SAWS III Workshop (2010)
Dr. Curtis N. JamesAssociate ProfessorDepartment of MeteorologyEmbry-Riddle Aeronautical UniversityPrescott, [email protected]://meteo.pr.erau.edu/
In-Flight Convective Weather Avoidance
© 2006 Curtis James
Embry-Riddle Aeronautical UniversityPrescott CampusDegrees offered: B.S. Aeronautics B.S. Aeronautical Science B.S. Aerospace Studies B.S. Air Traffic Management B.S. Applied Meteorology B.S. Aviation Business Administration B.S. Aviation Environmental Science B.S. Engineering (AE, EE, ME, CS, CE) B.S. Global Security and Intelligence Studies B.S. Space Physics M.S. Safety Science
Convective Weather Avoidance
1. Understanding
how hazardo
us convect
ive weather develop
s
2. Anticipating convective
weather before & during flight
3. Properly interpreting
weather guidance and imagery
4. Safely modifying flight to avoid hazardo
us weather
Avoiding Convective
Weather
1. Understanding Convective Weather
Deep summer convective boundary layer◦May extend up to 20,000’ or more◦Dust devils common in this environment◦Low-level turbulence (generally LGT – MDT)
Stronger when strong wind flows over or around terrain (mechanical or mountain wave turb.)
◦Gusty wind (indicates turbulence is present) Maximum surface gust = wind speed at top of
layer Maximum gusts are typically 40% higher than
sustained wind
Deep convective boundary layer
20,000’ MSL or more
(more stable air above)
Hot, dry, unstable air
dust devil
thermal thermal
Find the thermals and max wind gust:
ConvectiveBoundaryLayer
Forecast soundings: http://rucsoundings.noaa.gov
Understanding Convective WeatherThunderstorms (esp. June –
September)◦Turbulence (see Video)◦Microbursts (or downbursts)◦Surface wind gusts◦Dust storms & IMC conditions◦Icing, hail, lightning◦Flash flooding◦Tornadoes (rare)
Dry air evaporates precipitation and accelerates cold, dense air downward. After touchdown, a vortex ring spreads outward (Eric Edelbrock, 2001).
Microburst
A gust front is the leading edge of the cold air advancing along the
surface (may contain dust!)July 2003—Photo by Phillip Zygmunt
Gust Front
Lightning (Prescott, AZ)2008—Photo by Curtis James
Lightning
Hail, Prescott Valley 1999Photo: NWS Flagstaff
2. Anticipating Convective WeatherForecast Soundings rucsoundings.noaa.gov
◦ Look for CAPE (at least a couple hundred J/kg) Maximum updraft speed (kt) ≈ (2 CAPE)1/2
◦ Dry air (large dew point spread) for microbursts◦ Shear for severe TS (at least 20 kt from 0 - 3 km)
Convective Weather Guidance/Forecast Products◦ http://www.aviationweather.gov◦ http://www.nws.noaa.gov/
Visual observations or METARs in flight◦ Towering cumulus, cumulonimbus clouds◦ Shelf cloud or dust warns a gust front is coming◦ Virga warns of dry microbursts
FORECAST SOUNDING: http://rucsoundings.noaa.gov/
FORECAST SOUNDING: http://www.wxcaster.com/etaskewts.htm
LI: -3.1CAPE: 590.4
0-3 km Shear: 29.4 kt0-8 km Shear: 43.1 kt
Aviation Weather Center Guidancehttp://aviationweather.gov/adds/convection/
NWSGuidance
• Local forecasts
• Hazardous weather outlooks
• Flash flood watches
• Severe thunderstor
m and tornado
warnings
Sample Text Product1058 AM MDT MON APR 19 2010
THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTHEAST AND NORTH CENTRAL COLORADO.
.DAY ONE...TODAY AND TONIGHT
ISOLATED TO SCATTERED THUNDERSTORM COVERAGE CAN BE EXPECTED ACROSS THE AREA THIS AFTERNOON AND EVENING...WITH THE MOST NUMEROUS STORMS OVER THE EASTERN PLAINS OF COLORADO. THERE IS ALSO MORE INSTABILITY ACROSS THE EASTERN PLAINS WHERE BETTER LOW LEVEL MOISTURE RESIDES. AS A RESULT...SOME STORMS ALONG AND EAST OF A LINE FROM NEW RAYMER TO LIMON MAY APPROACH SEVERE LIMITS LATE THIS AFTERNOON AND EVENING WITH HAIL UP TO 1 INCH IN DIAMETER AND WIND GUSTS APPROACHING 60 MPH ALONG WITH UP TO A HALF INCH OF RAIN. FARTHER WEST INCLUDING THE I-25 URBAN CORRIDOR...A DRIER AIRMASS WILL PREVAIL SO THE MAIN THREAT FROM ANY STORMS WILL BE LIGHTNING AND GUSTY WINDS TO AROUND 40 MPH. DAYTIME HEATING AND AN UNSTABLE AIRMASS WILL PRODUCE THE AFTERNOON AND EVENING STORMS.
Visual Warning Signs
2009—Photo by Sam Greene
Cumulonimbus cloud
2000—Photo by Dan Willard
Approaching gust front
2009—Photo by Sam Greene
3. Interpreting Available ImageryWEATHER RADAR Radars transmit focused pulses of
microwave light NEXRAD: 10 cm Airborne: 3 cm
Solid & liquid scatterers return the signal Precipitation (rain, snow, etc.) Bugs / birds Terrain
Size and number of scatterers determines power returned Clouds, dust have low reflectivity Large hail has high reflectivity
λ
Key facts about scatteringDepends on sum of diameter to the sixth
power (D6) of all particles in sample (assumed spherical)
Water is more reflective than iceSmaller wavelengths (airborne radar) scatter
more than longer wavelengths (NEXRAD)Reflectivity (Z) calculated from returned
power dBZ=10 log10(Z)
Echo range computed from elapsed time between pulse transmission and reception
Sampling volume
Detectable areas at 10,000’ MSLDetectable areas at 16,000’ MSL
NEXRAD Weather Radar Network
NEXRAD Volume Coverage Pattern
Shown: Most common scan strategy used by NEXRAD.Time required: About 5 or 6 min per volumeSweep curvature is due to sphericity of earth (minus refraction)
This base sweepis shown in many
NEXRAD plots
Developingthunderstorms
undetectedby base sweep!
Cone of silenceaboveradar
Reflectivity values / color tables
> 53
23 - 33
40 - 53
33 - 40
< 23
Airborne
dBZ
Very heavy rain or hail> 2.0” per hour
Heavy rain0.5 – 2.0” per hour
Moderate rain (heavy snow) – 0.17 – 0.5” /h
Light rain (light to moderate snow)0.01” – 0.17” per hour
NEXRAD
dBZ
Drizzle, cloud, dust or bugs
BREF(Base
Reflectivity)
0.5° sweep CREF
(CompositeReflectivity)
Reveals echo at higher altitudes
Portrays maxecho intensity
at each location
Time may differ slightly from
base reflectivity
NEXRAD Composite Reflectivity
Download composite reflectivity to the cockpit (not base reflectivity)!
Radar beamwidth
0°
+10°
-10°
-20°
0 dB-10 dB-20 dB
-30 dBSide lobes Main lobe
Beamwidth is the angular region where the poweris at least -3 dB (or half) that of the center of the beam
+20°
The actual radar beam iswider than the beamwidth,resulting in echo fringing
and ground clutter.
Antenna focuses radarbeam like a flashlight.
The pilot controls the tilt!
Airborne radar perspective
26,500’
26,500’
53,000’
53,000’
50 n.m. 50 n.m.
10° beamwidth
Beyond 36 n.m. range, radar beamis about as wide as the depthof the troposphere (~36,000’)
tropopause
Width of beam approximation:Width (feet) 100 × beamwidth (°) × range (n.m.)Height of beam approximation: Height (feet) 100 × tilt (°) × range (n.m.)
Proper tilt management a must!
Capital Cargo International Airlines—Boeing 727-200. En route from Calgary to Minneapolis on August 10, 2006, encountered large hail over
Alberta at an altitude between 30,000’ and 35,000’ MSL.Source: www.wunderground.com
Sources of misinterpretationAnomalous propagation
◦ Beam bends towards colder airClutter and shadowing
◦ Terrain reflects beam or side lobes◦ Shadowing (blind areas) beyond mountains
Increasing sampling volume size with range
Non-precipitation scatterers (birds, bugs)Second trip echoesAttenuation (airborne radar in heavy
precip.)
31
Where’s the
weather?This radar scan
contains anomalous
propagation, ground clutter,
spokes (or second trip echoes),
possibly bugs/birds, and thunderstorms.
Southern Airways DC-94 April 1977 – 71 dead
Source: NTSB/AAR-78-03
Attenuation example
16:08:01 – “All clear left approximately right now, I think we
can cut across there now.” – CAM 1
“The penetration resulted in a total loss of thrust from both engines due to the ingestion of massive amounts of water and hail.” -- NTSB
Airborne radar is not aweather penetration device!
Blind alleys
10
20
30
40
50
Do not fly in blind alleys where you could potentiallybecome surrounded by convective cells
BLINDREGION
BLINDREGION
Crescent-shapedechoes
Stratiform precipitation(schematic representation)
0°CMELTING LAYER
RAIN
WET SNOW
SNOW
Fall streaks
In which layer will thestrongest echo occur?
Stratiform precipitation(reflectivity representation)
0°CMELTING LAYER Bright band
Stratiform precipitation(NEXRAD representation)
0°CMELTING LAYER
Weak gradient
Weak gradient
Bright bandExaggerates reflectivity
Reflectivity in ice & snow underestimatesprecipitation reaching ground
Best estimate below melting layer
Cool, dense air
Life cycle of convective precipitation(schematic representation)
1. Cumulusstage
2. Maturestage
3. Dissipatingstage
*
-15°C
Lightning is possible if storm tops
below -15°C
0°C
Warm, moist unstable air
Life cycle of convective echoes(reflectivity representation)
1. Cumulusstage
2. Maturestage
3. Dissipatingstage
*
0°C
-15°C
Stratiform
Convective
Echo top ≠ cloud top
Life cycle of convective echoes(NEXRAD representation)
1. Cumulusstage
2. Maturestage
3. Dissipatingstage
-15°C
0°C
Strong gradient
Strong gradient
Developing thunderstormsmay be above base scan
(use composite reflectivity) May not capture all details of storm
Dissipating cellsBest analyzed
below melting layer
Mesoscale Convective System
Storm motion
Trailing stratiformechoes
Leadingconvective
echoes
0°C
up
Squall line:Identify convective echoes
Outlinethe convective
echoes
Don’t penetrate anystratiform echoesthat are connectedto convective ones.
Circumnavigateconvective echoesby at least 20 n.m.
Mesoscale Convective System & Mesoscale Convective Complex
MCS
MCC
Avoid the following:Stratiform echoes connected to convective ones
Convective echoes (strong gradients)
Strong echoes (red and magenta)◦Avoid even weak echoes in dry air
Severe storm patterns (especially) ◦Hooks (or pendants)◦Bows◦Fingers◦Crescent-shaped echoes
Sample: Bow echo
Terrainshadow
Bow echo
Sample: Fingers and hooks
Fingers
Hooks
Lightning detection basicsLightning emits intense electromagnetic
pulses called sfericsThe time, intensity, direction, and
polarity of these pulses can be detectedUsing triangulation from ground-based
sensors, the strikes mapped precisely in real time
Only cloud-to-ground lightning is displayed
Very useful for weather avoidance◦ Should be integrated with radar imagery!
Airborne lightning detectionSferics are detected using a 360°
direction finding antenna on the aircraft (out to 200 n.m.)
Range is inferred from the intensity of the sferic
Hotter strikes will appear closer than they really are (especially severe storms)
Updates in real-timeDetects cloud-to-ground and in-cloud
strikes (even before the thunderstorm cell matures)
Correlating radar & lightning detection
These strikes appearcloser to a/c
than the cellsdepicted by radar.
These strikes are isolatedfrom any radar echoes
What might cause differences in radar echo and strike location?
Radar vs. lightning detection:Why do cell locations differ?RADAR:
◦ Are the data current? (Time lag may be 10 – 15 min.)
◦ NEXRAD: Are you displaying composite reflectivity?
◦ Are cells shadowed by terrain or out of range?
◦ Are thunderstorms developing?Ground-based lightning data:
◦ Are the data current?◦ Are strikes less than 5 miles from radar
echo?Airborne lightning detection:
◦ Are data current?◦ Severe storm (displaces strike position
radially)?◦ Is the thunderstorm still developing?
Avidyne & Garmin symbols
Stormscope (WX-500)x = lightning “strike”+ = lightning “cell”
(disappears after 3 min)
Avidyne EX5000
Summary
Safe weather avoidance involves:1. Understanding convective weather
hazards2. Anticipating convective weather
(before & during flight)3. Properly interpreting weather
guidance and imagery4. Safely modifying flight to avoid
hazardous weatherThe last step is up to you!