Transcript COMAP 97 - Brockport

Cloud Microphysics
Original Materials by Liz Page
NWS/COMET
(minor modifications/additions by SMR)
Introduction
• Meteorology and hydrology are linked by
the processes that produce precipitation
• A greater understanding of cloud
microphysics will help determine which
clouds will be most efficient in producing
precipitation
Vapor Pressure
• Dalton’s Law of Partial Pressure
• Saturation vapor pressure (es)
• Saturation is a dynamic process
Dalton’s Law of Partial Pressure
• Total pressure = partial pressure of dry air +
partial pressure of water vapor
• e  vapor pressure (actual)
• es  saturation vapor pressure [f (T) only]
• S  saturation ratio = e/es
• RH  relative humidity = S*100%
Condensation and Cloud
Formation
• Cloud Condensation Nuclei
–
–
–
–
dust
salt particles from sea spray
natural aerosols
human created pollution
• Hygroscopic nuclei
– ‘attract’ water
– allow saturation at RH < 100%
– different nuclei have varying ‘degrees’ of condensation
efficiency
Process of Cloud Formation
• Air rises and cools to saturation
• Most effective (hygroscopic)
nuclei are activated
• Saturation vapor pressure
decreases as parcel continues to
rise and cool
• The parcel becomes
supersaturated
• More CCN activate at the
higher humidity
• Recall that not all CCN are
created equally!
Cloud Droplet Growth by
Condensation (Diffusion)
• Driven by the difference in saturation vapor
pressures
– between droplet and environment
– between droplets
• Vapor is transported from higher to lower
saturation vapor pressure
• Recall that es is a function of temperature
only
Collision and Coalescence
• Two-step process
– Will the droplets
collide?
– If so, will they
coalesce?
Collision and Coalescence
• Collisions begin at radius of 18 microns
• Collision efficiency increases as the size of
the colliding drop increases
– why?
– larger drops mean more collisions
– faster terminal velocities
Collision and Coalescence
• Not all collisions result in coalescence
• Coalescence is affected by:
– turbulence
– surface contaminants
– electric fields and charges
• Broad droplet spectra (varying sizes) favor
more collisions
Marine vs. Continental
Environments 1
• Droplet concentrations
– marine ~ 100 cm-3
– continental ~ 300 cm-3
• Does this make sense?
– It should (more ‘crud’ [CCN] over land), however:
• Where would clouds/precip more likely form?
– Marine!
– Why?
Marine vs. Continental
Environments 2
• Droplet concentrations are not the whole story
• Size DOES matter!
• CCN:
– more numerous over land than over water, but…
– larger size range over water (many tiny CCN over land)
– more CCN competing for available moisture  unable
to grow via condensation (haze instead)
– marine environment (w/fewer CCN of larger size
range) better able to create precipitation
Marine vs. Continental
Environments 3
• Not only larger CCN in marine environment, but
larger droplets as well
– Larger (size range of) droplets means greater collision
efficiency (see previous slide)
– Smaller (continental) droplets more prone to
evaporation  cumulus clouds with ‘sharper’ edges
• Oceanic cumulus cloud can produce precipitation
more efficiently than a continental cumulus cloud:
– shallower cloud
– weaker updrafts
– almost counterintuitive, no?
Droplet Breakup and
Multiplication
• Falling drops sweep out a cone-shaped
volume
• Drops are unstable just after coalescence
• Droplet breakup broadens the spectra and
limits the maximum size
– most raindrops are 5 mm in diameter
– larger droplets prone to breakup (unstable)
Precipitation Formation through
Ice Processes 1
• Bergeron process
• Dependent upon different saturation vapor
pressures
– es (ice) < es (water)
– supercooled water and ice can (and do) coexist
in same cloud
– supersaturated wrt ice, but saturated wrt water
– ice crystals will grow at expense of droplets
Precipitation Formation through
Ice Processes 2
• Ice forms on Ice Nuclei (IN)
–
–
–
–
silicates
clays
combustion products
industrial products
• Similar in principle to CCN
– not as numerous as CCN
– must be similar in nature as ice crystal
Nucleation of Ice
•
•
•
•
IN activate as a function of
temperature (~ -10°C)
Heterogeneous (contact)
nucleation
– IN necessary
– more common
Homogeneous (spontaneous)
nucleation
– no IN needed
– occurs ~ -40°C
– less common
Warm-top clouds (> -10oC)
rarely have ice
Ice Crystal Growth
• Ice crystals grow by:
– vapor deposition
• growth at expense of water vapor (direct deposit?)
• dominant crystal growth mechanism
• ‘cold’ process
– accretion of cloud droplets
•
•
•
•
freezing of supercooled water onto surface of IN/crystal
growth at expense of liquid droplets
‘cold’ process
graupel forms via accretion
– aggregation
• snowflakes stick upon collision (‘wet’ snow)
• ‘warm’ process
Growth by Deposition
Ice Particle Multiplication
• Three processes
– Fracture (collisions of fragile crystals)
– Splintering during riming
• rapid freezing of supercooled water onto crystal
• ejects splinters upon freezing
• possibly most important/efficient process of the three
– Fragmentation of large drops during freezing
• ‘isolated’ drop freezes from outside in (forms shell)
• water expands on freezing
• shell cracks, forming splinters
Parting Thoughts 1
• Not all clouds are ‘cold’ or ‘warm’
– contain water in all three phases
– relative ‘lack’ of IN allows coexistence of ice
crystals and supercooled water in the same
cloud
– top of cloud dominated by cold-cloud
(Bergeron) processes
– bottom of cloud governed by warm-cloud
(collision/coalescence) processes
Parting Thoughts 2
• Neither process solely responsible for
precipitation development (BOTH
contribute)
– Bergeron process dominant in mid-latitudes and
polar regions
– most mid-latitude precipitation starts frozen
– collision/coalescence dominant in tropics
– C/C also important in increasing raindrop size