Imagery Embry-Riddle Aeronautical University Interpretation in the Cockpit ERAU’s Prescott Campus offers: { B.S. Aeronautics { B.S. Aeronautical Science SAWS II Workshop (2008) { B.S. Aerospace Studies { B.S. Applied Dr. Curtis N. James { B.S. Aviation Business Administration Associate Professor { B.S. Aviation Environmental Science Department of Meteorology { B.S. Engineering (AE, EE, ME, CS, CE) Embry-Riddle Aeronautical University Prescott, Arizona { B.S. Global Security and Intelligence Studies [email protected] { B.S. Space Physics http://meteo.pr.erau.edu/ { M.S. Safety Science { Ph.D. in Aviation

RAdio Detection And Ranging Overview (RADAR)

{ Radars transmit focused pulses of microwave light λ z NEXRAD: 10 cm z Airborne: 3 cm Radar { Solid & liquid scatterers return the Basics signal Weather Radar z (rain, snow, etc.) Imagery z Bugs / birds Interpretation z Terrain Weather { Size and number of scatterers Basics determines power returned z Clouds, dust have low reflectivity z Large hail has high reflectivity

Radar system (schematic) Key facts about scattering

{ Weather radars are pulsed and monostatic { Depends on sum of diameter to the sixth power 6 (i.e. antenna transmits and receives) (D ) of all particles in sample (assumed spherical) { Water is more reflective than ice { Consist of transmitter/receiver, moveable { Smaller wavelengths (airborne radar) scatter antenna, radome, signal processor, display more than longgg()er wavelengths (NEXRAD) { Reflectivity (Z) obtained from power returned Transmitter, receiver & processor [dBZ=10 log (Z)] antenna positioner 10 { Echo range computed from elapsed time between pulse transmission and reception controls antenna display signal radome

Airborne radar: 10 kW peak power Operating frequency of 9.375 GHz Sampling volume

1 Reflectivity values / color tables Particle type identification dBZ dBZ Particle type is related to reflectivity, but… Very heavy rain or hail > 53 { > 2.0” per hour Icing conditions may be undetectable z Clouds often invisible to radar (esp. airborne) { Icing occurs in clouds between 0 and -40°C Heavy rain 40 - 53 0.5 – 2.0” per hour z Need to know freezing level(s) in order to identify an echo that contains freezing rain 33 - 40 Moderate rain (heavy snow) – 0.17 – 0.5” /h { Light snow often undetectable (airborne) 23 - 33 Light rain (light to moderate snow) 0.01” – 0.17” per hour { Non-precipitation echoes often misleading { NEXRAD will have polarimetric capabilities < 23 in about 5 years (better particle ID) Drizzle, cloud, dust or bugs

NEXRAD Airborne

PPI—Plan Position Indicator RHI—Range Height Indicator

Convective

Stratif orm

PPI display (120° sector viewed from above) RHI display (vertical cross-section)

Radar beamwidth Airborne radar perspective

The actual radar beam is Antenna focuses radar wider than the beamwidth, beam like a flashlight. resulting in echo fringing 0 dB Beyond 36 n.m. range, radar beam and ground clutter. -20 dB -10 dB is about as wide as the depth +20° of the troposphere (~36,000’) 53,000’ -30 dB tropopause +10° Side lobes Main lobe 26,500’ 50 n.m. 50 n.m.

0° 10° beamwidth 26,500’ 53,000’ -10° Width of beam approximation: Width (feet) ≈ 100 × beamwidth (°) × range (n.m.) Beamwidth is the angular region where the power -20° Height of beam approximation: is at least -3 dB (or half) that of the center of the beam Height (feet) ≈ 100 × tilt (°) × range (n.m.)

2 Tilt management (airborne radar) Terminal tilt

{ Leave tilt angle at maximum (~15°) to z Four useful tilt angles: observe echoes above terminal areas. { Terminal tilt { { Zero tilt Within 10 nm of an airport, building echoes may be above 15° tilt (rely more { Low-altitude tilt on ATC and NEXRAD) { Normal tilt z However, it is best to regularly vary tilt, especially after turns 10 nm z Stratiform echo: Best viewed below bright band (freezing level) z Convective: Best between 18-25 kft

50 50 40 40 Zero tilt 30 Low-altitude tilt 30 20 20 Alt: 20,000’ AGL 10 Step 1 10 { How to find: { How to find: 1. Tilt beam down until strong ground clutter is z Tilt beam up half a beamwidth from zero tilt. seen at a range (in NM) equivalent to your { Bottom of beam horizontal (helps identify altitude (in kft AGL) ppprecipitation at or above your altitude) { Bott om of beam will be ab out 10° down 2. Then, tilt beam upward by the angle 10° - ( beamwidth / 2) Don’t forget to tilt up { Center of beam is now approx. horizontal frequently to monitor (good reference tilt) building storms!

50 40 Normal tilt 30 Results of improper tilt management 20 10 { For smaller beamwidths: tilt beam down until strong ground returns are seen on outer range of radar scope. { This is a great tilt when at cruise altitude. { Shadows in ground returns indicate strong TS.

Don’t forget to tilt beam down frequently to Capital Cargo International Airlines—Boeing 727-200. En route from monitor building storms 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

3 Sources of misinterpretation Attenuation

{ Anomalous propagation { Loss of radar’s power along radial z Beam bends towards colder air due to absorption and scattering { Clutter and shadowing { Larger particles and shorter z Terrain reflects beam or side lobes wavelength → more attenuation z Shadowing (blind areas) beyond mountains z NEXRAD (10-cm wavellth)ength): attenuation negligible { Increasing sampling volume size with range z Airborne radar (3-cm wavelength): blind beyond first strong echo { Non-precipitation scatterers (birds, bugs) RADOME CONSIDERATIONS: { Second trip echoes { Class of radome (should transmit > 90%) { Water, ice & paint on radome also attenuate! { Attenuation

Airborne radar: Attenuation example What you MUST know

16:08:01 – “All clear left approximately right now, I think we { Key meteorological information can cut across there now.” – CAM 1 { Maximum Permissible Exposure Level 2 “The penetration resulted in a z MPEL = 10 mW/cm (typically ~10’ from radar) total loss of thrust from both { Radar’s antenna size (beamwidth) engines due to the ingestion 10” → 10°; 12” → 8°; 18” → 6°; 24” → 4° of massive amounts of water and hail.” -- NTSB { Tilt management (pilot controls tilt) { Radar’s limitations (e.g. attenuation) Airborne radar is not a weather penetration device! WHEN IN DOUBT: z Refer to NEXRAD data (if available) Southern Airways DC-9 4 April 1977 – 71 dead z Contact ATC for guidance Source: NTSB/AAR-78-03

NEXRAD Weather Radar Network NEXRAD Volume Coverage Pattern

Detectable areas at 10,000’16,000’ MSL Cone of silence above radar

Developing thunderstorms undetected by base sweep!

This base sweep is shown in many NEXRAD plots 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)

4 NEXRAD in the cockpit: NEXRAD Composite Reflectivity What you MUST know

Download composite 0.5° { CREFBREF Whether you have composite or base reflectivity to the cockpit sweep (Composite(Base reflectivity data (not base reflectivity)! Reflectivity) { The age of the data

RlhtReveals echo at { Where data void regions are located higher altitudes { Key meteorological information z Freezing level(s) for anticipating icing Portrays max z Anticipated weather and trends (e.g. echo intensity thunderstorms, turbulence, fronts) at each location z If precipitation may mix with dry air (T- DP > 10°C) Æ microbursts Time may differ slightly from base reflectivity

Precipitation Two types of precipitation

{ Clouds consist of visible moisture { Two basic ways that precipitation grows: (small liquid drops or ice crystals) z Ice crystal growth { Layered or “stratiform” precipitation z Form when moist air rises and cools { Weak updrafts, usually smooth flight { Liquid water droplets exist in clouds at temperatures from 0 to -40°C z ICING! ∗ { Most clouds do not precipitate z Collision and coalescence growth z It takes a million cloud drops to make { “Convective” precipitation one rain drop! z Heavier rain rates or hail z Precipitation forms in 15–30 min. { Strong updrafts, turbulence { Stay clear!

Stratiform precipitation Stratiform precipitation (schematic representation) (reflectivity representation)

Fall streaks ∗ ∗ ∗∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ SNOW ∗ ∗ ∗ ∗ ∗ ∗ ∗ 0°C ∗∗ ∗ 0°C ∗∗ ∗ MELTING LAYER ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ WET SNOW MELTING LAYER ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ Bright band

RAIN In which layer will the strongest echo occur?

5 Stratiform precipitation Life cycle of convective precipitation (NEXRAD representation) (schematic representation)

Lightning is ∗ ∗ possible if ∗ ∗ ∗ ∗ ∗ * ∗ ∗ ∗ storm tops ∗ ∗ ∗ ∗ below -15°C ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ Reflectivity in ice & snow underestimates -15°C ∗ precipitation reaching ground ∗ ∗∗ ∗∗ ∗ ∗∗ 0°C 0°C ∗ MELTING LAYER ∗ ∗∗∗ Cool, dense air

Warm, moist unstable air Bright band Exaggerates reflectivity 1. Cumulus 2. Mature 3. Dissipating Best estimate stage stage stage below melting layer

Life cycle of convective echoes Life cycle of convective echoes (reflectivity representation) (NEXRAD representation)

Echo top ≠ cloud top ∗ Developing thunderstorms ∗ ∗ ∗ ∗ ∗ ∗ * ∗ ∗ ∗ may be above base scan ∗ ∗ ∗ ∗ Convective ∗ ∗ ∗ (use composite reflectivity) May not capture all details of storm ∗ ∗ ∗ ∗ ∗ -15°C ∗ -15°C ∗ ∗∗ ∗∗ ∗ ∗∗ 0°C ∗ 0°C ∗ ∗∗∗ Stratiform Dissipating cells Best analyzed below melting layer

1. Cumulus 2. Mature 3. Dissipating 1. Cumulus 2. Mature 3. Dissipating stage stage stage stage stage stage

Mesoscale Convective Systems Squall line (type of MCS)

Storm motion { Thunderstorms often form in groups z Lines (squall lines) or clusters (MCS or MCC) ∗ { Contain convective & stratiform echoes ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ Leading z Theeco convect ive eecoesae echoes are in t he ir cu mu uuslus ∗ ∗ ∗ convective and mature stages (strong updrafts) ∗ ∗ ∗ echoes ∗ ∗ ∗ z The stratiform echoes are dissipating cells ∗ (downdraft regions) ∗ ∗ ∗ ∗ Trailing stratiform ∗ ∗ { Unsafe to penetrate stratiform echoes ∗ ∗ ∗ 0°C that are connected to convective ones! echoes ∗ ∗ ∗ ∗ ∗

up

6 Squall line: Identify convective echoes MCS

Circumnavigate convective echoes MCC by at least 20 n.m.

Don’t penetrate any stratiform echoes that are connected Outline to convective ones. the convective echoes Mesoscale Convective System & Mesoscale Convective Complex

Beware of the following: Sample: Bow echo

{ Strong reflectivity gradients Terrain { Strong reflectivity echoes (red and shadow magenta) { Severe storm patterns: Hooks (or pendants), bows, fingers, and crescent- shaped echoes { Squall lines { Cells that produce shadows or attenuation (airborne radar) z Blind alleys (airborne radar) Bow echo

Sample: Fingers and hooks Finger echo (airborne radar)

Fingers

Hooks

7 Hook echo (airborne radar) Blind alleys

Do not fly in blind alleys where you could potentially become surrounded by convective cells

BLIND BLIND REGION REGION 50

40

30

20 Crescent-shaped echoes 10

In summary…

For safe interpretation of weather radar: { Need key meteorological information z Freezing level(s), expected weather conditions, etc. { Understand radar’s characteristics & limitations z NEXRAD: (use composite reflectivity; recognize data void regions and shadows; check time stamp) z Airborne Radar: (know it’s a crude instrument, use proper tilt management & beware of attenuation) { Recognize stratiform & convective echo z Don’t penetrate echoes associated with convection { Recognize signs of severe weather

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