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Download A Look at Low Level Wind Shear A Look at Low Level Wind Shear Southwest Aviation Weather Safety Workshop. Phoenix, AZ Southwest Aviation Weather Safety

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  • Slide 1
  • A Look at Low Level Wind Shear A Look at Low Level Wind Shear Southwest Aviation Weather Safety Workshop. Phoenix, AZ Southwest Aviation Weather Safety Workshop. Phoenix, AZ Ken Widelski Meteorologist NWS: Lubbock, TX
  • Slide 2
  • Objectives What is Low Level Wind Shear? What is Low Level Wind Shear? Speed versus Directional Speed versus Directional Non-Convective Wind Shear Non-Convective Wind Shear Convective Wind Shear Convective Wind Shear Pre-Flight Planning/In-Flight Precautions Pre-Flight Planning/In-Flight Precautions Recap Recap Questions?? Questions??
  • Slide 3
  • What is Low Level Wind Shear? A change in the wind direction or speed within 2000 ft AGL. A change in the wind direction or speed within 2000 ft AGL. This condition can present danger to aircraft, especially at landing, when a sudden shift from headwind to tailwind can cause a rapid loss of airspeed and lift. This condition can present danger to aircraft, especially at landing, when a sudden shift from headwind to tailwind can cause a rapid loss of airspeed and lift.
  • Slide 4
  • Many Sources of LLWS Low Level Jet (*) Low Level Jet (*) -Strong winds above an inversion -Strong winds above an inversion Weather Fronts (*) Weather Fronts (*) Thunderstorms: Microburst (*) Thunderstorms: Microburst (*) Mountain waves Mountain waves Gap winds Gap winds Sea and land breezes Sea and land breezes KEY: LLWS can be convective and non- convective
  • Slide 5
  • Speed versus Directional Speed Shear: is the change in wind speed with height. In the illustration, the wind is increasing with height Speed Shear: is the change in wind speed with height. In the illustration, the wind is increasing with height
  • Slide 6
  • Directional Shear is a change in wind direction with height. Directional Shear is a change in wind direction with height. Surface flow is southeast. Winds veer (clockwise) turning to south- southeast and south. Surface flow is southeast. Winds veer (clockwise) turning to south- southeast and south.
  • Slide 7
  • A Different Perspective
  • Slide 8
  • Non-Convective LLWS Change in wind direction or speed within 2000 ft AGL. Change in wind direction or speed within 2000 ft AGL. The most common type of LLWS The most common type of LLWS Occurs over a larger area Occurs over a larger area Longer periods of time Longer periods of time Easier to detect Easier to detect Usually associated with a Low Level Jet Usually associated with a Low Level Jet
  • Slide 9
  • Aspects of the Low Level Jet A narrow band of A narrow band of strong winds in the lower atmosphere. Stronger winds Stronger winds Typically above a Radiative inversion
  • Slide 10
  • LLJ Characteristics Under the inversion top Under the inversion top -Conditions are usually stable with -Conditions are usually stable with light winds. light winds. Above the inversion top Above the inversion top -Winds can suddenly change up to 90 degrees in direction and up to 40 KTS in speed. -Winds can suddenly change up to 90 degrees in direction and up to 40 KTS in speed. Can pose a serious hazard to aircraft. Can pose a serious hazard to aircraft.
  • Slide 11
  • LLJ Flight Implications Courtesy of Hong Kong Weather Service When an aircraft When an aircraft departing from the departing from the airport ascends and airport ascends and enters the jet, it enters the jet, it experiences increasing experiences increasing headwind and lift. headwind and lift. As it departs the jet, however the headwind and lift decrease. As it departs the jet, however the headwind and lift decrease.
  • Slide 12
  • Frontal Passages Frontal passages can bring dramatic shifts in Frontal passages can bring dramatic shifts in Surface wind strength Surface wind strength Surface wind direction Surface wind direction The more vigorous the front The more vigorous the front = Stronger the Wind shift = Stronger the Wind shift = Better chance for Low Level = Better chance for Low Level Wind Shear Wind Shear
  • Slide 13
  • Frontal Type-Cold Front Frontal Passages can be large scale Frontal Passages can be large scale -Dense cold air rapidly digs in (Back) -Dense cold air rapidly digs in (Back)
  • Slide 14
  • Frontal Type-Warm Front Warm air gently slopes over cooler Warm air gently slopes over cooler surface air (Veer). surface air (Veer).
  • Slide 15
  • Small Scale Boundaries Outflow boundaries form from Outflow boundaries form from decaying thunderstorms. decaying thunderstorms. Cooler and more stable Cooler and more stable air spreads out in all air spreads out in all directions. directions. Convection is suppressed Convection is suppressed in this area. in this area. Looks benign but can be Looks benign but can be the focus for LLWS and the focus for LLWS and new convection new convection
  • Slide 16
  • Convective Wind Shear Affects a localized area Affects a localized area Main form: Microburst Main form: Microburst What is a Microburst? A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface. A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface.straight-line windsstraight-line winds
  • Slide 17
  • Microburst Lifecycle Microburst Lifecycle
  • Slide 18
  • Microburst Size: Microburst Size: -less than 1 mile in diameter as it descends from the cloud base to about 1,000-3,000 feet above the ground. In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter. -less than 1 mile in diameter as it descends from the cloud base to about 1,000-3,000 feet above the ground. In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter. Microburst Intensity: Microburst Intensity: The downdrafts can be as strong as 6,000 feet per minute. Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst. These strong horizontal winds occur within a few hundred feet of the ground. The downdrafts can be as strong as 6,000 feet per minute. Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst. These strong horizontal winds occur within a few hundred feet of the ground.
  • Slide 19
  • What are the dangers? The scale and suddenness of a microburst makes it a great danger to aircraft, particularly those at low altitude which are taking off and landing. The scale and suddenness of a microburst makes it a great danger to aircraft, particularly those at low altitude which are taking off and landing. As the aircraft is coming in to land, the pilots try to slow the plane to an appropriate speed. As the aircraft is coming in to land, the pilots try to slow the plane to an appropriate speed.
  • Slide 20
  • When the microburst hits, pilots will see a large spike in their airspeed, caused by the force of the headwind created by the microburst When the microburst hits, pilots will see a large spike in their airspeed, caused by the force of the headwind created by the microburst An initial response would be to decrease the speed of the aircraft. An initial response would be to decrease the speed of the aircraft. The plane would then travel through the microburst, and fly into the tailwind, causing a sudden decrease in the amount of air flowing across the wings The plane would then travel through the microburst, and fly into the tailwind, causing a sudden decrease in the amount of air flowing across the wings
  • Slide 21
  • The sudden loss of air moving across the wings causes the aircraft to literally drop out of the air. The sudden loss of air moving across the wings causes the aircraft to literally drop out of the air.
  • Slide 22
  • Pre-Flight Planning The NWS offers a host of data for pre- flight planning. The NWS offers a host of data for pre- flight planning. Aviation Area Forecast Discussion: Aviation Area Forecast Discussion: -Generally at TAF issuance -Generally at TAF issuance TAF: Terminal Forecasts: TAF: Terminal Forecasts: -4 routine issuances per day -4 routine issuances per day TWEB: Transcribed Weather Broadcast TWEB: Transcribed Weather Broadcast -4 routine issuances per day -4 routine issuances per day
  • Slide 23
  • Aviation discussion More direct insight into the TAF Reasoning. More direct insight into the TAF Reasoning. Example: Example: NATIONAL WEATHER SERVICE NATIONAL WEATHER SERVICE 245 PM MDT WED APR 4 2007 245 PM MDT WED APR 4 2007 WIDESPREAD MVFR CIGS WITH SOME LCL IFR NEAR THE BLKHLS WILL CONTINUE THROUGH THE AFTERNOON. CONDITIONS ARE EXPECTED TO DETERIORATE LATER THIS EVENING FROM WEST TO EAST AS AREAS OF SNOW START TO DEVELOP AND CIGS DROP TO IFR IN SOME LOCATIONS.
  • Slide 24
  • Experimental AFD Page http://aviationweather.gov/testbed/afd/
  • Slide 25
  • LLWS in the TAF NWSI 10-813: Feb 2005 Only refers to non-convective LLWS Only refers to non-convective LLWS from the surface up to 2000 FT AGL. from the surface up to 2000 FT AGL. Reason: LLWS is always assumed to be present in convective activity. R

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