Transcript Precipitation Processes

NATS 101
Lecture 8a
Vertical Stability
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NATS 101
Lecture 8a
Vertical Stability
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Severe thunderstorm near San Pedro River Valley, east of Tucson
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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
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
Ahrens, Fig 5.3
Absolutely Unstable
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 or
above forest fires
Superadiabatic lapse rate
Ahrens, Fig 5.6
Ahrens, Fig 5.8
Conditionally Unstable
Ahrens, Fig 5.9
Conditionally Unstable
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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 average is 6.5o C/km
and thus ELR is frequently
conditionally unstable!
ELR is often absolutely
unstable in a thin layer just
above dry ground on hot,
sunny days
Ahrens, Meteorology Today 5th Ed.
Lapse Rates and Cumulus Types
Shallow
Moderate
Deep
Ahrens, Meteorology Today 5th Ed.
The ELR and depth of unstable layer modulates the type of Cu.
As depth increases, the vertical extent of Cu generally increases.
As temp difference between the air parcel and the environment
increases, the updraft speed and severity of Cb typically increase.
Take Home Points
Vertical Stability Determined by ELR
(Environmental Lapse Rate)
Absolutely Stable and Unstable
Conditionally Unstable
Temp Difference between Air Parcel and
ELR, along with Depth of Layer of
Conditionally Instability Modulates
Vertical Extent and Severity of Cumulus
NATS 101
Lecture 8b
Precipitation Processes
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Rim ice coats the observatory on the top of Mt. Washington, N.H. after a winter
storm. Accretion is important in the formation of precipitation from cold clouds.
http://www.craterranch.com/Mt_Washington/Mt_Washington_Images/IMG_2672.JPG
Cloud Droplets to Raindrops
106 larger
106 larger
Ahrens, Fig. 5.17
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?
Drop Size-Shape and Air Resistance
sphere
Air Resistance Terminal
parachute
Small Radius
< 2.0 mm
Large Radius
2.0-4.0 mm
Gravitational = mg
2 A v2
mg = (1/2)=K(1/2)
ρair AKvρTair
Air Resistance
vT = [2 mKg->/(K
A) ]1/2 object
K = 0.5 sphere;
2.0ρair
irregular
vT = [ 4/3 rdrop g (ρwtr)/(ρair) ]1/2 for sphere
Terminal Fall Speeds
Terminal Fall Speed (cm/s)
(Gravitational Force=Air Resistance)
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
6.5 m/sec
1.0 cm/sec
0.1 m/sec
0.0002
0.02
0.1
0.2
1
Diameter (millimeters)
CCN
Cloud Droplets -> Drizzle
1 km in 1010 sec
1 km in 105 -103 sec
2
5
Small -> Large Raindrops
1 km in 102 sec
Collision-Coalescence
Not all drops
in collection
path adhere
Ahrens Fig 5.18b
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
Collision-Coalescence
Drops can grow by this process
in warm clouds with no ice
Occurs in warm tropical clouds
Only 10-50% collisions cohere
Warm Cloud Precipitation
Ahrens, Fig. 5.19
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. 5.20
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 it
takes a H2O molecule
more energy to sublimate
than evaporation
So at equilibrium, more
vapor resides over cloud
droplets than ice crystals
Ahrens, Meteorology Today 5th Ed.
SVP near Droplets and Ice
More Vapor
Less Vapor
Ahrens Fig. 5.21
SVP is higher over supercooled water drops than ice
Ice Crystal
Process
SVP for a water drop is
too high for ice crystal,
so vapor next to drop
will diffuse towards ice
Ice crystals grow at the
expense of water drops,
which freeze on contact
Deposition
As ice crystals grow,
o
Effect is maximized near -15 C
they begin to fall
where SVP water-ice is largest
Ahrens, Fig. 5.22
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. 5.23
Accretion
Splintering
Aggregation
(Riming)
a.k.a. Bergeron Process after the meteorologist
who first recognized importance of the process
Take Home Points
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 => Not efficient
Cold clouds (with ice)
Ice Crystal Process => Very efficient
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
Snow - Temp colder than 0oC (almost) everywhere
Ahrens
Met. Today 9th Ed.
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
Depends 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
Next Class Assignment
Air Pressure, Surface Maps
• Reading - Ahrens
3rd: 139-146
4th: 141-148
5th: 141-148
• Homework05 - D2L (Due Wednesday Mar. 3)
3rd-Pg 162: 6.1, 7, 8
4th-Pg 164: 6.1, 7, 8
5th-Pg 165: 6.1, 8, 9