TTL Dehydration: Are Pileus The Missing Link

1
TTL Dehydration Are Pileus The Missing
Link? Timothy Garrett, U. Utah (tgarrett_at_met.utah.
edu) Christopher Webster, JPL Matthew McGill,
NASA Goddard Paul Bui, NASA Ames Elliot
Weinstock, Harvard U. Darrel Baumgardner, U.
Mexico, Brian Barnett, NASA JSC

• Introduction
• Possible mechanisms for TTL dehydration
• Gradual dehydration Freeze drying during gradual
  transport across isentropes, possibly enhanced by
  gravity waves1, 2.
• Convective dehydration TTL mixing with cold dry
  air from deep convection3.
• Problems with these mechanisms
• On small spatial scales the TTL shows wide
  spatial variability in d-HDO. Values lie between
  those expected from Rayleigh distillation and
  those found in surface airmasses4. Gradual
  dehydration alone cannot explain observed mixing
  ratios of d-HDO.
• Convective dehydration presumes all ice crystals
  in the tops of deep convection settle immediately
  rather than evaporate but ice crystals at such
  cold temperatures tend to be too small to settle
  quickly 5.

Hypothetical dehydration mechanism
Gravity wave
Pileus
Vellum

• Deep convection forces TTL isotherms at level A
  upwards, nucleating a pileus cloud layer at level
  B.
• The pileus forms a vellum over the convective
  dome.
• The pileus layer is punctured by the rising deep
  convection and mixes, increasing total water in
  the pileus, while decreasing its temperature.
• The convection subsides. Pileus cloud oscillates
  around level A, further decreasing its minimum
  temperature. Since the pileus is now moistened,
  it is now solid stratiform tropopause cirrus. The
  cirrus gradually precipitates as radiative
  heating lofts the cirrus across the tropopause.
  Air crossing the TTL is dehydrated to 1.6 times
  the saturation vapor mixing ratio at the
  temperature of the tropopause temperature minus
  the temperature perturbation due to oscillation
  and mixing.

Puncturing and mixing
Evidence 3 Cirrus formed in the TTL has values
of total water, d-HDO and potential temperature
all intermediate to values found in convective
outflow and TTL air. Mixing between the TTL and
convection has occurred.
Evidence 4 Parcel model simulations show mixing
between convection and pileus should deepen
dehydration and maintain a stratiform cloud, even
in the trough of the gravity wave.
Tropopause cirrus layer
Initiation of convective uplift of aerosol
particles atop Cb at t 0
Mixing
Nucleation
No mixing
Evaporation
Enrichment with convective air
Cooling
Particles mixed in from Cb
Nucleation
Convective outflow layer
Pileus survives in trough of wave!
Nucleation
Potential drying
WB-57F measurements from 28th July during
CRYSTAL-FACE. A Temperature. B Mixing ratios of
water vapor (black), saturation of water vapor
(gray), and total water (vapor ice) (red). C
Fractionation of HDO.