Aerosols, precipitation, and cloud seeding

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Aerosols, precipitation, and cloud seeding

• Water in the West
• Evolving Technologies and Emerging Issues
• Irvine, California
• November 15-17, 2006

http//www.ral.ucar.edu/projects/wyoming
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Motivation

• Worldwide water resource stresses
• Severe weather hazards
• New observational, computational, statistical
  technologies
• Operational programs with little scientific basis
• Population and demographic changes
• Inadvertent weather modification
• ? similar physical processes!

estimates of indirect forcing that include
feedback to the liquid-water path and cloud
amount from changes in cloud microphysics and
precipitation efficiency range from -1.1Wm-2 to
-4.8Wm-2
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The Paradox

• Operational programs in more than 37 countries
  worldwide
• At least 69 programs in 11 U.S. states in 2004

• Limited funding supporting research as part of
  operational programs
• Impact of human activity on weather and climate
  recognized and heavily funded

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ISSUES

• What dont we know? What do we need to know?
• Design
• Seeding conceptual model (Apply to certain
  conditions)
• Identification of seedable opportunities (Change
  in time and space)
• Targeting and method of seeding (Change in time
  and space surface versus airborne)
• Natural variability (daily, monthly, seasonal)
• Execution
• Decision making system (Optimization of seeding
  experiments)
• Evaluation
• Statistical (Quantification)
• Physical (Providing confidence for the
  statistical results)

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Projects
Current cloud physics and radar-upgrade projects
Recent intl projects involving
equipment/software tech transfer, training, and
outreach in weather modification.
Italy and Greece Upgrade radar infrastructure,
observational project training
Wyoming snowpack enhancement program
Saudi Arabia, Qatar, UAE, Oman aerosol cloud
interactions
Burkina Faso and Mali Upgrade radar
infrastructure and training
Mexico Radar upgrades for NAME, previous
rainfall studies
Indonesia Infrastructure building and
aerosol-cloud interactions
Argentina software upgrades for hail studies
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Key Uncertainties(Possible solutions)What dont
we know? What do we need to know?

• Cloud and precipitation microphysics issues
• Background concentration, sizes, and chemical
  composition of aerosols participating in cloud
  processes (in-situ and satellite measurements,
  models)
• Cloud dynamics issues
• Cloud-to-cloud and mesoscale interactions
  relating to updraft and downdraft structures and
  cloud evolution and lifetimes (Multi-parameter
  radar, models)
• Cloud modeling issues
• Combination of best cloud models with advanced
  observing systems in carefully designed field
  tests and experiments (data assimilation,
  development of two-way interactive aerosol and
  microphysical parameterizations, land-surface
  interactions, upgraded and new parameterizations)

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The aerosol/precip connection

• Aerosol environment has changed
• CCN/sulfates are about 70 anthropogenic with
  strong variation in emissions geographically
• Desert dust concentrations vary widely appear to
  be important IN
• Clear anthropogenic effects
• (e.g., satellite evidence)
• Well known climate connections
• Direct (reflect incoming solar radiation back to
  space)
• Indirect (modify properties and lifetime of
  clouds)
• Linkage to precip understood in principle, but
  hard
• evidence is scanty and scattered we lack
• quantitative/predictive skill

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Important Point

• Preliminary indications are that aerosol changes
  could have changed precipitation processes in
  significant ways depending on the background
  pollution and aerosols in a specific region.
  These results apply both to inadvertent and
  advertent weather modification.
• Potential differences between start and end of
  the rainy season because of washout of aerosols
  by rain providing for cleaner conditions towards
  the end (Analyses will help determining optimal
  time for seeding)
• During dry periods aerosol concentrations may
  increase and decrease effectiveness of natural
  clouds to produce precipitation. Seeding may have
  larger positive effects

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So where are we?

• What do we know?
• Much has been learned about natural rainfall and
  seeding
• Warm vs cold rain
• The fundamental weather mod result AgI seeding
  produces lots of ice crystals
• The effects of seeding on precipitation are
  essentially never as clear-cut as in the stratus
  cloud photo. Clouds are complex.
• Controversial results regarding AgI seeding for
  rain increase from convective clouds
• Re-analysis of experiments effects less
  clear-cut
• Better progress with continued efforts studying
  winter orographic snowfall enhancement
• Greatly decreased funding since the 80s

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Re-thinking an old method Examples/Case studies
of the use of technology

• Using salt to stimulate collision/coalescence
• Use of pyrotechnic flares
• Background

• Maritime air 20-300 per cm-3 (larger drops)
• Continental air 500-5,000 per cm-3 (smaller)

• It is well known that maritime clouds rain more
  easily (the larger droplets collide with each
  other more readily)

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Hygroscopic Seeding
Uniform distribution of droplets
Non-uniform distribution
Try to add larger droplets that will initiate
coalescence
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South African program (1990s)

• Initiated a randomized experiment using
  hygroscopic flares
• Some aircraft measurements
• New evaluation methods
• Used the storm as the experimental unit
• Radar estimated rainfall
• Many more experimental cases
• Objective storm-tracking software (TITAN)
• Allowed for study of time-resolved response to
  seeding

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Radar estimate of rainfall within the TITAN
framework
The storm
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The TITAN experimental unit
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TITAN identifies and tracks individual storms
based on a specified reflectivity threshold
Objective radar estimate of rainfall
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Results from South Africa (Mather et al.)
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Hygroscopic Seeding Results

• South Africa and Mexico
• Quartile analyses to exclude outliers (25, 50
  and 75)
• Randomized experiments
• Time history of rainfall (seeded-blue
  unseeded-red)
• To what extent would weather modification
  operations be dependent on aerosol concentrations
  (see next slide)

South African Experiment
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Aerosol and non-aerosol days classification for
Mexican hygroscopic seeding data
Typical aerosol day (gt.1 optical depth)
Typical non-aerosol day (lt0.1 optical depth)
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Hygroscopic seeding

• Attempt to duplicate the South African experiment
  in Mexico showed amazingly similar results
• The effect seems to arise because the seeded
  storms had a longer lifetime
• Not part of the original hypothesis
• Seems to imply a connection between seeding and
  storm dynamics
• This connection is not well understood
• Hypotheses involve downdraft forcing
• This situation leads to caution about accepting
  the results
• During the same time frame an experiment in
  Thailand using hygroscopic salts also showed
  promising results

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Is the rainfall from these storms hydrologically
significant?
Flux in cubic meters per sec
Lets compare with the flow in some major rivers
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Equivalent number of very large storms (5,000
cms) Columbia 2 Mississippi 4
Ganges 6 Zaire 8 Amazon
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(No Transcript)
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Attempt at replication in the United Arab Emirates
UAE

• Preliminary airborne measurement program
• Droplet distribution found to be continental

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Results of the randomized statistical experiment
No statistically significant difference between
seed and control cases
TITAN radar image and seeding track
The experiment failed to duplicate the previous
hygroscopic seeding results
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Tentative explanation
Particle images lots of drizzle drops
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Sulawesi, Indonesia

• Clean environment, maritime CCN
• Tropical sounding
• First echoes at 5 to15C (warm rain)
• Efficient natural coalescence process
• Freezing by -5C and ice multiplication
• No point in seeding

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WYOMING SNOWPACK ENHANCEMNET 5-YEAR PILOT
PROGRAM FOLLOWS NAS/NRC RECOMMENDATIONS
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Numerical model simulations of seeding
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Simulated radar reflectivity9 December 2004
10 min after seeding started
Natural case
Seeded case
90 min after seeding started
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TARGETING OF SEEDING MATERIALNE winds middle
of 7 Feb 2004 storm
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Strong SW winds early into 27 Feb 2004 storm
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Evaluation Issues

• Inadvertent weather modification
• Geographical meteorological differences
• Natural variability
• Inadequate understanding of physical processes
• Controversy Unsubstantiated claims
• Independent assessment and evaluation
• Old technology

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The Climate Problem
80 yr. Temp. Rise CMIP 80 yr. Precipitation
Trend ?
Covey et al. 2003
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Daily Precipitation at 2 stations
Monthly Amount 75 mm Amount 75 mm
A B
Frequency 6.7 Intensity 37.5
mm Frequency 67 Intensity 3.75 mm
drought wild fires
local wilting plants
floods soil moisture replenished virtually
no runoff
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S-PolKa Radar

• Mass, Latent Heating Rates, Profiles
• Hydrometeor Identification
• Detection of cloud droplets
• Raindrop size distribution
• Effect of Bragg scatter is less at Ka-band
• Improved cloud microphysical retrieval
  (precipitation type, shape, size and
  concentration) using both dual-wavelength and
  dual-polarization observations

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Real time Aerosol and Environmental Monitoring in
the United Arab EmiratesJoint Dept of Water
Resource Studies, NCAR, NASA, and NRL
DevelopmentAn Example September 12th, 2004
Sept. 12, 2004, had one of the best organized
dust storms of 2004. This coincided with the
UAE2 campaign
Step 4. Based on monitoring such events, improved
weather models are developed and improved.
Dust Concentration (mg m-3)
Eventually, satellite data and observations will
be assimilated directly into the models to
provide an up to the minute assessment of aerosol
and environmental conditions
NCAR
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SummaryWhat is the present state-of-the-art with
respect to this technology?

• Convective clouds produce large amounts of rain
• Comparable to flow in major rivers
• AgI seeding of cumulus clouds for rain
  enhancement is a well used technology in many
  countries
• However, it is not a well proven technology
• Based on experiments in S. Africa, Mexico, and
  Thailand, hygroscopic seeding seems promising
  under specific circumstances
• Though promising, it is not well understood
• There are many new tools available to establish
  confidence in seeding technologies

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Summary

• Many factors control the amount of rain from
  convective clouds

• Aerosols (both CCN and IN) clearly affect the
  cloud particle structure
• Very likely affect the rain also (but positively
  or negatively?)
• However, the exact relationship has proven
  difficult to establish

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Aerosol change natural and anthropogenic
Satellite images of pollution effects on clouds

• Can mask seeding effects
• Older experiments may not be relevant today
• Transferring results from one region to another
  is very problematic

Mexico 1998
Industries
Indonesian smoke from fires in 1997
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And so

• A program of research should be considered
  mandatory even for an operational project
• Preliminary measurements
• Aerosol, cloud structure, rainfall climatology
• Should include evaluation of seeding effect
• Need a well designed, randomized experiment to
  know if you are spending the sponsors money
  wisely
• Need to look at hydrologic aspects where does
  the water go?
• Cost/benefit study
• History tells us its rarely cost beneficial to
  take a short-cut around the RD

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Where do we go?

• What are its primary advantages?
• - If shown to be beneficial can be conducted in
  region where water is needed.
• - Can be suspended at any time in times of
  abundant water or potential flooding
• - Preliminary cost/benefit calculations are
  very attractive in general ranging from 51 to
  151 in some cases
• - Technologies now exist to assess its benefit
  and quantify the effect
• What are the most significant challenges or
  obstacles?
• - Natural variability
• - Climate change (Temperature and Aerosols)
• - Scattered Resources
• What social and economic issues have emerged?
• - Environmental impacts such as silver iodide
  seeding
• - Extra area seeding effects
• How important will this technology be in meeting
  future western water needs and What is its
  potential role in an integrated water resources
  supply augmentation plan?
• - Determined by water managers
• What should be the role of local, state and
  federal agencies in facilitating the future
  progress and development of this technology?
• - Should be coordinated to avoid duplication and
  pool resources

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WMO GUIDELINES FOR THE PLANNING OF WEATHER
MODIFICATION ACTIVITIESWhat technical and
environmental challenges must be overcome? What
steps should be taken to best advance this
technology?

1. These guidelines are addressed to Members
   requesting advice or assistance on weather
   modification activities.
2. Experimental programs should be planned on a
   long-term basis because the precipitation
   variability is generally much greater than the
   increases or decreases claimed for artificial
   weather modification.
3. It is strongly recommended that an objective
   evaluation be performed by a group independent of
   the operational one.
4. Acceptance of the results of a weather
   modification program depends on the degree of the
   scientific objectivity and the consistency with
   which the experiment was carried out and the
   degree to which this is demonstrated.
5. Weather modification should be viewed as a part
   of an integrated water resources management
   strategy. Instant drought relief is difficult to
   achieve.
6. WMO recommends that operational cloud seeding
   projects for precipitation modification be
   designed to allow evaluation of the results of
   seeding through physical measurements.

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Cont.Focused experiments for quantification and
understanding

• To increase the chances of success in a specific
  situation, it should be verified through
  preliminary studies that
• The climatology of clouds and precipitation at
  the site indicates the possibility of favorable
  conditions for weather modification
• Conditions are suitable for the available
  modification techniques
• Modeling studies support the proposed weather
  modification hypothesis
• For the frequency with which suitable conditions
  occur, the changes resulting from the
  modification technique can be detected at an
  acceptable level of statistical significance
• An operational activity can be carried out at a
  cost acceptably lower than the socio-economic
  benefit that is likely to result. All prospective
  studies require expert judgment and the results
  are expected to depend on the site chosen and on
  the season.

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SummaryWhat would you tell western governors
about this technology?

• Important Problem
• Water resource management
• Large societal investments
• Inadvertent weather modification effects
• Opportunities
• New observing technologies
• Better models and computing
• Recent interesting research

Establish program with major emphasis on
quantification of the effects of both advertent
and inadvertent weather modification and include
scientific community