1. Identifying regions of Aviation Icing using satellite imagery Bodo Zeschke (BMTC) Image from COMET Image from BOM

2. 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 NMOC forecasters, particularly Ronik Kumar, for helpful suggestions for this presentation. SCRIPT SLIDE

3. 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 National Meteorological 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.

4. The icing environment 0 to -15 C BEWARE - Freezing Rain ! Image courtesy BOM

5. 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. The clear 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

6. 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 in the 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 (>0°C) rain/drizzle falls into a subzero environment. SCRIPT SLIDE The icing environment

7. 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

8. 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 less clearly 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 content less 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

9. 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 or greater. A SIGMET must be issued for severe icing conditions. SCRIPT SLIDE

10. Icing effects on an aircraft Diagram from BOM Aviation Forecasters Handbook (AFH)

11. 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 and vertical speed. Interfere with communication systems Reduce visibility. Icing effects on an aircraft (from the Aeronautical Forecasters Handbook) SCRIPT SLIDE

12. Icing severity (from the Aeronautical Forecasters Handbook) Icing severity depends on: droplet size – larger 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 greatest LWC) air temperature – the 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

13. Cloud types and icing Cb, TCu - possible severe clear ice Ac/As – clear ice possible in lower levels, light to moderate rime ice Ns – moderate mixed icing in the lower levels Sc – moderate rime ice if freezing level is low enough Sc Ns Cb TCu As Images from Wikipedia

14. Different cloud types and icing The most severe icing can be expected in large cumulus and cumulonimbus clouds, which usually contain a large concentration of water and large drops. Severe icing should be forecast if the vertical extent of the convective cloud is greater than 10000ft. Note that in an Area Forecast 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 plane is irregular. Certain cases have the maximum at the bottom of the cloud, while other samples have their maximum in the upper part. Note that stratocumulus possesses both characteristics. Stable on the broad scale, unstable on a small scale. Water content therefore varies. SCRIPT SLIDE

15. Areas of increased icing threat are common with: Image courtesy BOM, satellite images courtesy BOM/JMA Troughs Upslide flow Frontal Boundaries Lows / TC’s Thunderstorms Orographic uplift Airmass blocking

16. 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 (TC’s). Thunderstorms, esp. Mesoscale Convective Complexes orographic uplift; air mass blocking; Areas of increased icing threat are common with: SCRIPT SLIDE

17. Icing regions about a low pressure system and associated fronts. Diagram from Aviation Forecasters Handbook (adapted from COMET).

18. Icing regions about a low pressure system and associated fronts. The slide shows the icing regions about a low pressure system to the 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. SCRIPT SLIDE

19. NW Cloud Band / Monsoon trough Both of these synoptic systems involve poleward moving air and the associated large scale up-motion of a moist airmass. Icing can be severe due to the widespread nature of the supercooled midlevel cloud. Storms can be embedded within these systems, which presents an additional icing hazard. images courtesy BOM/JMA

20. Orographic cloud and upslide cloud UPSLIDE Inversion at or slightly below ridge level Upslope cloud: Example – west to southwesterly winds over the Great Dividing Range, that often create widespread cloudiness as the air is forced 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

21. Orographic cloud and upslide cloud (from the NSW Icing Directive) One example of terrain effects are upslope west to southwesterly winds over 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 moisture are 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 of the 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. SCRIPT SLIDE

22. Orographically lifted cloud icing incident – 17 July 2007 Infrared satellite image over Victoria and New South Wales depicts orographically -lifted cloud (light grey) formed in a convergent northwest airstream, ahead of a cold front, resulting in an overcast, but not deep layer of cloud with cloud top temps ~-7 to -10°C. (AFH) AREA OF THE INCIDENT O = Melbourne Airport X = Wagga Wagga Airport Diagram from BOM Aviation Forecasters Handbook (AFH)

23. 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 July 2007, 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

24. 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

25. 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 100 – FL250) 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 RFC’s and NMOC is sometimes required. SCRIPT SLIDE

26. 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 icing associated 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 aviation icing 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

27. 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 (F160’s) 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 have the 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 Bureau’s Visual Weather software, also with web 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

28. 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 relative humidity of the air. If described correctly, it is also a parameter which can eliminate areas without icing potential. From a viewpoint of numerical models, relative humidity is also a frequently calculated parameter. NMOC rule is that 50-70% relative humidity results in possible icing, 70-90% likely icing potential, greater than 90% very likely icing potential. It is then logical that one associates icing potential to a cross reference temperature / relative humidity Vertical velocity Sometimes vertical velocity is used as a complementary parameter to discriminate icing conditions, especially when one does not have a model prediction of liquid water content. SCRIPT SLIDE

29. Icing potential parameters in NWP data (from WMO Tet1 prognostic variables - CAeM) Vertical velocity Sometimes vertical velocity is used as a complementary parameter to discriminate icing conditions, especially when one does not have a model prediction of liquid water content. NMOC forecasters do not like using the NWP vertical velocity data as it is often “noisy”. SCRIPT SLIDE

30. Example 1: Northwest Cloud Band 31 May 2013 30 May 2013 00 UTC 31 May 2013 00 UTC images courtesy BOM/JMA Question – is the cloud band developing or dissipating. What other feature may be playing a part in its development ?

31. Northwest Cloud Band 31 May 2013 30 May 2013 00 UTC 31 May 2013 00 UTC images courtesy BOM/JMA Exercise – indicate areas within the cloud where you might be expecting significant icing

32. images courtesy University of Wisconsin – CIMSS 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

33. ACCESS – T Icing product for 31 May 00 UTC Percent humidity (dark blue > 95%) Isotherms in red image courtesy BOM

34. 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

35. Icing evaluation on Port Hedland balloon sounding image courtesy BOM -15C 0C Want about 2500 – 3000 ft of depth 13500’ 23500’ 10000’ icing Note that the dewpoint depression is very small (less than 5 degrees) between 13500 and 23500 feet.

36. Port Hedland and Giles balloon soundings Port HedlandGiles images courtesy BOM -15C 0C -15C 0C Question – does the Giles sounding show worse icing conditions than Port Hedland ? Worse icingSame icingLess icing

37. NMOC SIGWX product 31 May 00 UTC

38. Northwest Cloud Band 31 May 2013 Summary of the exercise Moist tropical confluent flow from the northwest to the southeast across northwestern and central Australia is evident in the water vapour imagery. The QANTAS icing product at 500 hPa indicates a large area within this cloud band that has relative humidity greater than 95% at temperatures between -5 and -15 C. The NWP vertical cross section from the northwest coast to central Australia shows a deep region of high relative humidity between freezing level and 23000 ft. Examination of the Port Hedland and the Giles soundings verifies the model data, showing saturation through a depth in excess of 10000 feet between the temperatures of 0 and -15C. This is confirmed by the NMOC SIGWX product, where severe icing is analysed, extending from the northwest maritime region of Australia through central Australia and into southern Australia. SCRIPT SLIDE

39. Indonesian example, 6 July 2013 5 July 12UTC5 July 18UTC 6 July 00UTC images courtesy University of Wisconsin – CIMSS, bottom RHS picture BOM/JMA Jakarta Brunei

40. Indonesian example, 6 July 2013 5 July 12UTC5 July 18UTC 6 July 00UTC images courtesy University of Wisconsin – CIMSS, bottom RHS picture BOM/JMA

41. Indonesian example, 6 July 2013 images courtesy BOM, BMKG

42. Indonesian example, 6 July 2013 Satellite imagery and synoptic setting From examination of the infrared and enhanced infrared imagery we can see an extensive area of deep convection developing over the northwest coast and adjacent regions of Kalimantan during the night of 5/6 th July. The convective region and associated deep stratiform cloud extends approximately 5 degrees latitude at 00UTC on the morning of the 6 th July. 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 the Darwin RSMC gradient wind chart however. Upper level divergence is indicated in the Darwin RSMC 200 hPa chart with stronger winds, in excess of 20 knots located to the west of Kalimantan. It appears that we may be dealing with a Mesoscale Convective Complex, developing within a low level convergent zone. SCRIPT SLIDE

43. Hypothetical flight from Brunei to Jakarta 6 July 2013 00 UTC QANTAS icing product for 00UTC 6 th July 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

44. EXERCISE - Outline in the vertical cross section where you may expect severe icing (RH greater 90% for relevant temperatures) Indonesian example, 6 July 2013 Freezing level anotated by “FZL” Medium green >80% Relative Humidity Dark Green > 90% Relative Humidity A B A B

45. Brunei Airport (WBSB) sounding, 00UTC 6 July 0 C-15 C 16500’ 23500’ QUESTION – icing problem ? Yes No

46. Kuching (WBGG) sounding, 00UTC 6 July 0 C-15 C 14000’ 22500’ QUESTION – icing problem ? Yes No

47. Jakarta (WIII) sounding, 00UTC 6 July 0 C-15 C 14000’ 21000’ 18500’ QUESTION – icing problem ? Yes No

48. Comparison with SIGWX prognosis chart Jakarta Brunei QUESTION – what would you do ?. A: issue a SIGMET for Icing for the identified area. B: issue a SIGMET for embedded Cb for the identified area. C: Don’t worry. The forecast is close enough !

49. Indonesian example, 6 July 2013 Examination of the QANTAS icing product and other NWP data. Also the Brunei, Kuching and Jakarta soundings: The QANTAS icing product at 500 hPa indicates a large area over western Kalimantan that has relative humidity greater than 95% at temperatures between 0 and -10 C. The NWP vertical cross section from Brunei to Jakarta shows relative humidities in excess of 90% from the freezing level (around 600 hPa) to 400 hPa. Over most of the Brunei – Jakarta flight path. Examination of the Brunei, Kuching and Jakarta soundings show saturation through a depth of about 8500 feet between the temperatures of 0 and -15C for Kuching. This is less for the Jakarta and Brunei soundings. 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 Brunei – Jakarta route. You are asked to suggest a suitable forecasting strategy on the previous slide. The next slide shows useful ASH SIGMET rules to help you. SCRIPT SLIDE

50. Aviation Services Handbook SIGMET rules for thunderstorms SIGMET for thunderstorms are only issued when any one of the following 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 little or no separation between adjacent storms and covering more than 75% of the affected area. The area affected would be of the 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. thunderstorms along a line of about 100 nautical miles (161 km) or more in length, with little or no separation between the clouds. SCRIPT SLIDE

51. 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 an enhanced convection situation over Indonesia.

52. References Aeronautical Services Handbook http://web.bom.gov.au/spb/adpo/aviation/ash.shtml Bureau of Meteorology Aviation Forecaster Handbook http://web.bom.gov.au/spb/adpo/aviation/afh.shtml http://web.bom.gov.au/spb/adpo/aviation/afh.shtml Bureau of Meteorology: Hazardous Weather Phenomena – Airframe Icing http://www.bom.gov.au/aviation/data/education/hwp-icing.pdfhttp://www.bom.gov.au/aviation/data/education/hwp-icing.pdf Forecasting Aviation Icing: Icing Type and Severity. MetEd (COMET) module http://www.meted.ucar.edu/icing/pcu6/index_flash.htm?0http://www.meted.ucar.edu/icing/pcu6/index_flash.htm?0 WMO Commission for Aeronautical Meteorology (CAeM) www.caem.wmo.int www.caem.wmo.int