J. Qunnr.Spectm~c.Radinr.Transfeer.
Vol. 14,pp. 861-871.PergamonPress 1974.Printedin Great Britain.
RADIATIVE TRANSFER DUE TO ATMOSPHERIC
WATER VAPOR: GLOBAL CONSIDERATIONS
OF THE
EARTH’S ENERGY BALANCE*
ROBERT
D.
CESS
Department of Mechanics, State University of New York,
Stony Brook, New York 11790, U.S.A. zyxwvutsrqponmlkjihgfedcbaZYXWVUT
(Received
14 December
1973)
Abstract-A
simple analytical formulation is presented for describing radiative transfer due to
atmospheric water vapor. The radiative model is then applied to a global energy balance for
earth, and the net infrared flux to space is expressed in terms of the mean surface temperature
and atmospheric lapse rate. Water vapor and clouds are assumed to be the only sources of
infrared opacity. When compared with empirical information, and for a global mean surface
temperature of 288 K, the radiative model indicates a cloud top altitude for a single effective
cloud of 6.8 km. Alternatively, when applied to a more realistic three-cloud formulation, the
model predicts a comparable value of 6.5 km for an average cloud top altitude.
With respect to changes in mean surface temperature, again comparing with empirical
results, a discussion relating to the model suggests that the cloud top altitude decreases with
decreasing surface temperature, which results in the surface temperature being roughly twice as
sensitive to changes in factors such as planetary al&do than for the conventional assumption
of a fixed cloud top altitude. Implications of this are discussed with respect to possible albedo
changes due to atmospheric particulate matter as well as cloudiness as a climate feedback
mechanism.
1. INTRODUCTION
THE MAIN source of infrared opacity within the earth’s lower atmosphere is water vapor,
and numerous
computational
procedures have been presented for evaluating
the atmospheric infrared radiative flux (e.g. Chapter 6 of GOODY,(‘) RODGERS,(~) MANABE and
WETHERALD,(3) RASOOL and SCHNEIDER (4) and DOPPLICK(~)). It is not at all clear, however,
that these various methods are mu&ally
consistent.
Furthermore,
realistic radiativedynamical studies of the earth’s atmosphere require the incorporation
of a large number of
quite complicated phenomena,
and for this reason it would be useful to reduce the radiative
flux calculation to as simple a form as possible.
The purpose of the present paper is threefold. The first objective is to reexamine the
problem of calculating
the infrared radiative flux within the earth’s atmosphere
due to
water vapor, with emphasis upon incorporating
emissivity data which is based upon direct
laboratory measurements.
Secondly, the radiative flux formulation
will be expressed in an
extremely simple analytical form, employing a procedure which is quite analogous to that
used by CESS and KHETAN’~) in describing radiation due to the pressure-induced
opacity
of hydrogen within the atmospheres of the major planets.
Aside from computational
ease, the advantage of a simple analytical formulation
is that
it facilitates physical insight into problems for which radiation
is coupled with other
processes. In this regard, the third objective of the paper is to consider a global mean
*This work was supported by the National Science Foundation
861
through Grant No. K036988.
ROBERTD. CESS
862
energy balance upon the earth’s atmosphere,
with emphasis upon coupling mechanisms
involving cloudiness and albedo changes. The importance
of such mechanisms
has been
accentuated by recent concern over possible man-made modifications
to the earth’s climate.
2. WATER
VAPOR
SCALE
HEIGHT
A prerequisite
to investigating
the water vapor opacity within the earth’s atmosphere
involves specification
of the water vapor scale height. This differs from the atmospheric
scale height, since the water vapor mixing ratio is controlled by condensation,
which in turn
is governed by dynamical processes. As of now numerical models of the hydrological cycle
have not correctly reproduced the distribution
of water vapor within the earth’s atmosphere
(e.g. HUNT(~)).
Alternately,
MANABE and WETHERALD(3) have employed an empirical expression for the
vertical distribution
of relative humidity (global average) within the earth’s troposphere,
based upon a composite of experimental
data, and this expression is
= zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG
(0.77/0.98)(P/P, - 0.02),
PJPwJar
where P is atmospheric pressure, P, the surface pressure, and P, and P,,,,, the water vapor
partial pressure and saturation
pressure, respectively. For present purposes a sufficiently
accurate representation
within the troposphere
is of the form
P,IP,,,,t
= 0.77(P/P,).
(1)
Equation (1) does not apply to the stratosphere,
where the water vapor mixing ratio is
constant,
but as discussed in Section 6, within the limitations
of the present model the
stratosphere
plays an insignificant
role with regard to the overall opacity of the earth’s
atmosphere.
We now wish to rephrase equation (1) in terms of a water vapor scale height. and to
accomplish this P,,,, satmay be expressed as a function of temperature
T(K) by
P w,,,,(atm)
= (2.20 x 106)e-5385’T.
(2)
Equation (2) is simply the Clausius-Clapeyron
equation with the constants
fitting the equation to saturation vapor pressure data. Since
P/P, = exp( -gz/RT)
for a hydrostatic atmosphere, where g is the acceleration due to gravity,
R is the gas constant for the atmosphere,
then upon combining equations
that
P, = Pwse-z/Hw
with P,, , the water vapor partial
(1) and (2) to be
pressure
P,,(atm)
at the earth’s surface, following
= (1.69 x 106)e-s385’Ts,
while the water vapor scale height, H,,,, is
evaluated
by
(3)
z is altitude, and
(l)-(3), it follows
(4)
from equations
(5)
Radiative transfer due to atmospheric water vapor
Furthermore,
with l? = -dT/dz
the troposphere,
then
denoting
H, =
Since the atmospheric
the lapse rate, and taking r to be constant
RTlg
1 + (5385RT/gT,)
863
within
-
scale height is H = RT/g, it follows that
H/H,,, = 1 + 5385RT/gT,.
(7)
It is important to note that for this approximation
H/H, is independent of local atmospheric
temperature.
With g = 981 cm set-’ and R = 0.287 x lo7 cm2 sece2 K-‘, together with r = 6.5K
km-’ and T, = 288K, which are representative
of global mean conditions,
equation (7)
yields H,JH = 0.22, which compares with the value HJH = 0.25* as employed for example
by RASOOL and SCHNEIDER,(~)
and SCHNEIDER.@)
Consider now the application of the water vapor scale height to determining the effective
broadening
pressure P for the rotational lines of water vapor. The broadening pressure is
the total atmospheric pressure, and application of the Curtis-Godson
approximation
(p. 238
of GOODY(‘)) yields
p” = (l/P,,,) j-P dP,.
Employing
equations
(3) and (4), P” is related to P by
‘= (1
+iIJH)”
with p = 0.82P for H,IH
(8)
= 0.22.
3. WATER
VAPOR
EMISSIVITY
Use of the gas emissivity in radiative flux calculations is well known, and in this section
we present an analytic expression for the water vapor emissivity which greatly facilitates
such flux calculations.
Employing the strong-line limit, the water vapor emissivity, E, may be expressed in terms
of a single variable as zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
E
=
c(P,PH,,,).
A quantitative
appraisal of the applicability
of the strong-line limit is given by RODGERS.(~)
Furthermore,
the form of the strong-line parameter, P,P”H,, is consistent with the results
of PENNER and VARANASI’~’for water vapor at elevated temperatures,
as well as the band
absorptance
correlations
for water vapor by EDWARDS and BALAKRISHNAN.(~‘) MANABE
and WETHERALD,
on the other hand, have employed P, H,,,(P)o.7 as the strong-line
parameter.
With the water vapor emissivity expressed as a function of P,pH,,
then the emissivity
for atmospheric
applications
may be determined from laboratory data taken at pressures
other than those characteristic
of the atmospheric applications.
Laboratory
measurements
of the water vapor emissivity have been performed
by HOTTEL and coworkers,(“J2)
* This is equivalent to assuming that the water vapor mixing ratio varies as the fourth power of total
pressure for a hydrostatic atmosphere.
ROBERTD. CESS
864
SCHMITT,
and EcKERT,(‘~) while these data have been expressed by HoTTEL(‘~,‘~) in
terms of an emissivity chart for P” = I atm and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM
P,,,/p - - f 0. Since P,,,/p 5 lo-’
within the
earth’s atmosphere, then Hottel’s emissivity results, when expressed in terms of the strongline parameter, should be directly applicable to an atmospheric
formulation.
The water
vapor emissivity, as taken from HOTTEL’S chart, (15*‘6) is illustrated in Fig. 1 for T = 300K,
and this denotes a column emissivity.
As discussed on pp. 197-198 of GOODY,(‘) the temperature dependence of the water vapor
emissivity is quite small for the temperature range 220 K-300 K, which is consistent with the
results of RODGERS.(‘) Furthermore,
the modified emissivity (employing the temperature derivative of Planck’s function as the weighting function) results of MANABE and WETHERALD,
based on the earlier work of MANABE and STRICKLER,“~ are also quite insensitive to
temperature
within the same temperature
range. This relative invariance with temperature
refers to a fixed absorber amount (gm cm-‘), and since H,,, N T from equation (6), then
P, H,, as appears within the strong-line
parameter, is not a pressure path length, but is
instead proportional
to the absorber amount. Thus Hottel’s emissivity results may be
employed for temperatures other than 300 K providing H, corresponds to the same temperature as the emissivity information,
i.e. 300 K, and from equation (6)
HJkm)
=
8.78
(9)
1 + ( 10241~~) ’
It is interesting to note that in terms of a fixed absorber amount, Hottel’s emissivity
chart indicates a further invariance of the water vapor emissivity upon temperature
for
temperatures
from 300 K to over 550 K.
A large amount of information
concerning the water vapor emissivity is available in the
atmospheric
literature. We do not attempt to summarize here all this information,
but
instead we consider only two of the more recent emissivity formulations.
One of these is
due to RODGERS(‘) and employs the same strong-line parameter as in the present study,
so that a direct comparison
with Hottel’s emissivities is possible, and this is shown in
Fig. 1 (in Rodger’s nomenclature,
the present emissivity corresponds
to EJ. The second
formulation
is that employed
by MANABE and WETHERALD.
Since their strong-line
parameter is P, H,,,(P)“‘7, a general comparison in terms of a single parameter is not possible.
Such a comparison
may be made, however, for conditions
appropriate
to the earth’s zyxwvutsrqponmlkjihg
0.6
I
I
-
0.6
-
HOTTEL
I
I
-----RODGERS
-
--
MANAEE
6 WETHERALD
Iv
01
I
I
102
IO
P,FH,.
I
IO'
olmPCm
Fig. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
1. Comparison of emissivity models for water vapor.
Radiative transfer due to atmospheric water vapor
865
atmosphere.
For this purpose we choose zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH
T, = 288 K, H,,,/H = 0.22, and I’/P = 0.82, while,
from equations (5) and (9) P,, = 0.0128 atm and H, = 1.93 km. It then readily follows that
P,p”H,,, = 0.61 [P, Hw(~)o~7]1~05s,
and thus Manabe and Wetherald’s
emissivity values, as given in their Fig. 27, may be
converted to the present strong-line parameter. Their results are also illustrated in Fig. 1,
with their slab emissivities converted to column emissivities employing the diffusivity factor
1.66.
The differences between the three emissivity formulations
shown in Fig. 1 are not insignificant. Since Hottel’s chart is based upon direct laboratory measurements
and seems
to have stood well the test of time (e.g. discussions by PENNER
and TIEN(‘~)), we deem it
preferable to employ Hottel’s results. There is, in addition, theoretical justification
for the
dependence
of Hottel’s emissivities upon the strong-line
parameter. In the limit of nonoverlapping
strong lines, the emissivity should vary as the square root of the strong-line
parameter, and from Fig. 2 this is indeed the case, where the expression zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR
0.8
0.6
I
I
IO
IO'
x = P*Ptt*,
Fig. 2. Comparison
at12 cm
of a simple approximation
to Hottel’s emissivity results.
is an empirical fit to Hottel’s curve for small values of P,PH,,,; i.e. the limit of nonoverlapping lines. This proper reduction to the square-root limit is consistent with the interpretation by PENNER and VARANASI(9) of Hottel’s emissivity values at elevated temperatures.
Also illustrated in Fig. 2 is an empirical fit to Hottel’s curve given by
E = 0.75 [l - exp( - 0.096JPxw)].
(10)
The above expression is clearly a useful approximation
for the range of values of the
strong-line parameter which have been considered, and this range is characteristic
of the
earth’s troposphere.
Equation (10) will be employed in the following section with regard
to a simple analytical formulation
for the radiative flux.
4. RADIATIVE
FLUX
FORMULATION
There are a variety of ways, employing integration
by parts, by which the radiative flux
may be expressed in terms of emissivity, modified emissivity, and combinations
thereof
(see for example, p. 251 of GOODY,(‘) and RODGERS”)). For present purposes, since the
866 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
ROBERT D. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
CESS
water vapor emissivity is quite insensitive to temperature,
we will follow the same procedure as employed by CESS and KHETAN(6) in formulating
the radiative flux due to the
pressure induced hydrogen opacity for the atmospheres
of the major planets. Defining a
dimensionless
optical coordinate as
[ = (3/2)(0.096,/P&,)
(11)
with pressure in atm and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
H, in cm, the formulation
of the radiative flux due to water vapor,
making use of equation (lo), becomes essentially identical to that of CESS and KHETAN.‘~’
Letting qR denote the net infrared flux measured in the downward direction, it then follows
that
qR = 0.757
_J,j’-7
e
T4(5>
5d5
-T$(1/3
T4(t)ejt21.~2
+ eJts2-)]
(12)
where g is the Stefan-Boltzmann
constant, < is a dummy variable for [, and [,y denotes
the value of < at the planet’s surface. This equation
differs from that for the major
planets (6) through inclusion
of the last term, which accounts for surface emission. In
addition, the arguments
of the exponential
functions are different, since the appropriate
scaled quantity, (P, p), between levels is expressed as
O.l44J<P,
in accordance
with previous
discussion
5. GLOBAL
B)H,
= \:([2
- c2/
of the Curtis-Godson
ENERGY
(13)
approximation.
BALANCE
To illustrate the preceding radiative flux formulation,
we consider a global energy balance
for earth, which amounts to relating the mean surface temperature
to the outgoing infrared
flux. For this purpose, we initially adopt the same cloud model as utilized by RASOOL and
SCHNEIDER,(~)and SCHNEIDER.(*) This consists of a single cloud layer representing
a global
average of many cloud layers. The single cloud is assumed to be black in the infrared,
the cloud top is located at the altitude z,, and the cloud covers a fraction of sky denoted
by A,.
The global mean surface temperature
of the earth is roughly 288 K, as compared with
the effective temperature
of 253 K. The difference, 35 K, is the greenhouse effect due to
the opacity of the earth’s atmosphere.
Water vapor and clouds contribute
most of this
opacity, with the carbon dioxide contribution
being quite small. The analysis of RASOOL
and SCHNEIDER(~) for example, indicates that CO, contributes
only about 1 K to the
greenhouse effect. Correspondingly
we shall neglect the opacity contribution
due to CO,.
Equation (12) is directly applicable to the clear portion of the atmosphere,
while it
applies to the region above the black clouds providing T, and [, are replaced by T, and c,,
where the latter quantities refer to the cloud top. Letting F = -qR(0) denote the infrared
flux emitted to space, it then follows that
+ 0.75
(T/Ts’J4ebr di + (Tc/Ts)4(l/3 -t edit)
1
A,. zyxwvutsrqponmlkjihgfedc
(14)
Radiative transfer due to atmospheric
867
water vapor
Evaluation
of the integrals within equation (14) requires description of the atmospheric
temperature
profile. Since H = RT/g, the tropospheric
lapse rate, dT/dz = -I,
may be
rephrased as dT4/T4 = n de/c, where
(15)
and the temperature
profile within the troposphere
is in turn
T”(i) = Ts”(Ws)“.
(16)
For the present it will be assumed that equation (16) is applicable throughout the entire
atmosphere, although it is valid only for the troposphere.
We will confirm this assumption
in the following section by showing that inclusion of an isothermal stratosphere
does not
influence F. Typical values of [, are sufficiently large (I’,‘,2 6) such that the first integral
within equation (14) may be evaluated from the asymptotic expression for the incomplete
gamma function, and employing equation (16)
scs(TITs)
+ 1) - e-‘*, zyxwvutsrqponmlkjihgfedcbaZYXWVUT
(17)
4e-c d[ = [s-nr(n
0
where I(x) denotes the gamma function. Typical values of [,, on the other hand, are
sufficiently small (5,s 1) that the second integral in equation (14) may easily be evaluated
from
m
I0b’s)4e-idl = L-“j~o
(_
jr(n
l)j((,y+l+j
+
1
+j)
.
(18)
Equations (14), (17) and (18) thus completely describe the outgoing flux F for a specified
surface temperature
and cloud top altitude, with 5, evaluated from equation (11) together
with equations (5), (8) and (9). The cloud top temperature is related to cloud top altitude by
T, = T, - l-z,, while 5, = C,(Tc/TJ4’” from equation (16). For example taking I’ = 6.5 K
km-‘, T, = 288 K, and z, = 5.5 km, then 5, = 6.47, n = 0.274, 5, = 0.932, and
F/aTs4 = 0.656 - O.l37A,.
6. DISCUSSION
For comparative
purposes
equation
OF RESULTS
(14) will be expressed
F=c,
-c2Ac,
as
(19)
where c1 and cz depend upon T, and I, while cz is additionally
a function of z,. In the
following we take I = 6.5 K km-‘, and values for ci and cz for T, = 288 K, from both the
present analysis and other sources, are listed in Table 1.
Also included in Table 1 are results for which an isothermal (218 K) stratosphere
has
been included, and this accounts for the fact that H,,, = H within the stratosphere.(3)
Recall that the analysis of the previous section ignored the stratosphere and assumed that
equation (16) applied throughout
the entire atmosphere.
From Table 1, this is clearly a
useful assumption
and illustrates that the stratosphere
opacity does not affect the overall
energy balance of the planet.
SCHNEIDER@)has employed the empirical formulation Of BUDYKO(“) as a standard against
which a model atmosphere calculation may be compared, and we shall do the same. Budyko’s
QSRT
Vol.
14 No. 9-D
868
ROBERTD. CESS
Table 1. Values of cr and cz for T, = 288 K
cal cm-* min-
Present analysis, z, = 5.5 km
Present analysis, z, = 6.5 km
Present analysis, z, = 6.8 km
Present analysis, z, = 6.8 km
(218 K stratosphere included)
SCHNEIDER,(~)
z, = 5.5 km
BuDYKo,(~‘) empirical
formulation,
as
based upon monthly
c2,
Cl,
Reference
1
cal crnm2 min-’
0.361
0,367
0,361
0,368
0.077
0.096
0.102
0.102
0.399
0.366
0.107
0.102
data from 260 meteorological
stations,
F = 0.3 19 + 0.00327’, - (0.068 + 0.00237’,)4,,
may be expressed
(20)
with Fin cal cm-’ min-’ and T in degrees centigrade. From Table 1 we see that for T, =
288 K the present analysis is nearly identical to Budyko’s empirical expression for a cloud
top altitude of 6.8 km. SCHNEIDER,@) on the other hand, comparing his c2 value with that
of Budyko, suggests a cloud top altitude of 5.5 km.
The difference between the present cloud top value of 6.8 km and that of 5.5 km suggested
by Schneider is evidently due to the use of different water vapor opacity models. There
appears to be justification
for the present value, and for this purpose we consider the more
realistic three cloud model of MANABE and WETHERALD (see their Table l), which assumes
all three clouds layers to be black in the infrared, with the cloud top altitudes and fractions
of cloud cover listed in Table 2. Also included in Table 2 are values of A,, i , which represents
Table 2. Parameters for the three-cloud model of
MANABEand WETHERALD
z,, t , km
10
4.1
2.1
Ae. 1
A,.i
0.228
0.090
0.313
0.228
0.069
0.220
c,,,,calcm-*min-r zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON
0.171
0.053
0.033
the cloud cover fraction which is not overlapped by an upper cloud (or clouds), calculated
from the A,,i values in accordance with MANABE and STRICKLER,(‘~) together with c2, i
values which were calculated by treating each cloud layer individually
by the single-cloud
analysis of the previous section, i.e. evaluating c2 as a function of z, as in Table 1.
It readily follows that the present analysis may be extended to multiple clouds by simply
rewriting equation (19) as zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
F=c,
-&i&i
I
From this we further define an equivalent single-cloud aititude
and Wetherald’s three-cloud model by noting that an equivalent
may be defined by
corresponding
to Manabe
single-cloud parameter c2
(21)
Radiative transfer due to atmospheric water vapor
869
since xi A,, i represents the fraction of the cloud cover for the three-cloud
model. From
to an equivalent
Table 2, cp = 0.097 cal cmm2 min- ‘, and from Table 1 this corresponds
single-cloud
altitude of z, N 6.5 km. The present analysis thus predicts a single-cloud
altitude of 6.8 km when compared with Budyko’s empirical results and an equivalent
cloud altitude of 6.5 km employing
Manabe and Wetherald’s
three-cloud
model. We
therefore conclude that the single-cloud altitude of 6.8 km appears to be reasonable.
A partial comparison of the present radiation model with that employed by Manabe and
Wetherald may be made by noting that c1 represents the ‘clear sky’ outgoing flux. From
Fig. 10 of MANABE and WETHERALD, c1 N 0.37 cal cm-’ min-‘, which is in excellent agreement with present results (Table 1). This is somewhat surprising, however, in view of the
different emissivity models which have been employed (Fig. 1).
Again following SCHNEIDER,@’ a second test of the accuracy of the radiation model is to
compare the dependence of F upon surface temperature.
Employing Budyko’s empirical
expression, we have from equation (20) that
(22)
E = 0 0032 - 0 0023A
aT,
’
.
=’
while holding
the cloud top altitude
constant
and varying
T,in the present analysis gives
= 0.0028 + O.O006A,.
(23)
The first term is in reasonable agreement with Budyko’s equation (22), whereas the difference in the second terms is significant and cannot be reduced by merely changing z,.
Equation (23), however, appears to be consistent with other model atmosphere calculations.
SCHNEIDER”) gives aFIaT,
= 0.0033*
for A, = 0.5,
which is in good agreement with equation
(23), while from Fig. 10 of MANABE and WETHERALD(3J it is found that aFIaT,
increases
slightly with increasing cloud cover, and this again is consistent with equation (23).
All three analyses, i.e. equation (23), Schneider, and Manabe and Wetherald, have one
feature in common, which is that the cloud top altitudes are held constant while T, is
varied. This suggests that perhaps an alternate cloud model should be considered. A particularly simple possibility
is one in which the cloud top temperature
remains constant as
T, is varied, which would correspond to a decrease in cloud altitude for decreasing T,.
With T,= 288K and z, = 6.8 km, then T,= 243.8
K and, holding the cloud top temperature
constant at this value, the present analysis yields zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ
=
0.0028 - 0%-)025A,.
(24)
Equation (24) is in much better agreement with Budyko’s equation (22) than is equation
(23). This suggests that, at least to a first approximation,
an appropriate
single-cloud model
is one in which the cloud top temperature
is held constant when the surface temperature
is
varied.
l Schneider indicates agreement with Budyko’s result, stating that iJF/aT,
= 04031 from equation (20).
This is an error, however, since equation (20) yields aF/aT,= 04021 for A, = 0.5.
870 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
ROBERT D
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML
. Cess
7.
CONCLUDING
REMARKS
In this section we briefly and qualitatively
describe some implications
of the present
results to considerations
of global climate changes. For this purpose it will be convenient to
consider the sensitivity of surface temperature
upon the effective temperature zyxwvutsrqponmlkjihgfed
T,, where
F = aTe4. If the water vapor content of the atmosphere is held constant, by describing for
example a fixed distribution
of absolute rather than relative humidity,‘3) then in the absence
of clouds the opacity of the atmosphere will be independent
of surface temperature,
and it
may easily be shown that aT,/ aT, = 1.11. As discussed by MANABE and WETHERALD,
however, it is more realistic to employ a fixed distribution
of relative humidity, and this
results in a self-amplification
effect of water vapor, since a decrease in surface temperature
produces a decrease in water vapor content of the atmosphere,
which in turn results in a
decreasing greenhouse effect with the result that WJ3T, > 1.11.
Let us now compare this amplification
effect for two different models, assuming average
cloudiness A, = 0.5. Considering the cloud top altitude to be fixed, as has been conventional
in previous studies, then from equation (23) of the present analysis
=
zc=6.8
On the other hand, employing
constant,
equation
1.64.
km
(24). for which the cloud top temperature
=
is held
3.18.
This illustrates that the cloud model with fixed cloud top temperature,
which appears
reasonable in view of previous discussion, produces nearly twice the amplification upon surface temperature
than does the model employing fixed cloud top altitude.
This surface temperature amplification
is extremely important with regard to the problem
of possible global climate changes due to increased particulate
matter within the atmosphere. Preliminary studies of this have been conducted by RASOOL and SCHNEIDER,(~) and
YAMAMOTOand TANAKA.“‘) The essential point is that increased particulate matter could
alter the earth’s albedo, thus changing T,. The subsequent change in surface temperature
would thus depend upon the above mentioned amplification
due both to water vapor and
clouds.
A second mechanism
for global climate changes involves the possibility of increased
cloud cover. This has been discussed by SCHNEIDER,“’ and for a fixed cloud altitude he
shows that increasing A, from 0.50 to 0.58 reduces the surface temperature by 2 K. Budyko,
however, as quoted by Schneider, suggests that the effect of increased cloud cover might be
offset by a corresponding
increase in cloud altitude. But this is contrary to the suggestion
of the previous section that, within the context of a single cloud layer, z, decreases with
decreasing T, . In fact the present analysis indicates an amplification of Schneider’s calculated
reduction in surface temperature
from 2 K to 4 K. We do not imply that a positive amplification necessarily exists, but rather that the realities of global cloudiness probably defy
simple explanation.
Indeed, as pointed out by SCHNEIDER,@)a global-mean
energy balance
is probably insufficient for estimating average climate changes due to varying cloud cover.
Quite clearly cloudiness as a coupling mechanism needs much additional study, and the
present investigation
is intended merely as a guide in this direction. Since the water vapor
radiation
model formulated
in this paper combines both accuracy and simplicity,
the
results of Sections 4 and 5 should prove useful in such future endeavors.
Radiative transfer due to atmospheric water vapor
871
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