Transcript Slide 1

Lecture 15-16: Precipitation Processes
(Ch 7)
• scores just in, average score 10.9 (top score
19, eleven scores 15 or over)
• < 50% does not mean “fail” – quiz was too
long
NSF(Nat.Sci.Fnd.)
Cloud Research
NCAR Hercules, Electra
• scores will be posted after class
• cloud droplet size spectrum
Covered 13 Oct
• collision-coalescence process
• cold clouds over Edmonton today
• Bergeron process
• forms and distribution of precip
Water vapour + lifting parcel → CLOUD
Liquid water at the base of a cloud initially forms onto condensation nuclei (“CCN”),
of which there are a very large number. Within a few tens of meters of the LCL all the
available condensation nuclei have attracted moisture… they quickly attain diameters
of about a micrometer (p178; repeated p191)
These are in competition for water vapour… “cloud water is spread across numerous
small droplets rather than being concentrated in fewer large drops” (p190)
CLOUD
→
PRECIPITATION?
• How do some cloud droplets grow large enough to fall as precip?
Cloud droplet “size spectrum”
“Condensation can lead to rapid
droplet growth, but only up to a
radius of order 20 micrometer”
Fig. 7-2
An aside on cloud physics research: measuring cloud droplet size spectrum
Shadow images of cloud particles in a size range between 25 and 800 µm are
focused on a diode array with 32 elements which is read out with a maximum
repetition frequency of up to 5 MHz… calibration accomplished by a rotating glass
disk with opaque spots on it simulating particles. A similar device handles
precipitation particles (diam 0.2 - 6.4 mm)
An aside on meteorol. research
An “autonomous Unmanned Aerial
Vehicle” (UAV)
Technische Universitat Braunschweig
Growth in Warm Clouds: collision-coalescence process
• larger drops with greater terminal velocity assimilate smaller
• collision efficiency low for droplets of size nearly equalling that of the “collector”
droplet, and for droplets very much smaller than collector
Fig. 7-4
12Z Mon 16 Oct, 2006
• low cloud
• sub-zero
Stony Plain
Sounding
• slack flow over Ab
• T-Td < 2oC
Cumulonimbus clouds = ice (top, fuzzy cloud margins),
liquid (bottom, sharp margins) and mix of ice and liquid (middle)
Fig. 7-6
Growth in Cool and Cold Clouds: Bergeron process
- depends on co-existence of mixture of ice and liquid water
Fig. 7-5
• Equilib. vapour pressure over ice is less than
over supercooled water at same temp:
(H2O molecules bound in a crystal lattice
require more energy to “escape”)
T[oC]
es(T), mb
(Ice)
es(T), mb
(Water)
-1
562
568
-2
517
528
Fig. 7-7a
(See tutorial
on CD)
Fig. 7-7
Riming (accretion)
Ice crystal falls through
and
Aggregation
Ice crystal hits another
supercooled droplets
which freeze on (“graupel”
is heavily rimed ice crystal
- nucleus of hailstone
Aggregation is easier if there is a film
of water on the ice: bigger flakes
from warmer clouds
“What happens to these crystals as they fall determines
the type of precipitation that occurs”
Distribution & Forms of Precip
Snow:
• ice crystals in clouds can
have wide variety of
shapes (dendrites, plates,
columns)
• infinite variety of
snowflack forms (even
multiple forms within one
flake) because each regime
of T,Td will favour a
different structure
Lakeeffect
Fig. 7-10a
Rain:
• first drops tend to be large – because due to their larger size they fall fastest.
Their partial evaporation humidifies the air column so that the smaller drops,
initially greatly diminished by evaporation, survive to the ground
• due to droplet breakup, raindrop diameter seldom exceeds 5 mm
Graupel & Hail:
• graupel – heavily rimed ice crystals, diameter up to about 5 mm, may fall to
ground or if remain suspended, form nuclei of hailstones
Successive passages above/below freezing
level result in layered hailstone; not much
air due to the melting stages
LAKE-EFFECT SNOWS
• ice-free lake
• maximal Twater - Tair
• long overwater fetch
Heat &
Vapour
Slope-lift and
frictionalconvergence