Transcript The Neutral Atmosphere

The Neutral Atmosphere
Dan Marsh
ACD/NCAR
Overview
• Thermal structure
– Heating and cooling
• Dynamics
– Temperature
– Gravity waves
– Mean winds and tides
• Composition
–
–
–
–
Primary constituents
Continuity equation
Minor constituents - ozone, NO
Storm impacts
Thermal structure
Thermodynamic equation
T
T
Q
 (d  )w  v  T 
+…
t
z
cp
Heat advection
Adiabatic heating
Diabatic
heating/cooling
Sources of diabatic heating/cooling
• Absorption of solar radiation and energetic
particles (e.g. ozone)
• Chemical heating through exothermic
reactions (A + B -> AB + E)
• Collisions between ions and neutrals (Joule
heating)
• IR cooling (e.g. CO2 and NO)
• Airglow
Solar radiation energy deposition
Solar
Heat
Quantum
Internal
CO2(2), O2(1)…
Cooling
UV, Vis.,
IR Loss
Chemical
Potential
O, e,…
Airglow
After Mlynczak et al.
Solar UV Energy Deposition
Courtesy Stan Solomon
Schumann-Runge
continuum
S-R Bands
Hartley continuum
Global average heating rates
From [Roble, 1995]
Joule Heating (K/day)
Heating from collisions between ions and neutrals
Chemical heating through exothermic reactions
H + O3  OH + O2 (k4)
O + O2 + M  O3 + M (k2)
O + O + M  O2 + M (k1)
O + O3 +  O2 + O2 (k3)
OH + O  H + O2 (k5)
HO2 + O  OH + O2 (k6)
H + O2 + M  HO2 + M (k7)
K/day
Global average cooling rates
From [Roble, 1995]
Radiative cooling
• IR atomic oxygen emission (63 µm) in
the upper thermosphere
• Non-LTE IR emission of NO (5.3 µm)
120 to 200 km
• CO2 15 µm (LTE and non-LTE)
important below 120km
• IR emission by ozone and water vapor
in the middle atmosphere
Response to the
solar storm
during April, 2002
Mlynczak et al., Geophys. Res. Lett., 30(21), 2003.
TIMED/SABER
observations of
NO cooling
Airglow
O2
O2 (1∑) emission
O3
JH
JSRC, Ly-a
JH
1D
Q4
1∑
Q2
Q1
1∆
A1
630
nm
g
A2
762
762
nm
nm
Q3
1.27 µm
3P
O
After Mlynczak et al. [1993]
A3
3∑
O2
Burrage et al. [1994]
Vertical temperature structure (solstice)
WACCM simulations - solar max. conditions
130 K
Summer
230 K
Winter
Why is the mesopause not at its radiative equilibrium temperature?
Gravity waves 1.
ALTITUDE
80
65
50
After Holton & Alexander [2000]
•
•
Gravity waves are small scale waves mainly generated in the
troposphere by mechanisms such as topography, wind shear, and
convection.
Gravity wave amplitudes increase as they propagate upwards
(conservation of momentum).
Mean zonal wind at solstice
W
E
Summer
Winter
UARS reference atmosphere project
Summer
Fx> 0
80
ALTITUDE
Winter
Fx< 0
70
60
-90
EQ
+90
LATITUDE
After Holton & Alexander [2000]
•
•
Gravity wave momentum deposition drives a meridional circulation from
summer to winter hemisphere
Mass continuity leads to vertical motion and so adiabatic heating in the
winter and cooling in the summer. Observed temperatures are 90K
warmer in the winter and 60K cooler in summer than radiative
equilibrium temperatures
Transport affects constituent distributions
UARS reference
atmosphere project
Marsh & Roble, 2002
HEIGHT
O2 , N 2
HEATING
Schematic representation of
solar heating
-90
EQ
+90
O3
H 2O
HEATING
LATITUDE
HEATING
SR
After Forbes [1987]
Noon
SS
LOCAL TIME
Atmospheric Tides
• Atmospheric solar tides are globalscale waves in winds, temperatures,
and pressure with periods that are
harmonics of a 24-hour day.
• Migrating tides propagate westward
with the apparent motion of the sun
• Migrating tides are thermally driven
by the periodic absorption of solar
radiation throughout the atmosphere
(UV absorption by stratospheric
ozone and IR absorption by water
vapor in the troposphere.
• Non-migrating tides are also present
in the upper atmosphere and can be
caused by latent heat release from
deep tropical convection or the
interaction of tides and gravity waves
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Meriodinal wind at noon local time observed by UARS
McLandress et al. [1996]
GSWM-98 migrating diurnal tide (Equinox)
Hagan et al. [1999]
GSWM-98 migrating semi-diurnal tide
Migrating
thermospheric
tides
Hagan et al. [2001]
Many waves are always present
Migrating diurnal component
Combined field
Simulated winds at the equator
Composition
Primary constituent
Hydrogen
2,500 km
Helium
700 km
Oxygen
200 km
Nitrogen
0 km
Total density height variation
Ideal gas law
Hydrostatic balance
where
Which leads to:
In the “homosphere,” where eddy diffusion tends to mix the atmosphere,
the mean molecular weight is almost constant (m ~ 28.96 amu), and the
density will decrease with a mean scale height of ~ 7km.
Above about 90km, constituents tend to diffuse with their own scale
height (Hi = kT/mig) as the mean free path becomes longer. This is
the “heterosphere.” The ith constituent (assuming no significant
sources or sinks) will have the following gradient:
Constituents with low mass will fall off less rapidly with height, leading
to diffusive separation.
Diffusive separation
Turbopause
From Richmond [1983]
To recap…
• Above the turbopause (~105km), molecular
diffusion causes constituents to drop off
according to their mass.
• Below, the atmosphere is fully mixed:
• 78%N2, 21%O2, <1% Ar, <0.1% CO2
• Density decreases with a mean scale height:
H = kT/mg ~7km
• The lower thermosphere is also the transition
from a molecular to atomic atmosphere.
What about chemistry?
Recall:
If there’s chemical production or loss of a minor constituent then
this equality will not hold and a diffusive flux occurs. Above the
turbopause this will be:
Where Di is the diffusion coefficient:
From Richmond [1983]
Continuity equation
The total diffusive flux will include both molecular and
eddy diffusion terms:
So, neglecting transport, we now have a continuity
equation for the ith constituent:
The distribution of ozone
Chapman chemistry
O2  h  O  O
O  O2  M  O3  M
O3  h  O2  O
O  O  M  O2  M
O  O3  2O2
Zonal mean Ox loss rates 2.5ºN
Catalytic cycles
Mesosphere
O  OH  O2  H
H  O2  M  HO2  M
O  HO2  O2  OH
O  OH  O2  H
H  O3  O2  OH
net : O  O3  2O2
net : 2O  O2

Stratosphere

[GSFC, NASA]
Nitric Oxide in the lower-thermosphere
Production:
N( D) + O2  NO + O (fast)
2
Equatorial NO
N( 4 S) + O2  NO + O (slow)
Loss :
NO + h  N( 4 S) + O
N( 4 S) + NO  N2 + O
[a "canabalistic" reaction]
(Barth et al., 2003)
N(2 D) production mechanisms
electron impact:
N2  e*  N(2 D) + N
(e * secondary/photo- electron)
dissociative recombination:
e.g. NO+ + e  N(2 D) + O
Produced by (1-10 keV)
precipitating electrons and
solar soft X-rays (2-7 nm)
Thermosphere: SNOE Nitric Oxide Obs.
3 EOFs = 80% of var.
Solar forcing of the neutral
atmosphere
• UV/EUV
• Precipitating particles in
auroral regions
• Solar proton events (SPEs)
• Highly-relativistic electrons
(HREs) >1MeV
• Galactic cosmic rays
upper panel: WACCM temperature, ozone, and water vapor for July solar minimum conditions.
lower panel: Solar min/max percentage differences. Data only plotted where differences are statistically significant (95%
confidence level).
Mesosphere/Stratosphere response
MLS ozone (30S-30N) vs.
200-205 nm solar flux
DeLand et al., 2003
Hood & Zhou, 1998
[NASA LWS report]
Solar Proton Ionization Rates
Coupling processes
•
•
•
Downward transport of
thermospheric nitric oxide by
~1keV electrons
NOy production in lower
mesosphere and upper
stratosphere via energetic
electron precipitation (4-1000
keV)
Both processes lead to
stratospheric ozone destruction
POAMII ozone SH 30km
(2xAp)
Callis et al. 1998
Randall et al. 1998
The End