SAWS III Workshop (2010) Dr. Curtis N. James Associate Professor Department of Meteorology Embry-Riddle Aeronautical University Prescott, Arizona [email protected].

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!