1 BASIC CONVECTION By: Ken Harding NWS ABR

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

3 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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11 Some days, the sky is very inviting…..

12 Some days…not

13 Subject Overview What is Convection Initiating Convection Some “Rules of Thumb”

14 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

15 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.

16 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

17 Altering Stability Three ways to decrease stability: Upper Level Cooling Low Level Warming Increase in Atmospheric Moisture

18 Air Mass Boundaries Are ‘Favored’ areas for convection and Thunderstorm growth Often subtle differences between air masses… –have large impacts on convection.

19 Air Mass Boundaries Types –Fronts –Dry lines –Sea-Breeze fronts (Lake Breeze?) –Convergence Zone

20 “Triggering” Mechanism Starts the convection –Low pressure systems –Fronts –Thunderstorm ‘outflow boundaries’ –Orographic lift

21 Low Pressure Systems An upper low gives cooling aloft –This reduces stability Surface lows give low-level convergence –this supplies ‘Lift’ from below

22 Fronts Provide lift - Convection is likely –if air is unstable Cold Air

23 Thunderstorm Outflow Boundaries Colliding outflow boundaries often provide lift for additional convection.

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29 Rules of Thumb for Convection - 1 If cumulus tops are ‘crisp’ and ‘well defined’... –the cloud will continue to grow.

30 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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32 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

33 Supercell Thunderstorms

34 Wind Shear

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39 Convection Vs Turbulence Strength of Convection Cloud Type Type of Turbulence Weak Fair Wx CuLgt Chop Moderate Towering Cu (Tcu) Mod chop Mod Turbc Strong Tcu and Cb Sev Chop Sev Turbc Pilot reports in thunderstorms indicate +/ fpm UDDF

40 Convective Turbulence Avoid convective turbulence by flying above cloud tops… When possible. Smooth Ride Bumpy Ride

41 Weak Convection ? No problem –Except sometimes Normal Approach Affects of Convection

42 Rules of Thumb for Convective 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.

43 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 for Convective Turbulence

44 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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46 A thunderstorm is never as bad on the inside as it appears from the outside…it’s worse.

47 Convective SIGMETS For Severe Thunderstorms (50 Kts, ¾” hail, tornadoes) Embedded Thunderstorms Lines of Thunderstorms Over 40% coverage in an area > 3000 mi 2 of VIP level 4 Thunderstoms.

48 Convective SIGMETS

49 CCFP

50 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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53 Lightning Sequence

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55 ASOS Thunderstorm Reporting

56 KEFC – 330/1690kts

57 National Lightning Detection Network Near Selby, SD Near Morris, MN Near Rapid City, SD Near Minot, ND

58 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

59 How it Detects Detects ONLY Cloud to Ground (CG) 30 mile range Provides info for 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 3 rd CG

60 How It’s Reported Detection 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 Z AUTO 08011KT 10SM SCT048 28/17 A2992 RMK AO2 SLP122 T TSNO

61 Thunderstorms in TAFs hours out –VCTS or SCT030CB hours out –Shorter duration of thunder –PROB30 beyond 9 hours hours –TEMPO Thunder (less than 4 hours) –Prevailing Thunder (less than 2 hours)

62 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 – Us – –We’re always open – Visit us or bring your students by. Come and hanger fly with us.

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