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. Visible imagery IR imagery WV imagery Trop Atmospheric window
Emissions Vis - Albedo IR – temperature WV – emissions of water vapour depends on vapour and its temperature

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. Where is the source of radiation detected at the satellite?
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. Where is the source of radiation detected at the satellite?
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 loop 6-8 June, 2000

19. 1. Latitude Effect Whiter at the poles Higher contrast in tropics
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. WV Image MODEL: 5 March 03, 06Z 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. Potential Vorticity P = 1 a.   z
PV simply combines vorticity and static stability (vertical temperature gradient).
P = 1 a.  z 
density
absolute
vorticity
vertical
potential
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

51. The effect of a Potential Vorticity anomaly (cyclonic development)
A positive PV anomaly over a low level baroclinic zone induces a positive feedback mechanism
(eg depressions)

52. Potential Vorticity anomalies
00Z 2/11/92.
PV (colours). 900 hPa w (white) MSLP (black)

53. Potential Vorticity anomalies
00Z 3/11/92 (24 hours later)
PV (colours). 900 hPa w (white) MSLP (black)

54. PV and WV Imagery

55. PV and water vapour imagery
Stratospheric air has high PV and low humidity.
The upper troposphere in a tropical airmass has low PV and high humidity.
In mid latitudes PV values near the tropopause relate closely to radiances in the water vapour channel.

56. PV and water vapour imagery
In a developing depression, the tropical air in the warm conveyor belt will be white or pale grey in a WV image, and will have low PV.
The dry, cold descending air behind the system will be dark grey or black in a WV image and will have high PV.
Where the tropopause is changing height rapidly, there will be a sharp PV gradient.

57. PV and water vapour imagery

58. PV and water vapour imagery
This means that the PV field from an NWP model is almost like a forecast water vapour image.
If the PV distribution from the model is overlaid on a water vapour image, the quality of the analysis or forecast can be subjectively assessed.

59. PV and water vapour imagery
The model’s PV field at T+0 can be compared with water vapour imagery.
If they do not match well, the model analysis can be adjusted to give a better fit and therefore a better forecast.
This provides a means of evaluating and improving NWP forecasts.

60. Any Questions?