Using global models and chemical observations to diagnose eddy diffusion

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Using global models and chemical observations to
diagnose eddy diffusion
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• Goal determine the eddy diffusion rate in the
  upper mesosphere, including latitudinal and
  seasonal changes
• Proposal use a 3-D chemical model to determine
  which global measurements can best constrain the
  mean global diffusivity coefficient use the
  measurements and model to narrow the range of
  diffusion rates
• Why do we want/need to know diffusion?
• theoretical (how much turbulence diffusion is
  generated by gravity wave breaking?)
• without knowing diffusive transport, we dont
  know if/when our chemical simulations are
  correct
• What do we know now?
• current estimates from observations and numerical
  models differ widely
• Why use chemicals?
• different constituents are sensitive to diffusion
  over different altitude ranges
• matching profiles for multiple constituents
  provides a stringent test of our estimates

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WHAT MAKES A CHEMICAL USEFUL?

• concentration large enough to be measurable
• sensitivity to transport because of either
• long lifetime strong vertical gradient
• short lifetime but equilibrium concentration
  depends on transported species
• reactions and rate coefficients reasonably well
  known
• consistent response (for example, increases
  monotonically with increasing diffusivity)

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ROSE model

• vertical range tropopause to thermosphere
• driven by meteorological observations at lower
  boundary
• radiation and dynamics can be decoupled from
  chemistry (as in the present study)
• time-dependent chemistry
• easily changed for mechanistic studies
• simulates
• oxygen O(1D), O, O2, O3
• hydrogen H, OH, HO2, H2O, H2O2, H2, CH4
• nitrogen N, NO, NO2, NO3, HNO3, N2O5, N2O,
  HO2NO2
• chlorine Cl, ClO, HCl, HOCl, ClONO2, CFCl3,
  CF2Cl2
• carbon CO, CH2O, CO2
• Note thermospheric NO is specified based on
  SNOE empirical model (Marsh et al. 2004)

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eddy diffusion coefficient Kzz in ROSE model
chemical continuity eqn for mixing ratio c
Kzz is the eddy diffusion coefficient calculated
by the Hines gravity wave drag parameterization
and includes effective Prandtl number
m2/s
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How the model is used

• several model integrations with different levels
  of eddy diffusion in the chemical continuity eqn
  otherwise identical
• NOTE large-scale dynamics is identical in all
  runs because
• the dynamical Kzz does not change
• these runs are uncoupled (climatological
  radiative gases)
• comparison of averaged vertical profiles global
  at all local times or day-only and night-only
• subjective assessment of which provide the best
  constraints on diffusion assuming the
  availability of global measurements over all
  local times
• actual application will depend on extent and
  accuracy of measurements

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basic results

• source gases and stratospheric species that are
  not useful because concentrations are too small
  in mesosphere
• hydrogen family H2O2
• nitrogen family NO3, HNO3, N2O5, N2O, HO2NO2
• chlorine family ClO, HOCl, ClONO2, CFCl3, CF2Cl2
• carbon family CH2O
• other species with low concentrations
• oxygen family O(1D)
• species that are not useful because of weak
  vertical gradient
• O2 and H2
• species that cannot be tested due to specified
  thermospheric NO in ROSE model
• nitrogen N, NO, NO2,
• species considered below
• oxygen family O, O3
• hydrogen family H, OH, HO2, H2O, CH4
• chlorine family Cl, HCl
• carbon family CO, CO2

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sample for interpretation of the model results

• curves show global mean CO profiles from 4 model
  runs
• CO increases with altitude due to a source in the
  thermosphere
• higher diffusion leads to lower mesospheric CO
  due to upward transport of low-CO air
• the differences among the 4 cases increase with
  altitude in the mesosphere
• interpretation CO could provide a good
  diagnostic of diffusion near the mesopause

profile values range from zero to 2 x 10-4 vmr
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CO and CO2 (transport)

• With increasing diffusion, CO2 increases near the
  mesopause while CO decreases

Kzz0
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Obs (SABER) model (ROSE) of CO2
ROSE model
SABER v 1.06
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CO2 3 model cases for January-February
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Cl and HCl (transport)

• With increasing diffusion, HCl increases near the
  mesopause while Cl decreases

Kzz0
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long-lived hydrogen species

• With increasing diffusion, CH4 and H2O increase
  in the lower and middle mesosphere and H
  increases in the upper mesosphere

Kzz0
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atomic oxygen
percentage change with diffusion is small
the altitude of the rapid increase of O in the
middle mesosphere moves down slightly with
increased diffusion
Kzz0
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ozone (photochemistry)

• both day night ozone change with diffusion, but
  the signs are opposite
• at night, lower O3 with higher Kzz is dominated
  by the impact of eddy diffusion on H
• during day, diffusion increases O3 in the middle
  mesosphere through the increase in O
• a valuable diagnostic since the response differs
  in day night (easier to distinguish from other
  perturbations)

Kzz0
Kzz0
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night ozone 3 model cases for January-February
high diffusion brings up water, which leads to
ozone destruction
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Obs model of night ozone
ROSE model
SABER
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OH and HO2

• daytime increase with increasing diffusion in the
  vicinity of the vmr maximum
• nighttime differences not monotonic with changes
  in Kzz

Kzz0
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Summary of useful chemicals

• Useful in middle mesosphere
• CH4
• H2O
• O3, day night
• Useful in upper mesosphere/mesopause
• O3 night
• CO
• CO2

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information about vertical structure of diffusion
rate?
high diffusion better above the mesopause?
high diffusion worse at and below the mesopause?
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Problems with this approach

• molecular diffusion
• these tracers are also sensitive to molecular
  diffusion
• how best to treat the two together?
• numerical formulation of diffusion
• at present, models are being used to validate
  the upper mesosphere chemical observations from
  SABER