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Aviation Weather Hazards

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Weather center. Weather radar, observing equipment and balloon launching on roof ... Weather a factor in 697 or 30% of all GA fatalities ... – PowerPoint PPT presentation

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Title: Aviation Weather Hazards


1
Aviation Weather Hazards
  • Mark Sinclair
  • Department of Meteorology
  • Embry-Riddle Aeronautical University
  • Prescott, Arizona

Weather radar, observing equipment and balloon
launching on roof
ERAU Academic Complex
Weather center
2
Talk Overview
  • Survey of weather related accidents
  • Turbulence
  • Low-level turbulence and surface wind
  • Thermal turbulence
  • Microbursts
  • Mountain wave turbulence
  • IMC conditions

3
All weather related accidents
  • The following data are from the FAAs National
    Aviation Safety Data Analysis Center (NASDAC),
    Office of Aviation Safety, Flight Standards
    Service and are based on NTSB accident data.
  • Data from all accidents, the majority non-fatal
  • http//www.asias.faa.gov/aviation_studies/weather_
    study/studyindex.html

4
Weather related accidents
5
Nearly 87 or 7 out of 8 of these involved
general aviation operations
General Aviation
GA
Commuter
Ag
Air carrier
6
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7
19,562 total accidents 4,159 (21.3) weather
related Main cause wind
8
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9
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10
GA weather-related fatalities a study by D.C.
Pearson (NWS)
  • http//www.srh.noaa.gov/topics/attach/html/ssd02-1
    8.htm
  • Looked at NTSB data from 2,312 GA fatal accidents
    in the US during 1995-2000
  • Weather a factor in 697 or 30 of all GA
    fatalities
  • A similar study by AOPA showed an average of 35
    but declining
  • Weather a bigger factor in FATAL accidents than
    for non-fatal

11
GA weather related fatalities (cont.)
  • NTSB cited NWS weather support to be a
    contributing factor in only two (0.3) of the 697
    weather-related fatal accidents.
  • NTSB cited FSS support to be a factor in only
    five (0.7) of the accidents.
  • NTSB cited inadequate ATC support only nine times
    (1.3)
  • Combined, NWS, FSS and ATC 2.3
  • Pilot error accounted for remaining 97.7
  • Continued flight into IMC the leading cause of GA
    weather-related fatalities

12
Flight Safety and Weather
  • Clearly, the responsibility for flight safety is
    YOU, the pilot
  • You need to brief (up to 41 dont)
  • Clear sky and light wind now does not mean it
    will be that way
  • One hour from now
  • 50 miles from here
  • 1,000 ft AGL

13
Fatal GA accidents
14
Causes of
15
Aviation Weather Hazards
  • Surface wind is the major listed hazard in in ALL
    weather related GA accidents
  • Continued flight into IMC conditions (reduced
    visibility and/or low ceilings) the leading cause
    of FATAL GA accidents

16
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

17
Turbulence in PIREPs
Turbulence Frequency
Turbulence Intensity
18
(No Transcript)
19
Turbulence
  • Can be thought of as random eddies within linear
    flow

Hi! Im an eddy

20
Turbulence
  • Linear wind and eddy components add to gusts and
    lulls, up and down drafts that are felt as
    turbulence

21
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

22
Low-level Turbulence (LLT)
23
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

24
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

25
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

26
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 gt
    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

27
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

28
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

29
Deep summer convective boundary layer causes
thermal turbulence
(more stable air above)
up to 20,000 MSL
thermal
thermal
dust devil
Hot, dry, unstable air
30
(No Transcript)
31
Towering cumulus over Prescott Fall 2000 Photo by
Joe Aldrich
32
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 dont reach the ground)

33
Downburst (Phoenix, AZ) July 2003Photo by
Phillip Zygmunt
34
Downburst (Prescott Valley, AZ) 1999Photo by
Jacob Neider
35
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

36
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
37
Diurnal variation of surface wind
38
2. Mountain Wave Turbulence
39
(No Transcript)
40
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

41
Air flow over mountains
Airflow
Upwind
Downwind
Strongest wind speed and turbulence on downwind
side, also warm and dry
Orographic cloud and possible IMC conditions on
upwind side
Desired flight path
Actual flight path
Splat!
Mountain
42
Mountain wave turbulence (MWT)
  • Produces the most violent turbulence (other than
    TS)
  • Occurs in two regions to the lee of mountains
  • Near the ground and
  • 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

43
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44
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 gt 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)

45
Mountain wave terminology
Breaking waves
Wave clouds (altocumulus lenticularis)
Inversion
Fohn cloudwall
Hydraulic jump
rotor
46
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

47
Breaking Wave Region
  • Vertically-propagating waves with sufficient
    amplitude may break in the troposphere or lower
    stratosphere.

48
Rotor cloud
cap cloud
Rotor cloud
49
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.

50
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
51
Lee wave photos
Satellite photo of lee waves over Scotland
52
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

53
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

54
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

55
Turbulence PIREPs
56
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

57
B. Instrument Meteorological Conditions
58
Instrument Meteorological Conditions (Ceiling
and visibility below specified minimum values)
59
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.

60
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
61
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.

62
D. C. Pearson, 2002
IFR conditions are a factor in over half of the
General Aviation weather related accidents
63
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

64
1. Fog
65
Fog
  • Fog low cloud with base lt 50 ft AGL
  • Generally reported when vis lt5 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)

66
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."

67
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.

68
Foggy Weather
69
Fog types
  • Radiation fog
  • Air near ground cools by radiation to saturation
  • Also called ground fog
  • Needs clear night, light breeze lt 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

70
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 gt 0C)
  • Supercooled fog (-30C lt temp lt 0C)
  • Ice fog (temp lt -30C)

71
The COMET program
72
Radiation Fog Near Ground in Valley
73
Advection Fog over San Francisco
74
Fog Formation over San Francisco
75
Onshore Winds Advect Fog Inland
76
Types of Fog - Upslope Fog
  • Air is lifted by moving up to higher ground.

77
Upslope Fog Example
78
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

79
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?

80
Climatology of IMC
  • In west, highest frequency of IFR conditions
    occur in
  • Pacific northwest - lots of cyclones fronts
  • gt 40 in winter
  • California coast - coastal upwelling fog
  • LA basin - smog
  • Elswhere in west lt 10 IFR conditions
  • Higher frequency in east, particularly in midwest
    and south
  • In IL, IN, OH, PA, gt 50 frequency in winter
  • Also gt 40 along Gulf coast in winter

81
Climatology of IMC, winter
10-40
10-40
40-50
lt 10
40-50
40-50
10-40
gt 50
lt 10
10-40
40-50
10-40
lt 10
10-40
40-50
10-40
82
Identification of Current IFR Conditions
  • AWC - Aviation Weather Center
  • red dots IFR, magenta dots LIFR, blue dots MVFR
  • Also shows Icing and Turbulence reports

83
Other Sources of Current IFR Conditions
  • AWC Standard Brief Satellite with AFC
    AWC - Standard Brief
  • ADDS (Aviation Digital Data Service run by AWC)
    Metar regional plots are color coded for IFR
    conditions ADDS METARs
  • ADDS Interactive Java tool using sky cover ADDS -
    METARs Java Tool
  • NCAR-RAP Surface Observations (similar to ADDS
    site) RAP Real-Time Weather

84
IFR Forecast Products
  • Terminal Area Forecast (TAF) Text product
    issued by WFOs for selected airports. Hourly
    resolution of prevailing and temporary surface
    conditions for up to 24 hours into the future.
  • TAF provide visibility and cloud ceilings, which
    can be related to IFR conditions
  • TAF has standard format so can be decoded and
    displayed as graphics or plain text.

85
Sources of TAF Forecasts
  • ADDS TAFs Available as plotted maps for a
    single time for a given region for prevailing or
    tempo conditions. Also available in text form in
    raw or translated formats for a given single
    station (need to know 4 letter ID).
  • ADDS - TAFs Java Tool Mouse over map for raw
    TAF data at any station.
  • Aviation Weather Center (AWC) - TAF Graphics
    Mouse over times and data types showing US
    prevailing or tempo conditions (3 hour
    resolution) in graphical form for IFR conditions.

86
Area Forecasts
  • Text product generated by AWC. Covers state or
    part of state VFR conditions for 12 hours into
    future with 6 hour outlook.
  • Coded format not decoded into graphics.
  • Available at http//aviationweather.gov/products/f
    a/ NWS plans to develop graphical Area Forecast
    product in future.

87
AIRMET
  • AIRMET regularly issued for IFR or Mountain
    Obscuration conditions covering at least 50 of
    an area.
  • 6 hour forecast with 6 hour outlook
  • Text product with graphical products generated
    from decoding of from lines.
  • Available at ADDS - AIRMETs

88
Model Guidance
  • NCEP Short Range Ensemble (multiple model runs
    which generate probabilities). Aviation products
    at SREF Aviation Products. Available for 3 ½ day
    outlooks.
  • TDL Model Output Statistics (MOS) (statistical
    relationship of model parameters and observed
    conditions) for visibility and ceiling
    probabilities and most likely conditions.
    Available at MAV MOS Graphics. Available for 3 ½
    day outlooks.

89
Forecasting LIFR is Difficult
LIFRLow IFR
PODProbability of Detection It happened - was it
forecast?
FARFalse Alarm Rate It was forecast but did not
occur.
About 75 of the time LIFR was forecast, it did
not happen.
Less than half of the observed LIFR conditions
were forecast correctly at TUL.
90
Online Weather information and Forecasts to
reiterate
  • 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

91
Summary
  • Issues to do with low-level wind are the main
    weather hazard facing GA
  • Probably includes cross winds, low-level
    turbulence, mountain effects and shear
  • Continued flight into IMC conditions the main
    cause of GA fatalities
  • Get a weather brief from your FSS
  • Get a weather brief from your FSS
  • Get a weather brief from your FSS

92
Talk Web site
  • http//meteo.pr.erau.edu/aviation_weather_hazards.
    ppt
  • Embry-Riddle Aeronautical University has a degree
    program in Meteorology.
  • Check us out at http//meteo.pr.erau.edu

93
Thank you Any questions?
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