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BASIC CONVECTION

By:Ken HardingNWS ABR

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Who am I?• NWS meteorologist – 20 years• Private pilot – 13 years• In Aberdeen since 1996

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What do I talk about?• I know a lot about:

– The NWS– Aviation weather

• TAF/METAR• ARTCC Weather Support

• I know a little about:– ASOS– Aviation Weather Center Products

• I’d like to learn from you:– Weather concerns– ‘War stories’

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Some days, the sky is very inviting…..

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Some days…not

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Subject Overview

• What is Convection• Initiating Convection• Some “Rules of Thumb”

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What is Convection?• The transport of heat (and

moisture) by the movement of a fluid.

• In meteorology, describes vertical transport of heat/moisture...– by updrafts/downdrafts – in an unstable atmosphere

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Convection

• Air in the free atmosphere will continue rising…– until it reaches a level where it is no

longer buoyant.

• The more unstable the atmosphere…– the greater the convection.

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Initiating Convection• Atmospheric stability

– Is determined by the way temperature varies with altitude

• The ‘stability’ of a layer of air…– is the resistance to vertical

displacement.

• “Unstable” air promotes convection• “Stable” air inhibits convection

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Altering StabilityThree ways to decrease stability:

UpperLevelCooling

LowLevel

Warming

Increase inAtmosphericMoisture

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Air Mass Boundaries

• Are ‘Favored’ areas for convection and Thunderstorm growth

• Often subtle differences between air masses…– have large impacts on convection.

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Air Mass Boundaries

• Types– Fronts– Dry lines– Sea-Breeze fronts (Lake Breeze?)– Convergence Zone

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“Triggering” Mechanism

• Starts the convection– Low pressure systems– Fronts– Thunderstorm ‘outflow boundaries’– Orographic lift

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Low Pressure Systems

• An upper low gives cooling aloft– This reduces stability

• Surface lows give low-level convergence– this supplies ‘Lift’ from

below

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Fronts• Provide lift - Convection is likely

– if air is unstable

Cold Air

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Thunderstorm Outflow Boundaries

• Colliding outflow boundaries often provide lift for additional convection.

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Rules of Thumb for Convection - 1

• If cumulus tops are ‘crisp’ and ‘well defined’...– the cloud

will continue to grow.

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Rules of Thumb for Convection - 2

• If cumulus tops are ‘ragged’ and ‘ill-defined’...

– the cloud will not continue to grow.

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Rules of Thumb for Convection - 3

• The taller the cloud..– The stronger the convection, and…– The greater likelihood of turbulence– The greater likelihood of micro/down

bursts

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Supercell Thunderstorms

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Wind Shear

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Convection Vs TurbulenceStrength

of ConvectionCloudType

Type ofTurbulence

Weak Fair Wx Cu Lgt Chop

Moderate ToweringCu (Tcu)

Mod chopMod Turbc

Strong Tcu andCb

Sev ChopSev Turbc

Pilot reports in thunderstorms indicate +/- 4000 fpm UDDF

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Convective Turbulence

• Avoid convective turbulence by flying above cloud tops…When possible.

Smooth Ride

Bumpy Ride

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Weak Convection ?• No problem

– Except sometimesNormal

ApproachAffects of

Convection

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Rules of Thumb forConvective Turbulence

• Expect light turbulence in areas where dust devils are occurring.

• Expect light/Mod turbulence under areas of developing cumulus clouds.

• Thermals rarely exceed 10,000 feet AGL without forming clouds.

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• Expect moderate turbulence…– in areas where extreme heating and

low humidity are expected or..– where exceptionally large dust devils

are occurring.– In areas of Virga

Rules of Thumb forConvective Turbulence

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Convection vs. Icing

• Convective clouds tend towards clear icing

• Moderately growing cumulus can have mod/svr clear/mixed icing

• If the cloud is raining, it has supercooled water – clear icing

• Supercooled water can extend to great heights (>20,000 ft AGL) and temps < -15 C.

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A thunderstorm is never as bad on the inside as it appears from the outside…it’s worse.

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Convective SIGMETS

• For Severe Thunderstorms (50 Kts, ¾” hail, tornadoes)

• Embedded Thunderstorms• Lines of Thunderstorms• Over 40% coverage in an area >

3000 mi2 of VIP level 4 Thunderstoms.

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Convective SIGMETS

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CCFP

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Lightning Facts

• About 100 strikes per second worldwide• Carries approximately 100,000,000 Volts• Bolts over 110 miles long have been

measured• Both negative and positive lightning• 50,000 degrees C.• Approximately 1” in diameter

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Lightning Sequence

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ASOS Thunderstorm

Reporting

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KEFC – 330/1690kts

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National Lightning Detection Network

Near Selby, SD

Near Morris, MN

Near Rapid City, SD

Near Minot, ND

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How it Works

1. Sensors transmit lightning data to satellite; 2. Satellite relays information to earth station; 3. Data is transmitted to NCC via land-lines; 4. NCC processes data; 5. Processed data is relayed back to satellite; 6. Lightning data is displayed within seconds of occurrence 7. Accurate within +/- 1/2 mile

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How it Detects

• Detects ONLY Cloud to Ground (CG) • 30 mile range• Provides info for 30-10 miles, 10-5

miles, and 5-0 miles.• Reports once per minute• Detection Efficiency

– 90% of CG within 10 miles– 100% of thunderstorms after the 3rd CG

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How It’s Reported• Detection 10 - 30 miles:

– LTGDSNT DIR • Detection 5 – 10 miles:

– VCTS• Detection 0 – 5 miles:

– TS reported as present weather• TSNO remark: NLDN down, or no

ALDARS• KMBG 201855Z AUTO 08011KT 10SM

SCT048 28/17 A2992 RMK AO2 SLP122 T02830167 TSNO

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Thunderstorms in TAFs

• 12-24 hours out– VCTS or SCT030CB

• 06-12 hours out– Shorter duration of thunder– PROB30 beyond 9 hours

• 00-06 hours– TEMPO Thunder (less than 4 hours)– Prevailing Thunder (less than 2 hours)

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New Resources

• 24-hour TAF for Mobridge• Pilot resources CD

– Sign up if you want one mailed to you• Web Log (Blog) of aviation links

– http://pilotweather.blogspot.com• Us

– kenneth.harding@noaa.gov– 605-225-0519– We’re always open – Visit us or bring your

students by. Come and hanger fly with us.

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