Aviation Hazards: Thunderstorms and Deep Convection

Transcription

1 Aviation Hazards: Thunderstorms and Deep Convection TREND Diagnosis of thunderstorm hazards using imagery

2 Contents Satellite imagery Visible, infrared, water vapour Basic cloud identification Identifying cumulonimbus Thunderstorm movement through looped images RADAR imagery RADAR and precipitation return signals Thunderstorm signatures Qualifications to RADAR imagery interpretation 2

3 Satellite Imagery: what to look for Contrast and Compare the virtues of: VIS vs IR vs WV For cloud identification purposes Individual cloud types Is the air ascending /descending? Are there signs of stability / instability? Integrating cloud patterns with: Observations Model output Analyses 3

4 Infrared (IR) Imagery Brightness depends on: Effective cloud top temperature Coded into a greyscale Net effect: Bright / white cold & high level Dark / blackish warm & low level 4

5 Visible (VIS) Imagery Brightness depends on: Albedo [Outgoing VIS] / [Incoming VIS] as percentage Depends on: Illumination of cloud (e.g., sun angle) Angular position of cloud w.r.t sensor and sun Cloud s reflectivity: Cloud thickness Particle composition & size Character of cloud s surface Sensor system 5

6 Visible (top) and Infrared (lower) Imagery - 08/12/00 1 Ocean 2 Land surface 3 Distinct features in VIS 4 Bright thick cloud for VIS, high surfaces for IR 5 Low cloud darker tones in IR 6 Middle-level cloud mid-shades in IR but bright in VIS 6

7 High Resolution VIS and IR Imagery Visible image, top and Infrared image, below: *Low cloud detail *Cumulonimbus tops 7

8 Water Vapour (WV) Imagery Maximum WV response seen at ~ 300 hpa So, for low level cloud, water vapour does not show well, if at all! Brightness depends on: Moisture AND Low Temperatures Qualitatively, Bwv ~ [q] / [T] Net Effect (at ~ 300 hpa): Bright / white moist and cold Dark / blackish dry and warm 8

9 Basic Cloud Identification from WV Imagery A looped time-series of water vapour imagery: Water vapour imagery is best for high level systems, rather than individual thunderstorm cells However, thunderstorms may develop along a distinct dry / moist boundary 9

10 IR versus WV Images GMS5 Imagery: 21:32 UTC 17/01/01 WV images are suited to viewing upper systems IR image may show individual storm cells 10

11 Visible versus Infrared Images Ocean areas vary in temperature: In IR, shades of grey In VIS, uniformly dark (except for sun glint) Stratus / fog: In IR, indistinct, little difference in temperature compared with ground In VIS, bright (highly reflective) Thick cloud formations: Both cold tops and reflective, will appear bight in both IR and VIS! High and low layers: VIS: shadows and highlighted IR: temperature differences reveal layers 11

12 Typical Vertical Motions Stratiform clouds (e.g., Stratus) Greater horizontal extent, lesser vertical extent Vertical velocity is ~ 10 cm s-1 Cumuliform clouds (e.g., Cumulus, Towering Cumulus, Cumulonimbus) Greater vertical extent, lesser horizontal extent Vertical velocity is ~ 10 m s-1 (e.g., for Cumulonimbus) 12

13 Vertical Motion and Cloud Shapes: Plan View Stratiform cloud: Small dz/dt Large areas of flattish cloud e.g., Stratus, Stratocumulus, AltocumulusAltostratus Cumuliform cloud: Relatively large dz/dt Cell shaped clouds, cauliflower appearance looks like clumps, lumps etc. e.g., Cumulus, Towering Cumulus & Cumulonimbus 13

14 Satellite Imagery: Cumulonimbus Identification Form Globular or carrot shaped depending on vertical wind shear. Windshear: upwind sharp edge; downwind anvil edge is indistinct. Tone: Visual Very bright: distinct shadows and highlights when the sun angle is low. Tone: Infrared Bright white in centre or adjacent upwind boundary of anvil, darker toward downwind edge. 14

15 Indications of Thunderstorms Cumulus cloud bubbling Growing into Towering Cumulus Likely thunderstorm locations: Trough or front Hills or mountains Regions of low-level convergence 15

16 Looping Images Time series of images Linear extrapolation of thunderstorm movement essential for Weather Watch 16

17 RADAR and Precipitation RADAR is derived from the phrase, RAdio Detection And Ranging Focused beam of tuned radio waves Some pulses reflect off rain drops, etc., and are received by RADAR Location of rain can be determined from the orientation of the RADAR antenna and the time taken to reach the receiver 17

18 Precipitation Intensity Precipitation intensity is dependent upon the power of the return signal The return signal power depends on the size of the precipitation drops and their concentration For example: From light drizzle low power return signal Large hail stones high power return signal 18

19 Other RADAR Echoes Note that cloud droplets are too small to be detected by RADAR Clouds are invisible to RADAR Other spurious RADAR echoes may be, however: From aircraft, areas of smoke or ash from large fires, swarms of insects, flocks of birds, or obstacles on the surface (during unusual atmospheric conditions) Permanent anomalous echoes should be noted 19

20 RADAR Display Interpretation When precipitation is detected, the RADAR may display in a plan view: Rain may be detected falling from rain bands Showers of rain may be detected falling form cumuliform clouds Heavy showers of rain, hail or snow may be detected precipitating from thunderstorms 20

21 RADAR Return Signal Legend dbz units of the power of the RADAR return signal - reflectivity Rainfall rate can be related to dbz level RADAR colour legend links dbz, estimated rainfall rate to location and extent of storm cell(s) 21

22 RADAR and Thunderstorm Signatures RADAR echoes are very sharp-edged from rain and hail produced by thunderstorms An intense core indicates heavy rainfall Hail stones produce particularly intense echoes, (due to water coating on the ice and because of their large size) 22

23 Typical RADAR Echoes from Thunderstorms 23

24 RADAR Echoes from Thunderstorms Thunderstorms may appear as Isolated cells In clusters or in lines The typical thunderstorm life is about 30 minutes A severe thunderstorm may be indicated by: Fast moving cells, rapidly growing cells, a bow in the direction of movement of a line of cells, and or a long-lived cell moving in a markedly different direction to other storms Potential for flash flooding A very slow-moving cell or the repeated passage of a number of cells over a particular location 24

25 RADAR Image showing thunderstorm activity which accompanied a microburst Brisbane 17/01/01 25

26 Qualifications to RADAR Image Interpretation With greater distance from the RADAR. The intensity of the RADAR beam decreases The RADAR beam sees at greater heights above the surface (curvature of the earth) Causes of blocking (or partial blocking): The beam loses intensity through heavy rain Mountains RADAR determined rainfall rates are approximations for true levels of precipitation 26

27 Looped RADAR Images A time series of images A linear extrapolation of thunderstorm is useful for Weather Watch 27

28 Summary Thunderstorms can be detected through satellite imagery: Develop skills in recognising the distinctive shape and properties of the cumulonimbus cloud tops RADAR images provide distinctive thunderstorm signatures Looped satellite or RADAR imagery can provide a basis for linear extrapolation of thunderstorm movement, essential for Weather Watch Any thunderstorm activity provides multiple Aviation weather hazards to aircraft and therefore any thunderstorm should be avoided by all pilots. 28

29 Forward to Empirical thunderstorm forecasting techniques 29