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Defining an icing environment
- role of drop size 
As discussed in
detail in the Ice Is NOT Nice - Part 1
premium workshop, an icing environment is defined by three
basic
factors that include (1) temperature, (2) liquid water content (LWC)
and (3)
drop size. The September 21, 2009 e-Tip discussed
the role temperature plays in defining an icing environment.
In this e-Tip, we'll briefly explore how drop size defines
the icing environment.
While temperature is perhaps the biggest factor to determine
if icing
will accrete on the airframe, drop size will often dictate
the intensity or severity of the icing that does accrete.
Supercooled liquid water comes in all different sizes.
Sizes vary from very small cloud drops of 5 or 10 microns in
diameter (1,000 microns = 1 millimeter) to freezing rain that can be
3,000 microns in diameter or greater. Just to put this into
perspective, certification for flight into known icing conditions does
not protect the aircraft for an environment with median volumetric
diameter (MVD) drops sizes larger than  50
microns otherwise known as Supercooled Large Drops or SLD.
Just for reference, a human hair is about 100 microns in
diameter. In other words, by the time you can physically see
a drop, it is significantly larger than 50 microns. The picture to the
left is the NASA Twin Otter after encountering freezing rain over West
Virginia.
The larger the supercooled liquid drop, the
greater the collection efficiency. In other words, larger
drops are more likely to impinge on the airframe than smaller ones.
As the wing of the aircraft moves through an environment with
very small drops, the drops may simply ride over the flow of the wing's
boundary layer without impacting the airframe. Icing may only
be light or trace rime in this situation. Larger drops, on
the other hand, have a greater mass and tend to penetrate the boundary
layer. If the supercooled drops are large enough, they may
not only impact and freeze to the leading edges, but they may penetrate
the boundary layer impacting the wing behind the protected surfaces
creating a severe icing scenario for aircraft with a certified ice
protection system (IPS).
It is often
difficult, if not fundamentally impossible to predict (forecast) the
size of drops in a cloud. Moreover, drop size can be much
different at the
base of a cloud
versus the cloud top. Instead, we must rely on what
environment may produce larger drops and what environment will likely
produce smaller drops.
Stratiform clouds generally contain drops
on the order of 20 microns or less. These cloud decks are
often only a few thousand feet thick and have very smooth tops as shown
on the right. Icing intensity in these clouds can be trace or
light near the bases with moderate potential near the tops especially
in a more substantial (thicker) cloud deck. These thicker
stratus cloud decks can often produce drizzle-sized drops with drop
sizes exceeding 100 microns in diameter which is also considered SLD.
Stratocumulus clouds (shown below) are typically produced in
the wake of a strong cold front as a result of unstable conditions in
the boundary layer (near the surface). This rising air is
highly capped by a temperature  inversion
aloft which gives
the clouds a quilted-like look to the tops as the rising air tries to
penetrate (unsuccessfully) through the inversion. These clouds rarely
produce precipitation and like stratus clouds are also only a few
thousand feet thick in most cases. However, don't become complacent; they
can produce copious amounts of supercooled liquid water especially
right near the cloud tops given the right temperature profile within
them.
Climbing through such a cloud deck when
the static air temperature is below freezing often results in little or
no icing at the bases with a rapid transition to larger drops
approaching SLD sizes near the tops. When the cloud top
temperature is only slightly supercooled, expect the potential for
moderate or greater clear ice near the tops.
Of all of the cloud types,
vertically-developed cumuliform clouds will produce the largest cloud
drops including SLD. Cumulus clouds (as shown to the right)
are those white puffy cauliflower-looking clouds typically seen on fair
weather days, or in the unstable air mass ahead of a cold front.
Cumulus clouds can build into towering cumulus or
cumulonimbus clouds producing an even greater threat of SLD icing.
A cumulus cloud is very dense and can
contain very large drops including SLD. Moderate or greater
icing typically occurs in cumulus clouds due to the large drop sizes
within them. The updrafts in these clouds can take large
supercooled liquid water drops to much higher altitudes producing heavy
icing at temperatures down to -30°C or colder.
Cumulus clouds are often scattered or
broken so icing can be spotty at times. However, extended
flight in a cumulus deck with temperatures below freezing can quickly
overwhelm even the best ice protection system producing significant
runback. Larger drops take longer to freeze and this allows
the drops to strike the leading edge, run back and freeze on the
unprotected surfaces.
Freezing rain and freezing drizzle are
the producers of some of the largest supercooled liquid drops.
Freezing rain is often produced as snow falls into a layer
that is above freezing (called a warm nose) and completely melts into
rain. The rain then falls into a subfreezing layer usually
very close to the surface to produce freezing rain. Freezing
rain contains drops greater than 500 microns in diameter.
Unlike freezing rain, freezing drizzle
is typically produced by an entirely different all-liquid
collision-coalescence process. In other words, the
precipitation doesn't typically start out as snow. Instead,
supercooled liquid drops are sometimes produced as smaller cloud drops
collide or coalesce to produce larger drizzle-sized drops (100 to 500
microns in diameter). These drops become heavy enough to
eventually fall out of the base of the cloud as freezing drizzle
assuming the static air temperature is below freezing. As a
result, the temperature profile is often entirely below freezing from
the surface through the top of the cloud layer. Consequently,
freezing drizzle often extends to a much greater depth than freezing
rain.
In a future e-Tip we will examine how liquid water
content
also plays a role in defining an icing environment.
Want
to learn more about minimizing your exposure to structural icing?
The Ice Is NOT Nice - Part 1
and Part
2
premium workshops are a one-of-a-kind training program specifically
tailored to help pilots learn to minimize their exposure to structural
icing. To build these workshops, AvWxWorkshops.com
collaborated
with leading icing expert, Ben Bernstein. Ben has over 19
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of experience forecasting structural icing and was a lead developer of
the operational gridded icing tools, CIP and FIP found on the Aviation Digital Data Service
(ADDS). If you are still not convinced, click here to see a 19 minute
FREE
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PLEASE
NOTE: We strongly discourage purchasing Ice Is NOT Nice -
Part 2
separately. Part 2 consists of three flight planning
scenarios
that assume you fully understand the training delivered in Part 1.
Moreover, links to the various weather products used in Part
2
are provided only in Part 1 of the program.
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