Transcript lecture13erk - The University of Arizona Department of

NATS 101
Lecture 13
Precipitation Processes
Supplemental References for
Today’s Lecture
Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp.
McGraw-Hill. (ISBN 0-697-21711-6)
Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere.
535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6)
Review: Vertical Stability
Rising and sinking unsaturated (clear) air
o
Temp changes at DAR of 10 C/km
Dew Point (DP) changes at rate of 2oC/km
Rising and sinking saturated (cloudy) air
Latent Heating Mitigates Adia. Cooling
Temp and DP cool at MAR of 6oC/km
Water Vapor Condenses into Liquid
Review: Vertical Stability
Vertical Stability Determined by ELR
Conditionally Unstable
(MAR < ELR < DAR)
Temp Difference between Environmental
Air and Air Parcel, and the Depth of
Conditionally Instability Controls
Vertical Extent and Severity of Cumulus
Conditionally Unstable: Lower Rock
Ahrens, Fig 5.7
Environmental Lapse Rate
(ELR)
6.5o C/km
6.0o C/km
10.0o C/km
ELR is the Temp
change with height
that is recorded by
a weather balloon
ELR is 6.5o C/km, on
average, and thus is
conditionally unstable!
ELR is absolutely
unstable in a thin layer
just above the ground
on hot, sunny days
Ahrens, Meteorology Today 5th Ed.
Cloud Droplets to Raindrops
106 bigger
106 bigger
Ahrens, Fig. 5.15
A raindrop is 106 bigger
than a cloud droplet
Several days are needed
for condensation alone
to grow raindrops
Yet, raindrops can form
from cloud droplets in
a less than one hour
What processes account
for such rapid growth?
Terminal Fall Speeds
Terminal Fall Speed (cm/s)
(upward suspension velocity)
1.E+03
1.E+02
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
0.0002
0.02
0.1
0.2
1
Diameter (millimeters)
CCN
Cloud Droplets-Drizzle
2
5
Small-Large Raindrops
Collision-Coalescence
Area swept is
smaller than
area of drop
small
raindrop
Collection Efficiency 10-50%
Big water drops fall faster than
small drops
As big drops fall, they collide
with smaller drops
Some of the smaller drops stick
to the big drops
Collision-Coalescence
Drops can grow by this process
in warm clouds with no ice
Occurs in warm tropical clouds
Warm Cloud Precipitation
Updraft
(5 m/s)
Ahrens, Fig. 5.16
As cloud droplet ascends,
it grows larger by
collision-coalescence
Cloud droplet reaches the
height where the
updraft speed equals
terminal fall speed
As drop falls, it grows by
collision-coalescence to
size of a large raindrop
Mixed Water-Ice Clouds
glaciated region
Ahrens, Fig. 5.17
Clouds that rise above
freezing level contain
mixture of water-ice
Mixed region exists
where Temps > -40oC
Only ice crystals exist
where Temps < -40oC
Mid-latitude clouds are
generally mixed
SVP over Liquid and Ice
SVP over ice is less than
over water because
sublimation takes more
energy than evaporation
If water surface is not flat,
but instead curves like a
cloud drop, then the SVP
difference is even larger
So at equilibrium, more
vapor resides over cloud
droplets than ice crystals
Ahrens, Meteorology Today 5th Ed.
SVP near Droplets and Ice
Ahrens, Fig. 5.18
SVP is higher over supercooled water drops than ice
Ice Crystal
Process
Effect maximized around -15oC
Ahrens, Fig. 5.19
Since SVP for a water
droplet is higher than
for ice crystal, vapor
next to droplet will
diffuse towards ice
Ice crystals grow at the
expense of water drops,
which freeze on contact
As the ice crystals grow,
they begin to fall
Accretion-Aggregation Process
Small ice
particles will
adhere to ice
crystals
Supercooled
water droplets
will freeze on
contact with ice
snowflake
ice crystal
Ahrens, Fig. 5.17
Accretion
Splintering
Aggregation
(Riming)
Also known as the Bergeron Process after the
meteorologist who first recognized the
importance of ice in the precipitation process
Summary: Key Concepts
Condensation acts too slow to produce rain
Several days required for condensation
Clouds produce rain in less than 1 hour
Warm clouds (no ice)
Collision-Coalescence Process
Cold clouds (with ice)
Ice Crystal Process
Accretion-Splintering-Aggregation
Examples of Precipitation Types
Type
Drizzle
Size
< 0.5 mm
Rain
0.5 - 5 mm
Freezing Rain
0.5 - 5 mm
Sleet
0.5 - 5 mm
Snow
1 - 2 mm
Hail
5 to 10 cm
or larger
Description
Small uniform drops that fall
from stratus clouds
Size of drops generally vary
from one place to another
Rain that freezes on contact
with object
Ice particles from raindrops
that freeze during descent
Aggregated ice crystals that
remain frozen during descent
Hard pellets of ice from
cumulonimbus clouds
Temp Profiles for Precipitation
Ahrens, Meteorology Today 5th Ed.
Snow - Temp colder than 0oC everywhere (generally speaking!)
Sleet - Melting aloft, deep freezing layer near ground
Freezing Rain - Melting aloft, shallow freezing layer at ground
Rain - Deep layer of warmer than 0oC near ground
Summary: Key Concepts
Precipitation can take many forms
Drizzle-Rain-Glazing-Sleet-Snow-Hail
Depending on specific weather conditions
Radar used to sense precipitation remotely
Location-Rate-Type (liquid v. frozen)
Cloud drops with short wavelength pulse
Wind component toward and from radar
Assignment for Next Lecture
• Topic – Atmospheric Pressure
• Reading - Ahrens pg 141-148
• Problems - 6.1, 6.7, 6.8