icing lecture 2013 accessible

8/10/2019 Icing Lecture 2013 Accessible

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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.

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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.

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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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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.

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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.

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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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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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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.

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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


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


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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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.

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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)


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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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.

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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.

SCRIPT SLIDE


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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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icing lecture 2013 accessible

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