2. Aviation Weather Hazards Mark Sinclair Department of Meteorology Embry-Riddle Aeronautical University Prescott, Arizona Weather center Weather radar, observing equipment and balloon launching on roof ERAU Academic Complex

3. Aviation Weather Hazards ALLSurface wind is the major listed hazard in in ALL weather related GA accidents FATALContinued flight into IMC conditions (reduced visibility and/or low ceilings) the leading cause of FATAL GA accidents

4. A. Turbulence “Bumpiness” in flight Four types –Low-level turbulence (LLT) –Turbulence near thunderstorms (TNT) –Clear-air turbulence above 15,000 ft (CAT) –Mountain wave turbulence (MWT) Measured as –Light, moderate or severe –G-load, air speed fluctuations, vertical gust

5. Turbulence Can be thought of as random eddies within linear flow + Hi! I’m an eddy

6. Turbulence Linear wind and eddy components add to gusts and lulls, up and down drafts that are felt as turbulence + 15 kt wind 5 kt eddy 10 kt lull 20 kt gust downdraftupdraft

7. Low-level Turbulence (LLT) Occurs in the boundary layer –Surface layer of the atmosphere in which the effect of surface friction is felt –Typically 3,000 ft deep, but varies a lot –Friction is largest at surface, so wind increases with height in friction layer –Vertical wind shear  turbulence Important for landing and takeoffs Results in pitch, yaw and roll

8. Low-level Turbulence (LLT)

9. Factors that make low-level turbulence (LLT) stronger Unstable air – encourages turbulence –Air is unstable when the surface is heated –Air is most unstable during the afternoon –Cumulus clouds or gusty surface winds generally indicate an unstable atmosphere Strong wind –More energy for turbulent eddies Rough terrain When LLT is stronger than usual, the turbulent layer is deeper than usual

10. Low-level turbulence (LLT) Mechanical –Created by topographic obstacles like mountains, and by buildings and trees –Increases with increasing flow speed and increasing surface heating (afternoon) Thermal –Occurs when air is heated from below, as on a summer afternoon –Increases with surface heating

11. Mechanical Turbulence Created by topographic obstacles in flow Increases in both depth and intensity with increasing wind strength and decreasing stability. Worst in afternoon –Extends above 3000 ft for gusts more than 50 kt Strongest just downwind of obstacles Over flat terrain, mechanical turbulence intensity is usually strongest just above surface and decreases with height

12. Mechanical Turbulence (cont.) Over flat terrain –Maximum surface wind gusts are typically 40% stronger than the sustained wind –Moderate or greater turbulence for surface wind > 30 kt –When sustained surface wind exceeds 20 kt, expect air speed fluctuations of 10-20 kts on approach –Use power on approach and power on landing during gusty winds –Sudden lulls may put your airspeed below stall

13. Thermal turbulence Produced by thermals (rising bubbles of warm air) during day in unstable airmass Common on sunny days with light wind Stronger above sun-facing slopes in pm Turbulence intensity typically increases with height from surface and is strongest 3-6,000 ft above the surface

14. Thermal turbulence (cont.) Generally light to moderate –Commonly reported CONT LGT-MOD Usually occurs in light wind situations, but can combine with mechanical turbulence on windy days Often capped by inversion –Top of haze layer (may be Sc cloud) –~3,000 ft, but up to 20,000 ft over desert in summer –Smoother flight above the inversion

15. Deep summer convective boundary layer causes thermal turbulence up to 20,000’ MSL (more stable air above) thermal Hot, dry, unstable air dust devil

17. Towering cumulus over Prescott Fall 2000 Photo by Joe Aldrich

18. Dry microbursts from high based thunderstorms When precipitation falls through unsaturated air, evaporative cooling may produce dry microbursts Result in very hazardous shear conditions Visual clue: fallstreaks or virga (fall streaks that don’t reach the ground) Flight path of plane 45 kt downburst 45 kt headwind 45 kt tailwind

19. Downburst (Phoenix, AZ) July 2003—Photo by Phillip Zygmunt

20. Downburst (Prescott Valley, AZ) 1999—Photo by Jacob Neider

21. The nocturnal boundary layer Clear nights, moderate flow Shallow friction layer Greatly reduced turbulence Lack of mixing  possibility of strong vertical shear –Surface air decoupled from gradient flow in free air above friction layer –Surface flow often unrelated to pressure pattern (and flow above friction layer) May have super-gradient flow and turbulence at top of inversion

22. Strong turbulence during day means a deep layer is stirred Mixing means 3,000 ft wind better mixed down to surface Stronger turbulence, reduced vertical wind shear Reduced turbulence means only a shallow layer is mixed Suppressed downward mixing means surface wind falls to near zero at night Stronger vertical shear Friction layer during day 3,000 ft Deep turbulent friction layer Friction layer during night Shallow non- turbulent friction layer

23. Diurnal variation of surface wind Wind at 3,000 ft AGL Wind speed (kt) 0 10 20 30 Midnight6am6pmnoonMidnight Surface wind Surface wind is stronger and more turbulent during afternoon

24. 2. Mountain Wave Turbulence

26. In mountainous terrain... Watch for strong downdrafts on lee side –Climb above well above highest peaks before crossing mountain or exiting valley Intensity of turbulence increases with wind speed and steepness of terrain Highest wind speed directly above crest of ridge and on downwind side Maximum turbulence near and downwind of mountain

27. Mountain Strongest wind speed and turbulence on downwind side, also warm and dry Air flow over mountains Orographic cloud and possible IMC conditions on upwind side UpwindDownwind Airflow Desired flight path Actual flight path Splat!

28. Mountain wave turbulence (MWT) Produces the most violent turbulence (other than TS) Occurs in two regions to the lee of mountains: 1.Near the ground and 2.Near the tropopause –Turbulence at and below mountain top level is associated with rotors –Turbulence near tropopause associated with breaking waves in the high shear regions just above and below trop

29. Turbulent Layer 1 - SFC-~7kft above peaks Turbulent Layer 2 2kft above to 6kft below trop Turbulent Layer 2 2kft above to 6kft below trop Tropopause Roll Cloud Lenticular Cloud Cap Cloud 02468101214161820Miles Troposphere Stratosphere Mountain Wave (> 25kt perpendicular component /stable air are key) Rules of Thumb for Predicting Turbulence

30. MWT (cont) Severity increases with increasing wind speed at mountain crest –For mountain top winds between 25 and 50 kt, expect mod turb at all levels between the surface and 5,000 ft above the trop –For mountain top winds > 50 kt, expect severe turb 50-150 miles downstream of mountain at and below rotor level, and within 5,000 ft of the tropopause –Severe turb in boundary layer. May be violent downslope winds –Dust may indicate rotor cloud (picture)

31. Mountain wave terminology Fohn cloud wall Hydraulic jump rotor Breaking waves Inversion Wave clouds (altocumulus lenticularis)

32. Mountain Waves Mountain waves become more pronounced as height increases and may extend into the stratosphere –Some pilots have reported mountain waves at 60,000 feet. –Vertical airflow component of a standing wave may exceed 8,000 feet per minute Vertical shear may cause mountain waves to break, creating stronger turbulence –Often happens below jet streak or near front

33. Breaking Wave Region Vertically-propagating waves with sufficient amplitude may break in the troposphere or lower stratosphere.

34. Rotor cloud Wind Rotor cloud cap cloud

35. Lee Waves Lee waves propagate horizontally because of strong wind shear or low stability above.These waves are typically at an altitude within a few thousand feet of the mountain ridge crest.

36. Lee waves (cont.) Lee waves are usually smooth, however, turbulence occurs in them near the tropopause –Avoid lenticular cloud with ragged or convective edges –Watch for smooth (but rapid) altitude changes Lee wave clouds in NZ

37. Satellite photo of lee waves over Scotland Lee wave photos

38. Flow over/around mountains Strongest flow near top and on downwind side For stable air and/or lighter winds, air will tend to go around rather than over mountain For less stable air and strong winds, air will go over mountain

39. Mountain Wave Accidents In 1966, a mountain wave ripped apart a BOAC Boeing 707 while it flew near Mt. Fuji in Japan. In 1992 a Douglas DC-8 lost an engine and wingtip in mountain wave encounters

40. Example: Extreme MWT encounter DC8 cargo plane over Evergreen, CO 9 Dec 92 encountered extreme CAT at FL 310 Left outboard engine, 19 ft of wing ripped off 10 sec duration, 500 ft vertical excursions, 20 deg left/right rolls Safe landing at Stapleton

41. Turbulence PIREPs

42. Web sites for turbulence information http://adds.aviationweather.gov/ –Hit the turbulence button http://www.dispatcher.org/brief/adfbrief.html –Lots of aviation links to real time weather info –Look down to turbulence section These are tools to help pilots better visualize aviation weather hazards. Not intended as a substitute for a weather briefing from a Flight Service Station

43. B. Instrument Meteorological Conditions

44. Instrument Meteorological Conditions (Ceiling and visibility below specified minimum values)

45. IFR/MVFR/VFR VFR- Visible Flight Rules – Pilot must be able to see the ground at all times. MVFR – Marginal VFR conditions. Still legally VFR but pilots should be aware of conditions that may exceed their capabilities IFR – Instrument Flight Rules – Pilot has special training and equipment to fly in clouds. LIFR – Low IFR.

46. Fog-Visibility IFR/MVFR/VFR VFR – Visibility greater than 5 miles. MVFR – Visibility 3-5 miles. IFR – Visibility 1-3 miles. LIFR – Visibility less than 1 mile. Red IFR Magenta LIFR Blue MVFR

47. Cloud Ceiling IFR/MVFR/VFR VFR - Ceiling greater than 3,000 ft. MVFR – Ceiling 1,000 to 3,000 ft. IFR – Ceiling less than 1,000 ft. LIFR – Ceiling less than 500 ft. IFR may be cause by either (or both) ceiling and visibility restrictions.

48. D. C. Pearson, 2002 IFR conditions are a factor in over half of the General Aviation weather related accidents

49. Meteorological Causes of IFR Conditions Fog (radiation fog, advection fog) Precipitation (snow, heavy rain) Low Clouds (lifting, cooling) High surface Relative Humidity (RH) common factor in all causes of IFR

50. 1. Fog

51. Fog Fog = low cloud with base < 50 ft AGL Generally reported when vis <5 miles and there is no precipitation reducing visibility Formed by condensation of water vapor on condensation nuclei Longer-lived when layer of cloud above Need –A cooling mechanism –Moisture Either lower T (cool) or raise DP (add moisture)

52. Mist Mist (BR) is reported as "A visible aggregate of minute water droplets or ice crystals suspended in the atmosphere that reduces visibility to less than 7 statute miles but greater than or equal to 5/8 statute mile."

53. Fog Can be considered as a low stratus cloud in contact with the ground. When the fog lifts, it usually becomes true stratus. This photo shows fog over the Pemigewasset River basin with clear skies elsewhere.photo

54. Foggy Weather

55. Fog types Radiation fog –Air near ground cools by radiation to saturation –Also called ground fog –Needs clear night, light breeze < 5 kts and high surface relative humidity at nightfall Advection fog –Occurs when warm moist air moves over colder bodies of water (sea fog), or over cold land –Needs winds up to about 15 kt –Occurs mostly near coasts, day or night California coast (+ other upwelling regions) Near Gulf coast in winter in southerly flow

56. Fog types (cont.) Upslope fog –Occurs on windward side of mountains –Moist air moves upslope and cools Precipitation fog –Occurs with surface inversion during rain –Occurs over land areas in winter –Raindrops fall to cold ground and saturate the air there first Three thermodynamic types –Warm fog (temp > 0°C) –Supercooled fog (-30°C < temp < 0°C) –Ice fog (temp < -30°C)

57. The COMET program

58. Radiation Fog Near Ground in Valley

59. Advection Fog over San Francisco

60. Fog Formation over San Francisco

61. Onshore Winds Advect Fog Inland

62. Types of Fog - Upslope Fog Air is lifted by moving up to higher ground.

63. Upslope Fog Example

64. Types of Fog - Precipitation Fog Rain falling into layer of cold air Evaporation below cloud base raises the dew-point and lowers the temperature Typically occurs in winter when there is a surface inversion The precipitation itself can also lower visibility to below IFR criteria in heavy snow or rain conditions

65. Questions pilots should consider regarding fog before they take off: 1. How close is the temperature to the dew point? Do I expect the temperature-dew point spread to diminish, creating saturation, or to increase? 2. What time of day is it? Will it get colder and form fog, or will it get warmer and move further from saturation? 3. What is the geography? Is this a valley where there will be significant cold air drainage? Will there be upslope winds that might cool and condense? 4. What is the larger scale weather picture? Will it be windy, suppressing radiation fog formation? Is warm, moist air moving over a cold surface?