SAWS III Workshop (2010) Dr. Curtis N. James Associate Professor Department of Meteorology Embry-Riddle Aeronautical University Prescott, Arizona In-Flight Convective Weather Avoidance © 2006 Curtis James

Embry-Riddle Aeronautical University Prescott Campus Degrees 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 hazardous convective weather develops 2. Anticipating convective weather before & during flight 3. Properly interpreting weather guidance and imagery 4. Safely modifying flight to avoid hazardous 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

Find the thermals and max wind gust: Convective Boundary Layer Forecast soundings:

Understanding Convective Weather Thunderstorms (esp. June – September) ◦ Turbulence (see Video)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 1999 Photo: NWS Flagstaff

2. Anticipating Convective Weather Forecast Soundings rucsoundings.noaa.govrucsoundings.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 km) Convective Weather Guidance/Forecast Products ◦ ◦ 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:

FORECAST SOUNDING: LI: -3.1 CAPE: km Shear: 29.4 kt 0-8 km Shear: 43.1 kt

Aviation Weather Center Guidance

NWS Guidance Local forecasts Hazardous weather outlooks Flash flood watches Severe thunderstorm and tornado warnings

Sample Text Product 1058 AM MDT MON APR 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 Imagery WEATHER 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 scattering Depends on sum of diameter to the sixth power (D 6 ) of all particles in sample (assumed spherical) Water is more reflective than ice Smaller wavelengths (airborne radar) scatter more than longer wavelengths (NEXRAD) Reflectivity (Z) calculated from returned power dBZ=10 log 10 (Z) Echo range computed from elapsed time between pulse transmission and reception Sampling volume

Detectable areas at 10,000’ MSL Detectable 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 volume Sweep curvature is due to sphericity of earth (minus refraction) This base sweep is shown in many NEXRAD plots Developing thunderstorms undetected by base sweep! Cone of silence above radar

Reflectivity values / color tables > < 23 Airborne dBZ Very heavy rain or hail > 2.0” per hour Heavy rain 0.5 – 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 (Composite Reflectivity) Reveals echo at higher altitudes Portrays max echo 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 dB Side lobes Main lobe Beamwidth is the angular region where the power is at least -3 dB (or half) that of the center of the beam +20° The actual radar beam is wider than the beamwidth, resulting in echo fringing and ground clutter. Antenna focuses radar beam like a flashlight. The pilot controls the tilt!

Airborne radar perspective 26,500’ 53,000’ 50 n.m. 10° beamwidth Beyond 36 n.m. range, radar beam is about as wide as the depth of 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 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:

Sources of misinterpretation Anomalous propagation ◦ Beam bends towards colder air Clutter 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 echoes Attenuation (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-9 4 April 1977 – 71 dead Source: NTSB/AAR 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 a weather penetration device!

Blind alleys Do not fly in blind alleys where you could potentially become surrounded by convective cells BLIND REGION BLIND REGION Crescent-shaped echoes

Stratiform precipitation (schematic representation) 0°C MELTING LAYER RAIN WET SNOW SNOW Fall streaks In which layer will the strongest echo occur?                        

Stratiform precipitation (reflectivity representation) 0°C MELTING LAYER Bright band                        

Stratiform precipitation (NEXRAD representation) 0°C MELTING LAYER Weak gradient Bright band Exaggerates reflectivity Reflectivity in ice & snow underestimates precipitation reaching ground Best estimate below melting layer

Cool, dense air Life cycle of convective precipitation (schematic representation) 1. Cumulus stage 2. Mature stage 3. Dissipating stage * -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. Cumulus stage 2. Mature stage 3. Dissipating stage * 0°C -15°C                       Stratiform Convective Echo top ≠ cloud top             

Life cycle of convective echoes (NEXRAD representation) 1. Cumulus stage 2. Mature stage 3. Dissipating stage -15°C 0°C Strong gradient Developing thunderstorms may be above base scan (use composite reflectivity) May not capture all details of storm Dissipating cells Best analyzed below melting layer

Mesoscale Convective System Storm motion              Trailing stratiform echoes Leading convective echoes 0°C   up                  

Squall line: Identify convective echoes Outline the convective echoes Don’t penetrate any stratiform echoes that are connected to convective ones. Circumnavigate convective echoes by 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 Terrain shadow Bow echo

Sample: Fingers and hooks Fingers Hooks

Lightning detection basics Lightning emits intense electromagnetic pulses called sferics The time, intensity, direction, and polarity of these pulses can be detected Using 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 detection Sferics 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-time Detects cloud-to-ground and in-cloud strikes (even before the thunderstorm cell matures)

Correlating radar & lightning detection These strikes appear closer to a/c than the cells depicted by radar. These strikes are isolated from 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 hazards 2.Anticipating convective weather (before & during flight) 3.Properly interpreting weather guidance and imagery 4.Safely modifying flight to avoid hazardous weather The last step is up to you!