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Identifying regions of Aviation
Icing using satellite imagery
Bodo Zeschke (BMTC)
Image from COMET
Image from BOM
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I am Bodo Zeschke.
I worked for 8 years as a Forecaster at the Darwin Regional Forecasting
Centre. So icing forecasts have mainly been confined to icing occurring
within Northwest Cloud Bands and the monsoon.
Since 2009 have been facilitating at the Bureau of Meteorology Training
Centre in Docklands. Here I have been looking at low level icing during
our chart discussion sessions.
I enjoy collaborating with forecasters, and would like to thank NMOCforecasters , particularly Ronik Kumar, for helpful suggestions for this
presentation.
SCRIPT SLIDE
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Learning outcomes
Gain an understanding of different kinds of icing relevant to
aviation and the effects of this on aircraft performance.
Gain a better understanding of the clouds and synoptic settings
that are favourable for aircraft icing.
Gain familiarity with the procedure used by the NationalMeteorological and Oceanographic Centre (NMOC) and the
Regional Forecasting Offices of the Australian Bureau of
Meteorology for determining Aviation icing from satellite
imagery, soundings and model data.
Through participation in exercises gain a basic understanding of
aviation icing within a northwest cloud band and also an
enhanced convection situation over Indonesia.
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The icing environment
0 to -15 C
BEWARE - Freezing Rain !
Image courtesy BOM
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This diagram shows the gradation in water condensate phase with altitude.
Water freezes when its temperature reaches 0C or lower. All areas with
positive temperatures are not conducive to icing when an aircraft traverses
these regions.
On the other hand, supercooled water cannot exist below -38 C.
Clear ice forms from larger water droplets mostly at temperatures between
0 and -10 C, but can exist at temperatures as low as -25 C in Cb. At these
temperatures the supercooled water will freeze more slowly when it
contacts an aircraft, and extends further along the airfoil as it freezes. Theclear appearance of this ice means that it can be misinterpreted as a wet
surface by the pilot. This icing is very dangerous.
SCRIPT SLIDE
The icing environment
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Rime ice forms from smaller and colder water droplets, typically in the
range -15 C to -38 C, with substantially lower risk at temperatures colder
than -20 C. The droplets freeze quickly, trapping air bubbles and this
gives them a white appearance. They are generally confined to the
leading edges of the aircraft.
Mixed ice, a combination of the two, can also occur, and is most likely inthe temperature range of -10C to -20C.
According to WMO documentation (CAeM) most occurrences of icing are
at temperatures between -3 and -7C.
Beware however. The most severe form of icing, with ice covering the
aircraft in a matter of seconds, occurs when rain falls into a sub-cloud
base temperature inversion, or above cloud level where the warm (>0C)
rain/drizzle falls into a subzero environment.
SCRIPT SLIDE
The icing environment
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Rime Ice vs Clear Ice, various icing intensity
Image sourced from Meteo France and WMO 2005
Image sourced from NASA Lewis Research Centre, Meteo France + WMO
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Rime Ice vs Clear Ice, various icing intensity
The top left hand picture shows the white encrustations of Rime Ice on
the nose of the propeller cone. The clear ice deposits can be seen lessclearly spreading out behind this.
The top right hand picture shows an example of light icing on the
leading edge of an aircrafts wing. Accretion greater than 1 g/cm2/hour
but less than 6 g/cm2/hour. This corresponds to a liquid water contentless than 0.6 g/m3. Here the rate of accumulation may create a
problem if flight is prolonged in the environment (i.e. more than one
hour). Occasional use of deicing/anti-icing equipment removes or
prevents accumulation. It does not present a problem if the deicing/anti-
icing equipment is used.
The bottom right hand picture shows an example of moderate icing on
the leading edge of an aircrafts wing. Accretion greater than 6
g/cm2/hour but less than 12 g/cm2/hour. This corresponds to a liquid
water content between 0.6 and 1.2 g/m3.
SCRIPT SLIDE
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Rime Ice vs Clear Ice, various icing intensity
Moderate icing means the rate of accumulation is such that even short
encounters become potentially hazardous and use of deicing/anti-icing
equipment or diversion is necessary.
The bottom left hand picture shows an example of severe icing on the
leading edge of an aircrafts wing. Accretion greater than 12 g/cm2/hour.
This corresponds to a liquid water content greater than 1.2 g/m3.Severe icing means the rate of accumulation is such that deicing/anti-
icing equipment fails to reduce or control the hazard. Immediate
diversion is necessary.
Icing is only included on area forecasts if it is considered moderate orgreater.
A SIGMET must be issued for severe icing conditions.
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Icing effects on an aircraft
Diagram from BOM Aviation Forecasters Handbook (AFH)
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Accumulation of ice can lower aircraft performance in many ways. It can:
Increase the stalling speed of the aircraft by changing the
aerodynamics of the wing and tail as well as increasing the weight.
Make it almost impossible to operate control surfaces and landing
gear.
Destroy the smooth flow of air over the aircraft.
Increase drag and decrease lift. Cause engine failure.
Cause propeller vibrations.
Damage compressor blades of jet engines (chunks of ice can inject
into the engine). This can occur at temperatures above 0 Celsius.
Produce errors in instrument readings of air speed, altitude andvertical speed.
Interfere with communication systems
Reduce visibility.
Icing effects on an aircraft(from the Aeronautical Forecasters Handbook)
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Icing severity(from the Aeronautical Forecasters Handbook)
Icing severity depends on:
droplet sizelarger supercooled droplets lead to faster accumulation
rates, increasing the severity of icing potential (marine stratucumulus
very large droplets)
liquid water content (LWC)higher liquid water content leads to
greater icing potential (growing Cumulonimbus cloud has greatestLWC)
air temperaturethe closer to zero on the freezing side, the higher
the risk of larger drops and higher liquid water content leading to more
severe cases of icing.
particulars of the individual aircraft, including the effectiveness of
the de-icing equipment.
Aircraft icing is a serious hazard for many types of aircraft, especially
light, fixed wing or rotary aircraft due to their relatively slow cruising
speeds and limited altitude range. SCRIPT SLIDE
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Different cloud types and icing
The most severe icing can be expected in large cumulus and
cumulonimbus clouds, which usually contain a large concentrationof water and large drops.
Severe icing should be forecast if the vertical extent of theconvective cloud is greater than 10000ft. Note that in an AreaForecast severe icing in cumulonimbus (Cb) is assumed.
Stable cloud has less supercooled liquid water content.
For stable clouds in layers the water distribution in the vertical planeis irregular. Certain cases have the maximum at the bottom of thecloud, while other samples have their maximum in the upper part.
Note that stratocumulus possesses both characteristics. Stable onthe broad scale, unstable on a small scale. Water content thereforevaries.
SCRIPT SLIDE
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Areas of increased icing threat are common with:
Image courtesy BOM, satellite images courtesy BOM/JMA
Troughs Upslide flow Frontal Boundaries
Lows / TCs Thunderstorms
Orographic uplift Airmass blocking
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trough systems, including pre-frontal troughs & an active
monsoonal trough;
areas of warm air advection or upslide (e.g. a northwest cloud
band);
frontal boundaries (typically above and upstream of the main
surface feature);
lows, including cut-off lows, extra tropical lows, tropical lows
and Tropical Cyclones (TCs).
Thunderstorms, esp. Mesoscale Convective Complexes
orographic uplift;
air mass blocking;
Areas of increased icing threat are common with:
SCRIPT SLIDE
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Icing regions about a low pressure system and
associated fronts.
Diagram from Aviation Forecasters Handbook (adapted from COMET).
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Icing regions about a low pressure system and
associated fronts.
The slide shows the icing regions about a low pressure system tothe south of Australia and the associated fronts.
The warm front boundary is often located on the poleward flank of
the system. Freezing rain can be a hazard associated with these
fronts, as the precipitation from the cloud falls into very cold high
latitude low level air.
However the warm front boundary is often located too far to the
south of Australia, and large intercontinental flights generally fly over
the main area of icing.
The cold front boundary is more of a problem for southern Australia.The area of enhanced icing potential occurs within the cloud band
and may be hundreds of kilometers long and tens of kilometers
wide. Therefore the flight path in relation to the front relates to icing
threat.
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NW Cloud Band / Monsoon trough
Both of these synoptic systems involve poleward moving air and theassociated large scale up-motion of a moist airmass.
Icing can be severe due to the widespread nature of the supercooledmidlevel cloud. Storms can be embedded within these systems, whichpresents an additional icing hazard.
images courtesy BOM/JMA
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Orographic cloud and upslide cloud
UPSLIDE
Inversion at or slightly below ridge level
Upslope cloud:
Examplewest to southwesterly winds over the Great Dividing
Range, that often create widespread cloudiness as the air isforced eastwards over the gently rising terrain
Orographic cloud:
Develop along mountaintops and ridges, and can persist for days
if the winds and moisture are consistent.
image courtesy BOM
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Orographic cloud and upslide cloud(from the NSW Icing Directive)
One example of terrain effects are upslope west to southwesterly windsover the Great Dividing Range of Australia that often create widespread
cloudiness as the air is forced eastward over the gently rising terrain.
These clouds can result in broad areas of icing conditions. Icing hazards
can also develop in orographic clouds, which tend to develop along
mountaintops and ridges and can persist for days if the winds and moistureare consistent.
The effects of blocking by mountain barriers are significant, especially
during winter. Stable lapse rates and mountain top inversions may prevent
winds from ascending the terrain, leading to deceleration and deflection ofthe flow. These processes can cause low-level convergence zones, clouds,
and precipitation, upstream of the mountain barrier. These upstream
convergent regions are favored icing areas.
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Orographically lifted cloud icing incident
From a communication between Geoff Feren and the pilot (2007).
The incident occurred near Mt. Hotham Airport, on the morning of 17 July2007, ahead of the major front and associated deep cloud band responsible
for the recent intense cold outbreak.
The pilot left Moorabbin Airport early that morning, flying IFR in a twin-
engine aircraft, which was not equipped with anti-icing gear.
His major icing incident occurred between 7500 and 8000 feet at about
9.30am with ambient air temperature -4 oC, not far from Mount Hotham
Airport.
The plane became covered "from head to toe" in thick rime ice, resulting in
clogging of air intakes. After contacting ATC in Melbourne, he decided to
divert northwestwards towards Benalla, where he could safely descend to
6000 feet.
Large chunks of ice became dislodged from his aircraft, and he
subsequently landed at Albury. This may have been because the cloud base
was around 6000 feet. SCRIPT SLIDE
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Airmass blocking
Upstream air mass blocking can cause uplift of the oncoming air-stream well windward of the actual mountain barrier.
Favourable conditions for icing.
Have occurred on the western slopes and ranges of VIC and NSW.
Freezing rain has been reported in these events.
Diagram from BOM Aviation Forecasters Handbook
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Presenting the procedures used by the National
Meteorological and Oceanographic Centre (NMOC) of
the Australian Bureau of Meteorology for determining
Aviation icing from satellite imagery.
NMOC attends to Aviation Icing for altitudes above 20,000 ft. NMOC
issues SIGMET over areas above FL185 (they also forecast icing for
the mid level charts FL 100FL250)
The Regional Forecasting Centres attend to Aviation Icing for altitudes
below 20,000 ft within their areas of duty.
The highest altitude of icing that has been observed by NMOC
forecasters was at 23 - 24,000
Therefore, liaison between RFCs and NMOC is sometimes required.
SCRIPT SLIDE
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Presenting the procedures used by the National
Meteorological and Oceanographic Centre (NMOC) of
the Australian Bureau of Meteorology for determining
Aviation icing from satellite imagery.NMOC forecasters examine the following satellite images:
The visible images to determine the thickness of the cloud. Also to
detect overshooting tops corresponding to cumulonimbus.
Cumulonimbus cloud has implied moderate and/or severe icingassociated with it.
The infrared images to verify the locations of cirrus. Also to
discriminate between cirrus and alto cloud on the basis of cloud top
temperature ie. grayscale. The alto cloud will have the serious aviationicing associated with it.
The water vapour images to determine the mid to upper level flow, in
particular the moist and dry airmasses. Forecasters focus upon moist
confluent airstreams in poleward flow. SCRIPT SLIDE
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Presenting the procedures used by the National
Meteorological and Oceanographic Centre (NMOC) of
the Australian Bureau of Meteorology for determining
Aviation icing from satellite imagery.
Forecasters examine the balloon soundings (F160s) of stations
nearby or under cloudband. In particular the vertical depth of 0 to -15
C layer. Depths 2500 or greater are a major concern. This procedure
is necessary to ground truth the models. Models generally do not havethe required details in the sounding.
The QANTAS icing product, in particular pressure levels where the
relative humidity is above 90% and the ambient temperature is
between 0 to -15C. Note that this product is presently replaced with
icing products within the Bureaus Visual Weather software, also withweb based aviation icing products.
Cross section of moisture and freezing level using model data.
Note that an AIREP generally has the highest priority.
SCRIPT SLIDE
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Icing potential parameters in NWP data(from WMO Tet1 prognostic variables - CAeM)
Liquid water content
The content of liquid water is a parameter that gives an excellent
indication of the icing potential. Expressed in grams per cubic meter.
Relative humidity
Saturation of air with icing potential can be represented by the relativehumidity of the air. If described correctly, it is also a parameter whichcan eliminate areas without icing potential.
From a viewpoint of numerical models, relative humidity is also afrequently calculated parameter.
NMOC rule is that 50-70% relative humidity results in possible icing,70-90% likely icing potential, greater than 90% very likely icingpotential.
It is then logical that one associates icing potential to a crossreference temperature / relative humidity
Vertical velocity
Sometimes vertical velocity is used as a complementary parameter todiscriminate icing conditions, especially when one does not have a
model prediction of liquid water content. SCRIPT SLIDE
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Example 1: Northwest Cloud Band 31 May 2013
30 May 2013 00 UTC 31 May 2013 00 UTC
images courtesy BOM/JMA
Questionis the cloud band developing or dissipating. What other feature maybe playing a part in its development ?
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Northwest Cloud Band 31 May 2013
30 May 2013 00 UTC 31 May 2013 00 UTC
images courtesy BOM/JMA
Exerciseindicate areas within the cloud where you might be expecting
significant icing
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images courtesy University of WisconsinCIMSS
Confluent flow. In particular from a moist source(here streamlines have been fitted to the 600-400 hPa cloud drift winds)
31 May 2013 00 UTC
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ACCESST Icing product for 31 May 00 UTC
Percent humidity
(dark blue > 95%)
Isotherms in red
image courtesy BOM
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ACCESS-T vertical cross section through the cloud
band.
BLUE corresponds to greater than 90 percent relative humidity. Below
400 hPa and above the freezing level this can correspond to very high
possibility of icing
Icing conditions generally not seen above 400hPa
A
B
400 hPa
A B
images courtesy BOM
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Icing evaluation on Port Hedland balloon sounding
image courtesy BOM
-15C
0C
Want about
25003000 ft
of depth
13500
23500
10000 icing
Note that the dewpoint
depression is very small
(less than 5 degrees)
between 13500 and
23500 feet.
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Port Hedland and
Giles balloon
soundings
Port Hedland Giles images courtesy BOM
-15C
0C
-15C
0C
Questiondoes the Giles sounding show worse icing conditions than Port Hedland ?
Worse icing Same icing Less icing
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NMOC SIGWX product 31 May 00 UTC
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Indonesian example 6 July 2013
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Indonesian example, 6 July 20135 July 12UTC 5 July 18UTC
6 July 00UTC 6 July 00UTC
images courtesy University of WisconsinCIMSS, bottom RHS picture BOM/JMAJakarta
Brunei
Indonesian example 6 July 2013
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Indonesian example, 6 July 20135 July 12UTC 5 July 18UTC
6 July 00UTC 6 July 00UTC
images courtesy University of WisconsinCIMSS, bottom RHS picture BOM/JMA
I d i l 6 J l 2013
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Indonesian example, 6 July 2013
images courtesy BOM, BMKG
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Indonesian example, 6 July 2013
Satellite imagery and synoptic setting
From examination of the infrared and enhanced infrared imagerywe can see an extensive area of deep convection developingover the northwest coast and adjacent regions of Kalimantanduring the night of 5/6thJuly.
The convective region and associated deep stratiform cloud
extends approximately 5 degrees latitude at 00UTC on themorning of the 6thJuly.
The area of storms is located in a region of low level south /southwest confluence on the BMKG gradient wind chart.Confluence is annotated over northernmost Kalimantan in theDarwin RSMC gradient wind chart however. Upper leveldivergence is indicated in the Darwin RSMC 200 hPa chart withstronger winds, in excess of 20 knots located to the west ofKalimantan.
It appears that we may be dealing with a Mesoscale ConvectiveComplex, developing within a low level convergent zone.
SCRIPT SLIDE
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Hypothetical flight from Brunei to Jakarta
6 July 2013 00 UTC
QANTAS icing product for
00UTC 6thJuly 2013, at
500 hPa
Temperature contour = - 5C,
contour interval 5 degrees
Light blue = from 50 to 75%
RH
Medium blue > 75% RH
Dark blue > 95%
Jakarta
Brunei
-5 C -5 C
-5 C
I d i l 6 J l 2013
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EXERCISE- Outline in thevertical cross section whereyou may expect severe icing(RH greater 90% for relevanttemperatures)
Indonesian example, 6 July 2013
Freezing level
anotated by FZL
Medium green >80%
Relative Humidity
Dark Green > 90%
Relative Humidity
AB
A
B
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Brunei Airport (WBSB) sounding, 00UTC 6 July
0 C-15 C
16500
23500QUESTIONicing
problem ?
Yes
No
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Kuching (WBGG) sounding, 00UTC 6 July
0 C-15 C
14000
22500QUESTIONicing
problem ?
Yes
No
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Jakarta (WIII) sounding, 00UTC 6 July
0 C-15 C
14000
21000
18500
QUESTIONicing
problem ?
Yes
No
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Indonesian example, 6 July 2013
Examination of the QANTAS icing product and other NWP data. Also theBrunei, Kuching and Jakarta soundings:
The QANTAS icing product at 500 hPa indicates a large area overwestern Kalimantan that has relative humidity greater than 95% attemperatures between 0 and -10 C.
The NWP vertical cross section from Brunei to Jakarta shows relativehumidities in excess of 90% from the freezing level (around 600 hPa) to
400 hPa. Over most of the BruneiJakarta flight path.
Examination of the Brunei, Kuching and Jakarta soundings showsaturation through a depth of about 8500 feet between the temperaturesof 0 and -15C for Kuching. This is less for the Jakarta and Bruneisoundings. You are asked to slect which of the soundings shows
significant icing potential. Examination of the WAFC London SIGWX prognosis shows that EMBD
CB is not annotated over the BruneiJakarta route. You are asked tosuggest a suitable forecasting strategy on the previous slide. The nextslide shows useful ASH SIGMET rules to help you.
SCRIPT SLIDE
Aviation Services Handbook SIGMET rules for
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Aviation Services Handbook SIGMET rules for
thunderstorms
SIGMET for thunderstorms are only issued when any one of thefollowing conditions is observed or expected:
a. Obscured (OBSC TS) by haze or smoke,
b. Embedded (EMBD TS) within cloud layers and cannot be
readily recognised,
c. Frequent (FRQ TS), i.e. an area of thunderstorms with littleor no separation between adjacent storms and covering morethan 75% of the affected area. The area affected would be ofthe order of at least 3 000 square nautical miles (3000 square
miles = about , or 54 miles square or 87 km
d. Squall-line thunderstorms (SQL TS), i.e. thunderstormsalong a line of about 100 nautical miles (161 km) or more inlength, with little or no separation between the clouds.
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Summary
Have gained an understanding of different kinds of icing
relevant to aviation and the effects of this on aircraft
performance.
Have gained a better understanding of the clouds and
synoptic settings that are favourable for aircraft icing.
Through participation in exercises using procedures
used by NMOC and Bureau Regional Forecasting
Offices have gained a basic understanding of aviation
icing within a northwest cloud band and also anenhanced convection situation over Indonesia.
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