Radiative cooling in the NLTE region of the mesosphere and lower thermosphere—Global energy balance

Adv. Space Res. Vol. 7, No. 10. pp. (10)S—(10)15, 1987 Printed in Great Britain. All rights reserved.

0273—1177/87 $0.00 + .50 Copyright © 1987 COSPAR

RADIATIVE COOLING IN THE NLTE REGION OF THE MESOSPHERE AND LOWER THERMOSPHERE GLOBAL ENERGY BALANCE —

Robert E. Dickinson,* Raymond G. Roble** and Stephen W. Bougher*** *Atmospheric Analysis and Prediction Division, **High Altitude Observatory, * * * High Altitude Observatory and Advanced Study Program, National Centerfor Atmospheric Research, tP. 0. Box 3000, Boulder, CO 80307—3000, U.S.A. ABSTRACT Radiative cooling in the mesosphere and lower thermosphere is predominantly from 15—pm emissions of CO 2. Above 120 km, complete NLTE cooling from NO becomes more important. Above 100 km, both the CO2 and the NO cooling are proportional to concentrations of atomic oxygen which are dynamically controlled and poorly characterized by observations. Furthermore, the rate for energy exchange between O and C02(ti2 = 1) is very poorly known. CO2 is close to LTE throughout the mesosphere, but small departures from LTE between 65 and 80 km may be important for questions of remote sensing. Remote sensing for trace gases, e.g., 03 and H2O, must consider NLTE effects in the mesosphere. A global mean column model for aeronomy processes above 65 km gives a reasonable agreement with observed temperatures, suggesting that radiative balance may be possible without the need for including eddy cooling or gravity wave heating. INTRODUCTION The circulation and thermal structure of the mesosphere/lower thermosphere (i.e., the altitude range of about 60 to 140 km) are determined by forcing (absorption of solar radiation, auroral heating by currents and particles, release of chemical energy, and wave disturbances incident from below) balanced by temperature—dependent radiative cooling. Below 120 km, the dominant cooling is from the emission of 15—pm radiation by the ~‘2 band of CO2, and above 120—km, cooling by the 5.3—pm band of NO and molecular thermal conduction are the dominant terms balancing the forcing. In the past, the various physical processes in the mesosphere/lower thermosphere have been largely studied in isolation. Studies of the individual process continue to be important, but since they are highly coupled by motions, comprehensive three—dimensional models are becoming an additional important research tool. Application of such models to the thermosphere above 120 km or to the stratosphere below 60 km is now being done by several groups e.g., /1/, both because observations have been more abundant and because dynamical and radiative processes are less complex. In particular, the treatment of thermal infrared radiative cooling by CO2 has been especially difficult because of the importance of NLTE processes in conjunction with radiative transfer considerations. Many bands contribute to the total cooling by C02 treatment of line shape is complex. Above 100 km where the theory, in principle, becomes simpler, cooling is crucially dependent on energy exchange with atomic oxygen and thus depends on concentrations of 0 and the energy exchange rate, both poorly known. In a previous review of this topic, Dickinson /2/ made the following points: (i) We now know how to calculate with some confidence the energetic processes of CO2 molecules that are important for radiative cooling. (ii) The primary uncertainty for the calculation of CO2 cooling above 100 km is the rate at which CO2 LI2 = 1 vibrations are quenched by collision with atomic oxygen. Besides atomic oxygen concentrations being variable, dynamically controlled, and poorly characterized by data, the rate for C02(ti2 = i)+ O—+ CO2 + 0 is uncertain by an order of magnitude.

t The National Center for Atmospheric Research is sponsored by the National Science Foundation. (10)5

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R. E. Dickinson, R. G. Roble and S. W. Bougher

(iii) Between 100 and 140 km, solar heating is balanced primarily by the complete NLTE radiative cooling of CO 2 and NO, which is linearly proportional to atomic oxygen concentrations, as well as to the concentrations of CO2 and NO, respectively. The mixing ratio of all these species is modulated by dynamic processes. Hence, a complete solution for radiative cooling requires a three-dimensional dynamic model. (iv) A current major challenge is to improve the efficiency of the calculation of C02—NLTE cooling to allow inclusion in three-dimensional dynamic models, without seriously degrading the accuracy of the calculation. The total CO2 cooling calculated in Dickinson /2/ using the CIRA 19723Standard Atmosphere (January— 1 is shown in Figure 1. July) and an energy exchange rate between 0 and CO2 of 2.10_Is cm CO 2 COOLING (Kday~) 110

I I

100

L_~Jl I

_~_.....—l 0

60

50

60

40

0

20

20

July

40

60

January LATITUDE

Fig. 1. The zonal average infrared cooling due to CO2 (K day—’) from /2/ using pressures and temperatures from the CIRA July—January profiles. The hatched negative area indicates warming.

This paper is intended to update the Dickinson /2/ review. Another review by Leovy /3/ is also noted. First, the physics and computation of C02—NLTE radiative cooling are briefly summarized. Then, new information from recent studies relevant to mesospheric radiation is reviewed. Finally, progress toward interfacing the NLTE code with a dynamical model is reported. This progress consists of linking the radiative code with a wide range of other aeronomical processes in a one—dimensional global average column model. This model is intended as a test bed for algorithms to be incorporated into our three— dimensional thermosphere GCM, the lower boundary of which is now being lowered to an altitude of 65 km. PHYSICS OF NLTE CO2 RADIATION The various low—lying energy levels of CO2 important for mesospheric radiation are shown in Figure 2. A CO2 molecule is excited to the i.’2 = 1 bending mode vibration by energy supplied from the molecular kinetic energy, by absorption of a 112 photon or by transfer of a vibrational quanta from a higher energy level. The mode then emits probabilistically a 15—pm photon with an average inverse lifetime of A = 1.5 s ~. It is also deexcited by collisional processes. Collisional energy transfer is slow, requiring many thousands of encounters to occur. Nevertheless, the rate of collisional deexcitation in the lower atmosphere is fast (i.e., A > A), so that the U~ = 1 and higher energy levels are maintained in a Boltzmann distribution. This condition is referred to as local thermodynamic equilibrium (LTE), and radiative transfer is readily calculated according to bulk radiation theory with emission proportional to the Planck function.

NLTE Radiative Cooling

I

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NEAR IRBANDS

~(0001)

2000

~ 1,000

_(O3~O) _(O3’O)

_(O2~OI (02°01

—(II’O)

(1000)

—

15 MICRON COOLING

—(OI’O) Bending

Symmetric Stretch

Asymmetric Stretch

V 1

V5

CO0 VIBRATIONAL STATES 0

—(000)

Fig. 2. The lower CO2 energy levels responsible for infrared emission in the mesosphere and lower thermosphere. The topmost level shown, 113 = 1, and higher levels absorb solar radiation during the daytime, and may thus influence the population of the v~and 112 states through relaxation or energy transfer to these levels.

At a pressure of around 10 pb (altitude 80 km), A A /2/ and the population dynamics of the CO2 molecular levels must be appropriately treated. For some purposes, such as remote sensing, even a 1% departure from LTE populations may be significant and such a difference becomes possible above 1 mb (— 50 km). However, departures from LTE may not be realized for the stronger bands except at much lower pressures, because with optically thick conditions, repopulation by absorption of photons incident from nearby levels helps maintain LTE. For such conditions, departure from LTE is indicated not by the largenes8 of the ratio A/A but rather by the ratio 0.5AP/A, where P is the probability that an emitted photon escapes to space. For example, this term does not become 0.1 for the strongest 15—pm band until an altitude of 86 km is reached, so that LTE cooling is expected for this band throughout the mesosphere. On the other hand, weaker bands, still significant for radiative cooling, begin to show significant NLTE effects by an altitude of 65 km (0.1 mb). The transition from LTE NLTE 1 toper CO is seen by noting that the cooling rate Q from a given band at a given level (in photons s 2 molecule) is given by

Q

—

A(Pexp(—960/T)

—

E) + R

(1)

1+~AP/A

—

where T = temperature, AE = the net rate of exchange of photons with other layers, and R the net exchange of photons to other isotopes or to higher energy levels. A degeneracy factor of 2 multiplying the exponent has been combined with a factor of 1/2 for the fraction of photons emitted upward. The term R depends on population differences from LTE and on the rate of collisions pressure p. Hence, R goes to zero for sufficiently large or small p, where the different bands become uncoupled. For large p (LTE limit, all bands below 65 km):

Q

A(Pexp(—960/T)

—

E),

(2)

and for the small p complete NLTE limit, appropriate above 100 km

Q

2Aexp(—960/T).

(3)

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R. E. Dickinson, R. G. Roble and S.

W.

Bougher

As seen in equation (1), there are essentially three computational issues that must be addressed: (i) The radiative transfer calculation of P and E, made complex by the need to allow for Voigt line shapes and many lines in many bands (neglect of E gives the more simply calculated NLTE cool— to—space aproximation). (ii) Calculation of R, the net exchange with other excited energy levels. This process is important for determining the populations of higher energy levels and 112 = 1 levels of the less—abundant CO 2 isotopes. However, R appears to contribute only a small fraction to the total net cooling at any level and to be somewhat insensitive to the uncertainties in the relevant exchange rates. (iii) Calculation of A, the exchange with molecular kinetic energy. Past computations have emphasized the excitation of 112 = 1 by conversion from kinetic energy of N2 and 02, whose rate at the low temperatures of the mesosphere was inadequately characterized until Allen et a!. /4/. However, as discussed by Dickinson /5/, V—T exchange with atomic oxygen 0 is at least a factor of 100 faster, and so for terrestrial concentrations of 0, it becomes the dominant cooling mechanism above 100 km. Knowledge of this rate is also crucial for modeling the thermosphere of Venus /5, 6/. This 0 + C02(v2 = 1) exchange rate is one of the key unsolved observational issues in study of the lower thermosphere. Little progress has been made on this issue in the last 15 years in spite of a large effort devoted to less important questions. The earth’s lower thermosphere will remain the “ignorosphere” until we know this rate! REVIEW OF RECENT PROGRESS IN THE STUDY OF MESOSPHERIC

NLTE-RADIATION Ozone The 9.6—pm (113 = 1) band of ozone gives significant cooling only up to the lower mesosphere (p > 0.2pm) as shown by Fomichev and Shved /7/ who develop an LTE model for ozone radiative cooling. However, NLTE effects begin to become significant about 1 mb, at least for remote sensing, and are large for p < 0.1 mb /8, 9/. The populations of ozone 113 vibrations are pumped by upwelling radiation and release of chemical energy into vibrations upon recombination of 0 + 02 into 03 (Figure 3). 1.10.9

\~ ._Night 0.3

0 00

Level of NLTE

_—

Oay

20

1.0

I 0.5

0.1

Fig. 3. The daytime and nighttime ozone NLTE source term (ratio of emission to LTE emission) inferred from Solomon et a!. /8/. An equatorial profile for ozone and temperature from the LIMS data was used. Depopulation occurs by spontaneous emission, but radiative excitation maintains temperatures not too far from LTE. During the day, chemical excitation drives ozone V~= 1 populations to values much higher than LTE. The light vertical lines indicate ±10%departure from LTE. The dots indicated the levels where the day and night populations first cross this boundary, near 0.3 mb for both day and night. Water Vapor Manuilova and Shved /10/ have treated the question of NLTE emissions from the 2.7—pm and 6.3—pm bands of H20 from 40—115 km, including a correct radiative transfer treatment of the optically opaque 6.3—pm band. During the day, net solar heating occurs and at night thermal cooling. Maximum rates of ~ 0.5°day’ are found, making these terms of marginal significance for radiative balance considerations.

NLTE Radiative Cooling

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Couoling to CO~Solar Heating Lopez—Puertas et at. /11, 12/ have developed an NLTE model for CO

2 radiation that includes coupling of 112 levels (i.e., 15—pm radiation) to absorption of near—infrared solar radiation, especially in the 2.7— pm and 4.3—pm bands. These authors (assuming a U.S. 1976 Standard Atmosphere temperature profile) find that above 75 km 15—pm emission is significantly enhanced by quanta from the solar absorption (Figure 4). However, the transfer of kinetic energy (i.e., cooling) to 15 pm is apparently only slightly influenced by the solar coupling. They also find overhead sun near—IR heating rates> 1°day’ between 60 and 100 km and peaking at 2°day’ around 75 km (Figure 5). A global average would divide this rate by approximately 3, but this heating is still significant since it maximizes where other solar heating and 15—pm cooling are minimum. The conversion of a 4.3—pm quanta into heat results primarily from the rapid transfer of its energy to N2 vibrations which do not radiate, but are slowly quenched by transferring vibrational or kinetic energy to other species, especially 0 in the latter case. 20

p-

, I

20

I

110

I

I

I

I

I

110

-

00

00 Night

Net! ‘~

90

KE Loss

i

:From Solar Absorption

w ~80.

.~

~70-

90

-

‘~

.

w ~60

-

~7o

-

~‘

:

-

2 60

60

50

50

Net Near IR Heating (Overhead Sun)

0

I

2

3

400

COOLING RATE (Degrees/Day)

Fig. 4. From Lopez—Puertas et at. /11/, using a 1976 U.S. Standard Atmosphere. Curve 1 indicates the noontime cooling rate from CO2 15—pm emission, curve 2 that representing vibrational transfer from other levels, Net KE loss the actual cooling from kinetic energy, and Night the cooling (from kinetic energy) during nighttime.

0.5

.0

1.5

2.0

2.5

3.0

HEATING RATE (Degrees/Day)

Fig. 5. From Lopez—Puertas et at. /12/, the net near—JR heating at noon.

Effect of Waves on Cooling Because of the nonlinear dependence of radiative emission on temperature, spatial oscillations in temperature increase the radiative cooling. For example, Dickinson and Bougher /6/ found that the large day—night temperature contrast in the Venus thermosphere significantly enhances the global mean cooling for a given global mean temperature. Fomichev et a!. /13/ have examined the likely effect of gravity wave and tidal temperature oscillations in enhancing radiative cooling in the lower thermosphere. Gravity waves were found to contribute ‘—‘O.S°day’around 100 km and semidiurnal tides over 1°day~at that level. Recent Studies of Latitude—Height Radiative Cooling Haus /14/ has developed a reasonably accurate model for 15—pm CO2 cooling from 30—110 km. Fomichev et a!. /13/ have calculated cooling rates for C02, H2O, and 03 over the same altitude range and summed these into latitude—height cooling plots comparable to our Figure 1. COMMENTS ON REMOTE-SENSING QUESTIONS The study of Solomon et a!. /8/ indicates that, for remote sensing of O~,NLTE effects start to become significant at an altitude of around 50—55 km. Remote sensing of temperature from CO2 emission now readily reaches altitudes of 65 km. Can small departures from LTE account for some of the disagreeJASR 7:10—B

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R. E. Dickinson, R. G. Roble and S. W. Bougher

ments between satellite and rocket or lidar measurements? For example, Remsberg /15/ shows LIMS temperatures between 55 and 64 km are typically cold by 1—2°C compared to lidar measurements and by 6—7° at 67 km. The question is how different are vibrational temperatures TVIB from kinetic temperatures. Figure 6 shows TVIB T for the fundamental 15—pm band of CO 2 from 50—80 km from the model of Dickinson /2/. In the upper mesosphere, vibrational temperatures are warmer over the cold sunimer pole, and are colder over the warm winter pole. Over the equator, the difference is less than a degree below 75 km, and over the poles less than a degree below 70 km. Below 65 km, vibrational temperatures are colder by a few tenths of a degree, especially in winter high latitudes. Hot and isotopic bands are expected to be farther from LTE. We examine this departure below 65 km (Figures 7, 8, 9). Again, departures are less than 1°except some of the isotopic bands can be cold by 2—3°over the winter pole in the altitude range 60—65 km.

—

—

20~°(~

T~15 T [C’

2 I)]

T515

::

-

—

T [C°O~°(vz

_~.2

I)]

IIIII~]~V\I\

-0.08

-

w

=

w

47O~/ >0

65

60

I

40

20

0

JULY

004

20

40

I

60

50

60

40

JANUARY

20

0

‘

204060

JULY

JANUARY

LATITUDE

LATITUDE

20~61/2 = 1 level from LTE in the mesosphere expressed as vibrational Fig. 6. The departure of the C’ from the same calculation as Figure 1; (a) shows the range 65—80 km temperature-kinetic temperature, and (b) the range 50—65 km. Tvis

—

T [C120’60’7(v

2OIGOIS(v 2 I)]

Tvis — T [CI

i

2I)]

-0.5

-ü

~

50 ~ 60

~~O2

-

40

20 JULY

0

20 40 JANUARY

LATITUDE

60

2O’60’T.

5~

I~J

60

___

I

40

I

20 JULY

I

T~I

0

20 40 JANUARY

60

LATITUDE

Fig. 8. Same as Figure 6a but for C12O580’8.

Fig. 7. Same as Figure 6a but for C’ Our calculated NLTE emissions are not directly applicable to remote—sensing approaches, which generally sense a spectral region rather than a particular CO 2 band. However, the contribution of the weaker bands to a sensed signal should generally not be greater than their contribution to cooling rates. If so, we can infer that neglect of NLTE effects should give negligible error for remote sensing of temperatures from CO2 radiances below 65 km, to the extent that emissions from layers above 65 km are not important, or adequately accounted for. Conversely, any inference of temperature from CO2 radiance above 70 km should correct for departures from LTE. Note that we consider here smooth mean temperature profiles. NLTE effects may be significant to somewhat lower levels for short vertical-scale wave disturbances.

NLTE Radiative Cooling

HOT BANDS 20~°(v

T~ 65

(10)11

1a T [C’

2 2)]

I’I’I’I’

(~

~0.I5

-0~5

\\ -025

c~

~

~sQ~ ~~0.05 60

4020

0 I

JULY

20

40

I

60

JANUARY LATITUDE

120~6~2

=

2 (first hot band) levels.

Fig. 9. Same as Figure 6a but for the C A ONE-DIMENSIONAL COLUMN MODEL FOR STUDY OF MESOSPHERETHERMOSPHERE COUPLING AND GCM PARAMETERIZATION We are currently extending our thermospheric general circulation model (TGCM) down to 65 km and coupling into it not only a realistic treatment of CO 2 radiation but also the chemistry and transport of ionic and minor neutral species. As a preliminary step in this development, we are studying the relevant processes in a one-dimensional global average column model that extends from about 65 to 500 km. Roble et a!. /16/ has described the model as formulated for the thermosphere above 100 km. We have recently extended this model down to 65 km and included the chemical, radiative, and dynamic processes appropriate for the mesosphere. As previously /1/, we included 0, 02, and N2 coupled through a major constituent equation. We add as minor species with transport and appropriate photochemistry: 4S), NO, diffusion H N( 2D), NO 2O, H, H2, CH4, CO, CO2, and in photochemical equilibrium with the above: 0~,O(’D), N( 2 OH, H3O2, HO2. For ions in the E— and F—ionospheric regions, we calculate O~,Ot, NO+, N+, N~in photochemical equilibrium. The ion—neutral chemical reactions and rates used in the model are obtained from Torr /17/ and the minor neutral constituent chemical reactions and rates that are used in the mesosphere are the same as those prescribed by Solomon et a!. /8/ and Allen et at. /19/. The model has separate thermodynamic equations for electron, ion, and neutral temperatures, and appropriate energy exchange processes. Besides collisional exchange from thermal electrons and ions, the neutral gas al8o receives energy from Joule dissipation and auroral electron precipitation and from solar photoelectrons, 02 absorption in the Schumann—Runge continuum and bands, and 03 absorption in the Hartley bands, excess energy from exothermic ion—neutral and neutral—neutral chemical reactions including atomic oxygen recombination and O(’D) quenching. Cooling includes not only the CO2 emission but also that of NO, some 63—pm 0 emission, and cooling by molecular conduction. The model uses solar flux values appropriate for solar minimum conditions /20/. The model is timedependent and self—consistently calculates the global mean structure from an arbitrary initial state by marching forward in time until a steady state is achieved. The calculated global mean electron, ion, and gasFigures temperature solar for minimum con2 Hz’) areneutral shown in 10 andprofiles 11. Alsoforshown comparison ditions are globally (F10.7 averaged = 70 x values 1022 obtained Wm from the MSIS—83 empirical model of Hedin /21/ and in Figure 10 also from the 1976 U.S. Standard Atmosphere. For this calculation, we assumed an energy exchange rate between 0 and CO 3 s1. The calculated exospheric temperature is 730 K in reasonable agreement with that 2 ofpredicted 4•10~~ cm by the MSIS—83 model (740 K). The calculated profile in the lower thermosphere is somewhat colder than MSIS—83 between 140 and 160 km and warmer than MSIS—83 between 90 and 120 km. Given the uncertainty in the atomic oxygen vibrational energy exchange rates with CO 2 and NO, these differences are probably not significant. The calculations do not include thermospheric eddy cooling and gravity wave heating thus suggesting that radiative balance may possibly be achieved in the global mean throughout the mesosphere and thermosphere. This balance between molecular thermal conduction and solar heating with thermal IR cooling is shown in Figure 12.

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R. E. Dickinson, R. G. Rob)e and S. W. Bougher

GLOBAL MEAN 150 I

20

~:

I

I

GLOBAL MEAN

EE

I

I

MS~~”

~

~

I

200

I

300 400 TEMPERATURE (K)

I

500

65

I

I

—30

I

I

I

I

I

-20

-10

I

I

I

-

240 -

80 -

-

E 280

II: I

I

I

~IIIIiII

I

I

j

-

/Te

I~

600 800 1000 TEMPERATURE

400

I

/

j

200

600

Fig. 10. Global mean temperature profile (solid line) calculated using the new NCAR column aeronomy model /16/. Dots indicate ion temperature, dot—dashes indicate electron temperature, and dashes the observed neutral temperature profile, based on the MSIS atmosphere above 105 km and the 1976 U.S. Standard Atmosphere below that. 115

I

I

1200

I

I

1400

Fig. 11. Same as 10, but showing also the upper thermosphere.

I

I

I

I

I

0

I

I

tO

I

20

30

DEGREES PER DAY Fig. 12. Global mean energy balance for the computation of Figure 10, showing the balance between net heating (solar, chemical, joule, etc.) heating by molecular conduction from higher levels and the thermal JR cooling from CO 3 and NO. The calculated major and minor constituent distributions consistent with the calculated thermal structure are shown in Figures 13, 14, and 15. There is reasonable agreement between the calculated distri3 near 93 km. bution of 0, 02, and N2 with that of the MSIS—83 model above 90 km with the disagreements largely resulting from the differences of in N(2D), temperature Atomic oxygen peaks at and 3 x 1011 cm The calculated distributions N(4S),profile. and NO in the thermosphere mesosphere are shown in Figure 14. These distributions were obtained assuming a 50:50 branching ratio of N(2D):N(4S) for both photoelectron and auroral electron dissociation of N 3 near 107 km. 2 NO peaks at io~cm

NLTE Radiative Cooling

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The calculated distributions of H

2O, OH, HO2, H, 03, and O(’ D) in the mesosphere and lower thermosphere are shown in Figure 15. Also shown in the figure is the H distribution obtained from the MSIS—83 model for comparison. The H distribution was calculated considering Jean’s escape mechanism and contributions from the polar wind and exospheric charge exchange as discussed by Liu and Donahue /22/. Finally, the calculated global mean ion structure in the thermosphere is shown in Figure 16, along with a globally averaged profile obtained from the Chiu /23/ empirical model of electron density. The molecular ions NO+ and O~dominate the lower ionosphere up 2to s~, 180 km andmean) O~dominates that(0.35 altitude. global and Jouleabove heating ergs In these calculations, auroral heating ergs cmwere included in the calculation of the global mean cm2 s~1,global mean) in addition to (0.25 solar heating temperature structure in order to bring the calculated exospheric temperature into agreement with that predicted from MSIS—83. GLOBAL MEAN

i

~

GLOBAL MEAN

~

~‘QMSIS

345678 9101II21314I5I6 LOG~o (Major Species Composition, cm3>

Fig. 13. Calculated profiles of major species compared to observation.

-5—4-3-2-1012345678 LOG 3) 10 (Minor Nitrogen Species/cm

Fig. 14. Calculated odd nitrogen profiles.

GLOBAL MEAN I

I

20

I

I

I

I

I

GLOBAL MEAN I

I

I

I I

I

ISO

I I

I

I

I

I

I I

I

I

I

I

I

I I

I

I

I I

I I

:__

~VcKi,

LOG

0> 10 (ton/Electron Concentration/cm

Fig. 15. Calculated global mean odd hydrogen H 20 and odd oxygen profiles.

60 —6

I

I

—4

I

I —2

I

I

0

I

2

I

4

6

8

10

LOGIO (H-Species/cm3>

Fig. 16. Calculated ionic species and electron concentration, the latter compared to observation.

DISCUSSION AND CONCLUSIONS Infrared cooling by 15—pm radiation is the dominant energy loss process below 120 km. The complex energy exchange processes important around the mesopause are now tractable but still require further simplification for use in dynamical models. Near—infrared solar heating with CO2 and possibly H2O as absorber appears significant for the energy balance around the mesopause and should be included for accurate radiation calculation. However, these terms involve poorly known energy exchange processes, so their precise calculation is difficult.

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R. E. Dickinson, R. G. Roble and S. W. Bougher

Remote sensing of H

2O and 03 in the mesosphere should consider NLTE processea, and departures from LTE of CO2 emissions may be significant for remote sensing of temperature above 65 km, depending on the fraction of radiation being sensed from the hot and isotopic bands. Conversely, in the region of complete NLTE above 80 km, measurement of CO2 emissions, e.g., /24, 25, 26/, together with simultaneous measurement of 0 and CO2 concentrations and temperature, could improve our understanding of the energy exchange processes. The radiative processes above 100 km are, in principle, simpler, being approximated by complete NLTE. However, dependence on variable 0, NO, and CO2 concentrations implies that a three—dimensional dynamics/compositional model is required to calculate them. Preliminary results from a one-dimensional global mean column version of such a model suggests that it should be possible to construct such a three—dimensional model. Reasonable agreement with observed global mean temperature profile is achieved without the need to include any heating from wave absorption or cooling by eddy mixing. However, more detailed examination of energy balance allowing for the latitudinal variation of cooling processes as in a multidimensional model is certain to show that questions concerning energy sources remain. REFERENCES 1.

R. E. Dickinson, E. C. Ridley, and R. G. Roble, Thermospheric general circulation with coupled dynamics and composition, J. Atmo8. Sci. 41, 205—219 (1984).

2.

R. E. Dickinson, Infrared radiative cooling in the mesosphere and lower thermosphere, J. Atmo8. Terr. Phys. 46, 995—1008 (1984).

3.

C. B. Leovy, Infrared radiative exchange in the middle atmosphere in the 15 micron band of carbon dioxide, in: Dynamics of the Middle Atmo8phere, eds. J. R. Holton and T. Matsuno, Terra Scientific Publishing Company 1984, 355—366.

4.

D. C. Allen, T. Scragg, and C. J. S. M. Simpson, Low temperature fluorescence studies of the deactivation of the bend—stretch manifold of CO2, Chem. Phys. 51, 279—298 (1980).

5.

R. E. Dickinson, Venus mesosphere and thermosphere temperature structure, I. Global mean radiative and conductive equilibrium, Icarus 27, 479—493 (1976).

6.

R. E. Dickinson and S. W. Bougher, Venus mesosphere and thermosphere, I. Heat budget and thermal structure, J. Geophys. Re8. 91, 70—80 (1986).

7.

V. I. Fomichev and C. M. Shved, Parameterization of the radiative flux divergence in the 9.6pm 03 band, J. Atmos. Terr. Phys. 47, 1037—1049 (1985).

8.

5. Solomon, J. T. Kiehi, B. J. Kerridge, E. E. Remsberg, and J. M. Russell, III, Evidence for nonlocal thermodynamic equilibrium in the p, mode of mesospheric ozone, J. Geophys. Res. in press (1986).

9.

W. T. Rawlins, Chemistry of vibrationally excited ozone in the upper atmosphere, J. Geophys. Res. 90, 12283—12292 (1985).

10. R. 0. Manuilova and G. M. Shved, The 2.7 and 6.3 pm H20 band emissions in the middle atmosphere, J. Atmo8. Terr. Phys. 47, 413—422 (1985). 11. M. Lopez—Puertas, R. Rodrigo, A. Molina, and F. W. Taylor, A non—LTE radiative transfer model for infrared bands in the middle atmosphere, I. Theoretical basis, and application to CO2 15 pm bands, J. Atmo8. Terr. Phy8. in press (1986). 12. M. Lopez—Puertas, R. Rodrigo, J. J. Lopez—Moreno, and F. W. Taylor, A non—LTE radiative transfer model for infrared bands in the middle atmosphere. II. CO2 (2.7 and 4.3 pm) and water vapour (6.3 pm) bands and N2 (1) and 02 (1) vibration levels, J. Atmos. Terr. Phys. in press (1986).

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