Transcript Chapter VIII - U Wyoming Atmospheric Science

Chapter 4 (cont.)
Precipitation
How does precipitation form?
Why do some clouds generate precipitation and others do not?
What factors determine the various types of precipitation?
To answer these questions, let’s start by examining the
sizes of typical cloud and raindrops.
1. Growth by condensation
Fog and cloud drops form by the aggregation of water vapor
molecules by condensation on hygroscopic cloud
condensation nuclei (CCN), a sub-set of the aerosol.
Initially, small droplets grow rapidly in an air parcel that is
cooling, but as they become larger, their rate of growth
decreases rapidly, so that this process (condensational
growth) becomes too slow to produce raindrop size water
drops. (It would take days!)
The excess water vapor grows mainly on other activated CCN
2. Growth by collision and coalescence.
In this process, large
drops fall through
smaller drops, and
collect many of those
in their path, thereby,
growing even larger.
This mechanism works
because large drops fall
more rapidly than small
drops.
What determines the fall speed of drops?
There are two forces that act on a falling object, namely
(1) the force of gravity which tends to accelerate the
object towards the Earth’s surface, and
(2) the frictional drag caused by the air resistance.
When these two forces are exactly balanced, the
object falls at a steady, constant speed, the terminal
velocity.
Terminal velocities of larger drops are greater than
those of small drops.
Collision and Coalescence Mechanism of Raindrop Growth
Updrafts in clouds
tend to hold falling
drops aloft. Drops
with terminal velocities
less than the updraft
velocity are swept up
higher into the cloud.
Drops only fall from the cloud when their terminal velocity
exceeds the updraft velocity.
Collision and Coalescence Mechanism of Raindrop Growth
Growth by collision and coalescence is enhanced by:
A wide spectrum of drop sizes, which therefore implies
a range of drop terminal velocities.
Collision and Coalescence Mechanism of Raindrop Growth
Growth by collision and coalescence is also enhanced by:
A high concentration of drops.
Strong cloud updrafts which hold the drops aloft in
the cloud and give them more time to grow.
The collision and coalescence process explains the
production of precipitation in “warm” clouds.
Only relatively shallow clouds that do
not extend high into the troposphere
contain nothing but water drops.
e.g. stratus cloud
Deep clouds extend up into regions of the troposphere
where temperatures are well below freezing.
These ‘cold’ glaciated clouds consequently contain ice,
as well as supercooled water drops.
Ice Formation in Cold Clouds
Small, pure water drops do NOT freeze at 0°C.
As the temperature cools below 0°C, larger water
drops tend to freeze first, before the smaller drops.
As smaller water drops require colder temperatures to
freeze, the drop size of supercooled water drops in a
cold cloud tends to decrease with height.
Spontaneous nucleation of ice (homogeneous freezing)
only occurs at very low temperatures. Most ice
formation in clouds results from the action of Ice Nuclei.
The saturation vapor
pressure increases rapidly
with increasing temperature
How does the saturation
vapor pressure vary with
temperature below 0C?
Ice Particles and the Formation of Precipitation
svp (ice) < svp (liquid)
vapor
3. The Bergeron Process
90% RH
When supercooled liquid water drops and ice
particles co-exist in the same air parcel, the
liquid drops near ice “feel” the drier shell
around ice, and they start to evaporate.
100% RH
This water vapor deposits on the ice particles
as fast as it evaporates from the water drops.
The net result is that the ice crystals grow at
the expense of the water drops.
This ice crystal growth process, which
promotes the rapid growth of ice crystals, is
known as the Bergeron process. This process
is the dominant precipitation forming
mechanism in most places.
90% RH
100% RH
90% RH
100% RH
Ice Particles and the Formation of Precipitation
Ice particles sometimes collide and stick
together, forming a larger particle. This
process is called aggregation and leads to
the formation of ice crystal aggregates,
commonly known as snowflakes.
Some collisions result in splintering of the
ice crystals, thereby forming ice crystal
fragments, which in turn act as embryonic
ice crystals.
Supercooled cloud droplets sometimes collide
with ice crystals and snowflakes. They
freeze, and stick to their surface. The
resulting ice crystal is said to be rimed.
Rime Ice
Rime forms when supercooled water fog and cloud drops
impact a cold object. It not only leads to the growth of
graupel and hail but can form on objects at the surface.
Types of Precipitation: Rain
Falling raindrops are usually wrongly depicted by artists
as being shaped like teardrops.
Cloud and small drizzle drops are roughly spherical.
As falling drops become larger, they flatten on their
underside and look
more like a hamburger bun. This
results from the
higher air pressure
under the falling
drop.
Types of Precipitation: Rain
Virga are falling streaks of drizzle and rain that evaporate
before reaching the ground.
Donna Charlevoix U. of Illinois
Types of Precipitation: Rain
Drizzle is composed of drops of diameter less than 0.5 mm.
Raindrops have diameters between 0.5 mm and about 5 mm.
Showers fall from convective (cumiliform) clouds.
Continuous rain invariably falls from stratiform clouds (e.g.,
nimbostratus).
Types of Precipitation: Snow and Ice
Single ice crystals form in an infinite number
of shapes. All however exhibit hexagonal
symmetry.
Most of them fall into three main categories,
namely
columns (hollow and solid)
plates (thick and thin)
dendrites
The type of crystal that forms reflects its growth
environment (primarily temperature).
Plates grow at 0 to -4C, -10 to -12C, and -16 to -22C
Dendrites grow at -12 to -16C
Columns at -4 to -10C and -22 to -50C
Sector Plates
Rasmussen & Libbrecht, 2003
Plates and Sector Plates
Dendrites
Dendrites
Rasmussen & Libbrecht, 2003
More dendrites
More dendrites
Rasmussen & Libbrecht, 2003
Columns, Needles, and
Rosettes
Rosettes
Rasmussen & Libbrecht, 2003
Types of Precipitation: Snow and Ice
Snow Grains are the ice equivalent of
drizzle. They are opaque, white ice
particles of diameter <1 mm. They are
usually associated with stratus clouds.
Snow pellets (graupel) are opaque, white ice
particles formed by riming. Their diameter
is 2 – 5mm. They fall from cumiliform clouds.
Hail and hailstones are transparent or
partially opaque ice particles of diameter
5 – 140 mm. They form by riming of both
graupel and frozen drops. To grow larger
than 5 mm, a hailstone must make several
up-and-down cycles in the cloud.
5 km
3 km
1 km
The Giant Coffeyville
Hailstone!
Diameter: 14 cm (5.5”);
Weight: 0.7 kg (24 oz)
Cumulonimbus are the only clouds with sufficiently
strong updrafts to produce hail.
Hail Damage
Types of Precipitation: Snow and Ice
When below-cloud temperatures are below freezing,
falling snow reaches the ground.
Types of Precipitation: Snow and Ice
When surface temperatures are significantly above freezing, falling
snow melts about 1000 ft (300 m) below the freezing level, reaching
the surface as rain. This reflects the time required to acquire
sufficient latent heat to effect melting.
Types of Precipitation: Freezing Rain
Freezing rain occurs when rain or drizzle freezes on contact with a
frozen surface.
Freezing rain mechanism
Types of Precipitation: Freezing Rain
Freezing rain is hazardous because it coats surfaces with a very
slippery layer of clear ice. The ice can bring down power lines and
cause both personal and other accidents.
Types of Precipitation: Freezing Rain
Ice storms can break down trees, cause livestock to slip and fall, and can even cause suffocation due to
ice clogging their nostrils. Birds have been found frozen to trees, with their beaks frozen shut, and
unable to fly due to the ice build-up on their wings.
Types of Precipitation: Freezing Rain
Freezing rain occurs in the Front Range
area when cold-air is dammed up against
the mountains
Types of Precipitation: Sleet
Sleet consists of small ice pellets formed by the re-freezing of
small drops and/or partially melted snowflakes. Re-freezing occurs
when the drops fall into a deep cold layer at the surface.