded clouds, using new observational tools and high-performance computing.
Divide Peak
Webber Springs Old Battle
Sandstone RS Little Snake
Whiskey Park
Sand Lake
S Brush Cr
N French Cr
Brooklyn Lake
Cinnabar Park
Battle Pass HY47
Hog Park 5502E
Towner Lake GLEES
Douglas Cr
Rob Roy
Upper Cedar Cr
Savery
Saratoga x
0 20 40 km
Target
Target
Control
Control
Wyoming Weather Modification Pilot Project (2006-2012)
Radiometer
Radiometer Sounding Sierra Madre
Medicine Bow
- AgI Generator - SNOTEL
- Project Snowgauge
King Air instruments
FSSP 2DP CIP
PCASP
Icing on the instruments
Shallow orographic cloud over Medicine Bow mountain
Flight pattern over Medicine Bow mountains
- geographically fixed flight legs relative to 3 AgI generators
- 2 full ladders flown before seeding, 2 ladders during seeding
:
strategy to detect impact of AgI seeding: non-simultaneous comparison SEED – NOSEED *
downwind includes natural changes in time
quasi- simultaneous control SEED – NOSEED * upwind
removes these natural changes
* SEED and NOSEED refer to time periods
Why this case?
• Not because it produced much snow … rather, because it was rather simple (“natural laboratory”):
• Temperature was cold (-12 oC at 700 mb) with moderate LWP (~ 0.07 mm)
• No seeder-feeder source aloft
• Not much natural variation in LWP, cloud structure, or precip rate during the flight
Case study – Feb 12, 2013
cloud top mountain top
Pokharel et al. 2013
10 m/s
along-wind leg – Feb 12, 2013
west east 45 km
Medicine Bow mountains rada
r re
flec
tivi
ty
vert
ical
ve
loci
ty
lidar Medicine Bow mountains
liquid water cloud top
Pokharel et al. 2013
Feb 12, 2013
• leg #2 • ~3 km downwind of
AgI generators • 4 passes
• 2 before seeding • 2 during seeding
key strength: vertical perspective gives
reflectivity data very close to ground over complex
terrain Pokharel et al. 2013
Reflectivity FAD Feb 12, 2013
• Cloud is very shallow
• Very light snowfall generated naturally near the surface
• Note shift in near-surface reflectivity to the right during SEED
NOSEED downwind legs
SEED downwind legs
Reflectivity FAD Feb 12, 2013
• Cloud is very shallow
• Very light snowfall generated naturally near the surface
• Note shift in near-surface reflectivity to the right during SEED
NOSEED downwind legs
SEED downwind legs
Reflectivity FAD Feb 12, 2013
• Cloud is very shallow
• Very light snowfall generated naturally near the surface
• Note shift in near-surface reflectivity to the right during SEED
NOSEED downwind legs
SEED downwind legs
SEED - NOSEED
Conclusions
• This is just one case study. Studies such as ASCII should be done more in the context of commercial seeding operations. The observational technology, esp. cloud radar and lidar, are far advanced compared to the weather modification research heydays in the 1960s-1980s.
• High-resolution cloud-resolving, eddy-resolving simulations should be conducted in the context of these observational studies, for model validation of basic dynamics and cloud processes, and to inform about the likely magnitude of the seeding signal on various measurements.
• So there is hope …