1. Seasonal variability of the tropical tropopause dehydration
Holger Vömel
Strateole-2 Workshop 18 March 2015

2. Relative humidity at the tropopause Mid latitude Tropics
Outline
Motivation
How and Where
Trends
Uncertainties
Relative humidity at the tropopause
Mid latitude
Tropics
Tibetan Plateau
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3. Stratospheric Fountain
From Newell and Gould-Stewart, J. Atmos. Sci. 1981
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4. Brewer Dobson Circulation
From Holton et al., Rev. Geophys. 1995
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5. Walker circulation

6. Different regions in the tropics
Challenge:
Identify and quantify regions, that contribute to dehydration
Quantify the importance of the different dehydration processes:
Convective dehydration
Slow scale dehydration
Wave driven dehydration
Superstaturation in clouds
Increase the reliability in water vapor observations
Increase the number of observations
Validate global scale systems
Provide continuity to these observations
From Vömel et al., 2002
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7. Tropics (Costa Rica)
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8. Water Vapor Climatology: Costa Rica
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9. Water Vapor Climatology: Costa Rica
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10. RH (ice) Climatology: Costa Rica
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11. RH (ice) Climatology: Costa Rica
Altitude relative to mean tropopause [km]
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12. Tropopause statistics
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13. Seasonal Cycle: Temperature
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14. Seasonal Cycle: Water Vapor
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15. Seasonal Cycle: RH Ice
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16. RH (ice) Frequency at the Tropopause
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17. COBALD sounding
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18. COBALD sounding
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19. RH (ice) Frequency at the Tropopause
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20. Summary
Tropics:
Tropopause temperature  Monthly mean shows strong seasonal cycle
Tropopause water vapor mixing ratio  Monthly mean shows strong seasonal cycle (tape recorder)
Tropopause relative humidity over ice  Monthly mean nearly constant at 100%
Tibetan Plateau (July / Aug):
Tropopause relative humidity over ice  Monthly mean slightly below 100%
Tropopause relative humidity over ice in clouds  Monthly mean peaks at 100%
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21. NCAR EOL

22. Increase reliability of water vapor observations
Challenge
Increase reliability of water vapor observations
Increase number of observations
Validate satellite observations
Provide continuity of observations after the end of the satellite record
Problem: The processes immediately around the cold point matter and are hard to profile.
In particular they are a very difficult challenge for satellite observations, which mostly don’t have the vertical resolution in that region.
And water vapor and ozone vary strongly in the troposphere and stratosphere.
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