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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 (d T
z)w vT Q
c p
Adiabatic heating
Heat advection
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
HeatChemicalPotentialO, e,…
QuantumInternal
CO2(2), O2(1)…
UV, Vis.,IR Loss
AirglowCooling
After Mlynczak et al.
Solar UV Energy Deposition
Courtesy Stan Solomon
Hartley continuum
Schumann-Rungecontinuum
S-R Bands
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
TIMED/SABER observations of
NO cooling
Mly
ncz
ak
et
al.,
Geop
hys.
Res.
Lett
., 3
0(2
1),
20
03
.
Response to the solar storm during April, 2002
Airglow
O2 O3
O2O
Q4
Q2
Q1
Q3
A1
630 nmA2
762 nm
JHJSRC, Ly-a
JH
3P
1D
1∑
A3
1.27 µm
1∆
3∑
g
762 nm
After Mlynczak et al. [1993]
O2 (1∑) emission
Burrage et al. [1994]
Vertical temperature structure (solstice)
Why is the mesopause not at its radiative equilibrium temperature?
Summer Winter
130 K 230 K
WACCM simulations - solar max. conditions
Gravity waves 1.
• 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).
ALT
ITU
DE
65
80
50
After Holton & Alexander [2000]
Mean zonal wind at solstice
UARS reference atmosphere project
E W
Summer Winter
• 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
After Holton & Alexander [2000]
LATITUDE
EQ-90 +90
ALT
ITU
DE
Summer Winter
70
80
60
Fx> 0 Fx< 0
Transport affects constituent distributions
Marsh & Roble, 2002UARS reference atmosphere project
O2, N2
O3
H2O
EQ-90 +90
NoonSR SS
HE
AT
ING
HEATING
HE
AT
ING
LOCAL TIME
LATITUDEHE
IGH
TSchematic representation of
solar heating
After Forbes [1987]
Atmospheric Tides• Atmospheric solar tides are global-
scale 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 aGIF 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
Hagan et al. [2001]
Migrating thermospheric
tides
Many waves are always present
Simulated winds at the equator
Migrating diurnal component Combined field
Composition
Primary constituent
Nitrogen
Oxygen
Helium
Hydrogen
0 km
200 km
700 km
2,500 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
From Richmond [1983]
Turbopause
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.
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:
Recall:
What about chemistry?
From Richmond [1983]
Continuity equationThe 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
O OH O2 H
H O2 M HO2 M
O HO2 O2 OH
net : 2O O2
O OH O2 H
H O3 O2 OH
net : O O3 2O2
Mesosphere
[GSFC, NASA]
Stratosphere
Nitric Oxide in the lower-thermosphere
Production:
N(2D) + O2 NO + O (fast)
N(4 S) + O2 NO + O (slow)
Loss:
NO + h N(4 S) + O
N(4 S) +NO N2 + O
[a "canabalistic" reaction]
Equatorial NO
(Barth et al., 2003)
Produced by (1-10 keV) precipitating electrons and solar soft X-rays (2-7 nm)
N(2D) production mechanisms
electron impact :
N2 e* N(2D)+N
(e * secondary/photo - electron)
dissociative recombination :
e.g. NO+ + e N(2D)+ O
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).
MLS ozone (30S-30N) vs.
200-205 nm solar flux
Hood & Zhou, 1998
Mesosphere/Stratosphere response
DeLand et al., 2003
[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
Callis et al. 1998
POAMII ozone SH 30km
Randall et al. 1998
(2xAp)
The End