The Warm Rain Process

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The Warm Rain Process

• Dr. Sonia Lasher-Trapp
• Dept. of Earth Atmos. Sciences
• 6 Jan 2005

Photo taken by C. Knight
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The Warm Rain Process
Nucleation (on CCN)
ICE
condensation
Cloud Temp 0 C -important in
tropics -important in midlatitude
summer -important in midlatitude winter?
collision coalescence
drop breakup
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Why do we care about warm rain?

• Our ability to predict precipitation amount
  depends upon understanding cloud microphysical
  processes
• The radiative effects of clouds depend upon their
  composition and longevity
• The influence of clouds in regulating climate is
  one of the primary unknowns in understanding
  climate change

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Background Particle Scales
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Cloud and Cloud Field Scales
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Relevant Spatial Scales

• 10-8 m size of a cloud condensation nuclei
• 10-5 m size of a cloud droplet ( 10
  micrometers)
• 10-4 m size of a drizzle drop
• 10-3 m size of a raindrop, ice crystal (width
  of fingernail edge)
• 10-2 m size of a snowflake ( 1 cm)
• 10-1 m size of a large hailstone
• 102 m- 103 m size of a small cumulus cloud
• 104 m size of a cumulonimbus
• 105 m size of an orographic cloud
• 106 m size of stratiform cloud decks

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Relevant Temporal Scales

• Terminal velocity of drop (negligible to nearly
  10 m s-1)
• Updraft speed of cloud
• (cm s-1 to 10 m s-1)
• Cloud lifetime
• (15 min to several days)

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Observations
Aircraft, laden with microphysical
instrumentation, penetrate clouds to determine
their microphysical properties on smaller scales
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Instrument Placementon Aircraft
We avoid mounting instruments near propellers.
Why?
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Collecting Small-scale Observations of
Precipitation Development
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Aircraft data across a Florida Cumulus Cloud
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Radar Recording cloud evolution at larger
scales
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Numerical Modeling

• Useful for hypothesis-testing
• Can be used to interpret observations
  observations used to evaluate model
• Same difficulty in representing all scales
  computationally impossible

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Numerical Modeling
Ebert visualization group, Purdue Univ.
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Detailed Microphysical Calculations along
Trajectories
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Source of Clouds

• Air becomes saturated (100 relative humidity)
  by cooling and/or adding water vapor
• When the relative humidity of the air is 100 we
  say the air is supersaturated
• and a cloud forms vapor in excess of 100 RH
  condenses to form cloud droplets

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Source of Clouds

• The supersaturation relative to a droplet(S) is
  increased by two factors
• The size of a droplet (Kelvins Law)
• For a given bulk supersaturation, a droplet
  (having a curved surface) has a lower relative
  supersaturation
• A solution droplet (Raoults Law)
• For a given bulk supersaturation, the larger
  amount of solute dissolved in the droplet, the
  higher the supersaturation relative to the
  droplet

Solution effect
Curvature effect
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Background The Warm Rain Process

Soluble aerosol deliquesce
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Activation of CCN

• Consider a rising air parcel in which the RH just
  increased above 100
• As the parcel continues to rise, the RH (or S)
  continues to increase, and solution droplets
  containing the largest nuclei would grow larger
  than r and activate, growing into cloud droplets
• The supersaturation S continues to increase and
  more and more of the smaller droplets are
  activated
• As the droplets are growing, they are decreasing
  the amount of water vapor in the parcel,
  offsetting the increase in S from the rising
  (cooling) air parcel
• At some point the cloud droplets are taking up so
  much vapor that S starts to decrease in the air
  parcel-- the max S has occurred

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Droplet Growth by Condensation

• Droplet size distribution narrows in time
• Why isnt this the end of the story, i.e., does
  condensation alone produce raindrops?

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Droplet Growth by Collection

• Envision two drops a larger one with radius R,
  and a smaller one with radius r
• They each have their own fall velocity, with the
  larger droplet falling faster than the smaller
  one
• They eventually collide, and sometimes coalesce
  to form one even larger droplet
• As the bigger drop collects smaller drops, it
  becomes larger and falls faster, collecting even
  more drops before eventually falling out of the
  cloud as rain

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Drop Breakup

• Drop breakup speeds the production of rain
• It enhances the ability of the collision-coalescen
  ce process to act through a larger region of the
  cloud by forming multiple collector drops from
  individual collector drops
• Spontaneous breakup droplet becomes too large
  to by hydrodynamically stable in the airflow and
  breaks up on its own
• Collisional breakup drop shatters as it
  collides with other large drops

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RICO Data Scrutiny

• Instrument malfunction
• Instrument limitations (sample volume, known
  regions of poor performance)
• Sampling representativeness
• Much of our time spent here in the field is to
  look at the data being collected, to screen for
  potential problems in the data or our collection
  strategy

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So why are some of us here at RICO?

• classical calculations of droplet growth
  suggest that natural clouds should take longer
  time to produce precipitation than they do
• Classical calculations have a much slower speed
  mainly because of the time required for
  condensational growth of drops capable of
  collecting other drops

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Comparison of Observations and Adiabatic Model
Predictions
OBSERVED N 481 cm-3 , s 17.7, 7.3 mm
MODELED N 467 cm-3 , s 17.9, 0.24 mm
Modeled distributions are too narrow.
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Search for the mechanism by which coalescence
significant for precipitation onset begins in
warm clouds

• Factors that may be important
• Details of aerosol and CCN number concentrations,
  composition, sizes, including giant/ultragiant
  aerosol particles
• Entrainment and mixing
• Turbulence
• Successive thermals
• Preconditioning of cloud environment
• Time zero?

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CCN Concentrations

• Cleaner, more maritime air masses contain fewer
  aerosol particles and CCN than more polluted,
  continental air masses
• Fewer CCN result in fewer, but larger cloud
  droplets, accelerating rain production

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Giant/Ultragiant Aerosol Particles

• Giant aerosol particles with diameters between
  2 and 20 micrometers
• Ultragiant aerosol particles with diameters
  20 micrometers
• Soluble, giant aerosol particles (like sea salt!)
  do not have to grow long by vapor diffusion to be
  large enough to collect smaller droplets
• Ultragiant particles, if 45 micrometers, dont
  even have to be soluble!

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Entrainment and Mixing

• The mixing in of dry air from outside the cloud
  via the clouds own motions is called entrainment
• It is widely acknowledged that entrainment can
  lead to the production of smaller particles in
  the droplet size distribution
• It has been hypothesized that entrainment can
  actually lead to the production of larger drops,
  by significantly reducing the number of droplets
  in regions of the cloud that then experience less
  competition for the vapor

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Turbulence

• Numerous investigators have hypothesized that
  turbulence inside the cloud can lead to higher
  collection efficiencies among droplets than the
  currently accepted values

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Successive Thermals

• Some investigators have suggested that the drops
  from previous thermals within the same cloud may
  not completely evaporate, leaving some drops
  behind that may then be ingested by new thermals,
  giving them a head start

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Preconditioning of Cloud Environment

• Numerical models of precipitation formation often
  start from pristine conditions in an undisturbed
  environment, but it is likely that earlier clouds
  change the local environment for the later clouds
• Is this enough to accelerate precipitation
  formation in later clouds?

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Time Zero

• Numerical models of precipitation formation have
  a time zero, a definite starting time at which
  the cloud first forms
• What is time zero in an observed cloud? We
  always hope that the radar sees time zero to
  put temporal constraints on our calculations, but
  even K- band on SPOL doesnt see things as early
  as aircraft crew