Transcript Class Slides - Department of Atmospheric Sciences

ATMO 336
Weather, Climate and Society
Vertical Stability
Precipitation Processes
Concept of Stability
Stable Rock
always returns
to starting point
Unstable Rock
never returns
to starting point
Conditionally Unstable
Rock never returns if rolled
past top of initial hill
Ahrens, Fig 5.1
Archimedes’ Principle
• Archimedes' principle is the law of buoyancy.
It states that "any body partially or completely
submerged in a fluid is buoyed up by a force
equal to the weight of the fluid displaced by the
body."
• The weight of an object acts downward, and the
buoyant force provided by the displaced fluid
acts upward. If the density of an object is
greater/less than the density of water, the object
will sink/float.
• Demo: Diet vs. Regular Soda.
http://www.onr.navy.mil/focus/blowballast/sub/work2.htm
Absolutely Stable: Top Rock
Stable air strongly resists
upward motion
External force must be
applied to an air parcel
before it can rise
Clouds that form in
stable air spread out
horizontally in layers,
with flat bases-tops
Ahrens, Fig 5.3
Absolutely Unstable: Middle Rock
Unstable air does not
resist upward motion
Clouds in unstable air
stretch out vertically
Absolute instability is
limited to very thin
layer next to ground
on hot, sunny days
Superadiabatic lapse rate
Ahrens, Fig 5.5
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.
Summary: Key Concepts II
Vertical Stability Determined by ELR
Absolutely Stable and Unstable
Conditionally Unstable
Temp Difference between ELR and Air
Parcel, and Depth of Layer of
Conditionally Instability Modulates
Vertical Extent and Severity of Cumulus
ATMO 336
Weather, Climate and Society
Precipitation Processes
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
1 km in 1010 sec
1 km in 105 sec
2
5
Small-Large Raindrops
1 km in 102 sec
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
Weather Conditions Associated
with Precipitation Types
Gedzelman, The Science and Wonders of the Atmosphere
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