Water Vapour Imagery and Potential Vorticity

1. Water Vapour Imagery and Potential Vorticity

2. Questions • How can you visualize the wind? • How can you see the upper air flow? • What colour is the wind?

3. OUTLINE • Some Physics • Imagery Characteristics • WV Interpretation • NWP Verification • Potential Vorticity – Introduction • PV Anomalies • PV and WV Imagery

4. Visible

5. Infrared

6. Water Vapour

7. IR imagery WV imagery Visible imagery Trop Atmospheric window Atmospheric window

8. EM Spectrum • Visible 0.8 m • IR 10.8 m • WV 6.2, 7.3 m

9. Distribution of WV Completely moist atmosphere Emitting water molecules

10. Distribution of WV Completely moist atmosphere Emitting water molecules Where is the source of radiation detected at the satellite?

11. Distribution of WV Dry upper troposphere

12. Distribution of WV Dry upper troposphere Where is the source of radiation detected at the satellite?

13. Moisture profiles and radiation

14. Display converted to temperature • White indicates upper tropospheric moisture • Grey indicates dry upper troposphere and moist middle levels • Black indicates dry air at middle and upper levels

15. Variation of contribution with humidity in water vapour images Note that moisture here will not be detected

16. EM Spectrum • Channel 5 (6.2m) strong absorption, centred around 300 hPa • Channel 6 (7.3m) less strong absorption, centred near 500 hPa

17. WV imagery characteristics

18. Water vapour loop6-8 June, 2000

19. 1. Latitude Effect • Whiter at the poles • Moisture from colder source • Higher contrast in tropics • Can be cold or warm • More moisture variability • Higher tropopause • Moist air appears dark when it is warm

20. 1. Latitude Effect

21. 2. Seasonal Effects • Whiter in mid-latitude winter • Lower temperatures for given height • Range reduced • Higher contrast in mid-latitude summer • Higher, colder tropopause • Larger range

22. 2. Seasonal Effects Winter

23. 2. Seasonal Effects Summer

24. 3. Crossover effect • All radiation detected from 700-200hPa • A given intensity may come from different profiles • It’s been found that … • cloud at mid levels contributes more radiation than higher levels

25. Imagery interpretation • Broadscale upper flow patterns • Jetstreams • Troughs/Ridges • Areas of vertical motion • Short-wave features • Convection

31. Water Vapour and NWP

32. Verification • WV can be used to identify upper air features in the flow • Position • Orientation • Shape • Speed of movement • Development with time • Compare these to NWP analyses and forecast frames • Assessment of model performance

33. 5 March 03, 06Z WV Image MODEL: Relative vorticity, 300hPa heights, MSLP

34. Recapitulation on WV Water vapour imagery … • shows upper level flows and humidity patterns in cloud-free areas • can be directly compared to model fields (height, vorticity, vertical motion) • can show developments before cloud formation is evident on VIS/IR

35. Any Questions(so far)?

36. Potential Vorticity A refresher!

37. Objectives • to write down the equation for PV and understand the meaning of the terms • to describe the effects of a PV anomaly on atmospheric development • to describe how PV can be related to water vapour imagery and NWP

38. P = 1 a.   z vertical potential temperature gradient density absolute vorticity Potential Vorticity • PV simply combines vorticity and static stability (vertical temperature gradient).

39. How does PV vary? • Density decreases with height so PV tends to increase slightly upwards. • f, the Coriolis parameter increases with latitude, so PV increases slightly towards the poles. • The major change in PV occurs at the tropopause where the static stability increases very rapidly.

40. How does PV vary? • Typical values of PV in the troposphere are generally less than 1.5 “PV units”. • In the stratosphere PV increases rapidly to in excess of 4 “PV units”. • Therefore there is a large gradient of PV at the tropopause (1 PV unit = 10-6 m2s-1 K.kg-1)

42. Potential Vorticity PV is a conserved quantity, which changes only slowly with time.

43. Potential Vorticity • This fits with what we already know about vorticity • If we stretch a column of air it spins more rapidly • If we squash an air column it spins less rapidly

44. Invertibility • PV contains information about both the dynamics (through vorticity) and thermodynamics (through potential temperature) of the atmosphere. • This is enough information to give all the other atmospheric fields if we have a boundary condition and a balance state.

45. Invertibility • So if you know the PV distribution in the atmosphere together with say the MSLP field, you can get all the other fields. • You could write an NWP model using PV and it would be cheaper to run than a conventional model.

46. PV Anomalies

47. Potential Vorticity anomaly A PV anomaly near the tropopause. The thick line is the PV = 2 surface. Thin lines are isentropes. The dotted and solid contours show circulation (out of and into the page). strato- sphere tropo- sphere

48. The effect of a Potential Vorticity anomaly A column of air passing beneath the PV anomaly is stretched and so gains some cyclonic vorticity. In reality the upper level features move faster than low level air. strato- sphere tropo- sphere

49. The effect of a Potential Vorticity anomaly • An upper level PV anomaly induces low level vorticity. • Upper level PV anomalies occur where the tropopause changes height rapidly. • Tropopause height changes rapidly in the vicinity of fronts, developing depressions, upper lows or cold pools.

50. An upper low / cold pool