Water Vapour Imagery and Potential Vorticity

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

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

Visible

Infrared

Water Vapour

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

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

Distribution of WV Completely moist atmosphere Emitting water molecules

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?

Distribution of WV Dry upper troposphere

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?

Moisture profiles and radiation

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

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

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

WV imagery characteristics

Water vapour loop 6-8 June, 2000

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

1. Latitude Effect

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

2. Seasonal Effects Winter

2. Seasonal Effects Summer

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

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

Water Vapour and NWP

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

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

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

Any Questions (so far)?

Potential Vorticity A refresher!

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

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

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.

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)

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

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

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.

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.

PV Anomalies

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

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

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.

An upper low / cold pool

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)

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

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

PV and WV Imagery

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.

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.

PV and water vapour imagery

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.

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.

Any Questions?