The effect of stratospheric aerosols on the earth's monthly zonal radiation balance is investigated using a model layer consisting of 75% H2SO4, which is the primary constituent of the background aerosol layer

HARSHVARDHAN

English

NASA Technical Reports Server

PaperNo.43
PERTURBATIONFTHEZONALRADIATIONBALANCEBYA
STRATOSPHERICAEROSOLLAYER
Harshvardhan,University of Maryland, College Park, Maryland
ABSTRACT
The effect of stratospheric aerosols on the earth's monthly zonal ra-
diation balance is investigated using a model layer consisting of 75%
H2 SO4, which is the primary constituent of the background aerosol
layer.
INTRODUCTION
The role of stratospheric aerosols in enhancing the reflection of solar radiation
has been investigated recently by numerous authors (e.g., Pollack, et al. 1976).
Model calculations have shown that the dominant climatic effect of these aerosols
is enhanced solar reflection with increased infrared opacity playing a smaller role.
However, Herman, et al., (1976) have also pointed out that the degree of albedo
enhancement is a strong function of the albedo of the underlying surface and the
zenith angle of the incident solar radiation.
In this work, the effect of the aerosol layer at solar wavelengths will first be
presented in terms of the albedo sensitivity, which may be defined as dot/dr, where
t_ is the unperturbed system albedo and r is the optical depth computed at some
reference wavelength, here 0.55#m. The zonal variation of albedo sensitivity by
month, together with the seasonal variation of solar irradiation yields the solar per-
turbation. With the inclusion of the effect of the stratospheric aerosol layer on
the infrared radiation leaving the top of the atmosphere, the net energy deficit on
a zonal basis is obtained.
AEROSOL LAYER
There is a persistent tenuous layer of submicron size aerosols in the strato-
sphere that is composed primarily of an aqueous solution of sulfuric acid. The
number concentration of this aerosol layer increases after major volcanic events,
and as the residence time of these aerosols is of the order of a year or more, ra-
diative perturbations may be expected. Initially this enhanced concentration is
limited to the vicinity of the explosion but in a matter of months may spread
globally.
One result of this severe enhancement is the additional reflection of solar en-
ergy back to space. A radiative model of the aerosol is needed to compute the re-
flectivity of the layer. The aerosol composition is assumed to be 75% H 2SO 4 and
a modified gamma size distribution is adopted. Since the optical depth of the aer-
osol layer is small (_0.1) even after major volcanic events, the optically thin ap-
proximation is used to compute the reflectivity of the layer. Use of the Henyey-
Greenstein phase function yields analytical expressions for the planar albedo, a(/a o ),
259
https://ntrs.nasa.gov/search.jsp?R=19790012505 2020-03-20T18:31:07+00:00Z
and global albedo, _, which are used here. The altered albedo of the earth-
atmosphere system with the addition of a non-absorbing aerosol layer is
a'(/_ o) [1 -a(/lo)] (1 -a-)
a_o) = a(_t o) + (1)
1 - _ a'(#o)
where a'_o) is the unperturbed system albedo. A similar relationship exists for
absorbing aerosols.
The albedo sensitivity averaged over the solar spectrum for the model aerosol
layer is presented in Figure 1. Use has been made of the monthly zonal albedo
given by Ellis and Vonder Haar (1976) to compute the sensitivity. The isopleths
are in units of change in albedo in percent of incident solar energy for an optical
depth perturbation of Ar = 0.01. In the optically thin limit, the response is linear
and the effect of other layer thicknesses or non-uniform spread can be easily de-
duced. The dominant feature is seen to be the zenith angle dependence translated
here in a monthly and latitudinal dependence. Over high latitudes the effect of
surface reflectivity may also be noticed, such as the lower values of l_a/Ar in late
winter and early spring when there is snow cover on the ground.
RADIATION BALANCE
The net zonal radiation balance is
D(_) = Q(_) [ 1 - c_(_b)l - I(_b) (2)
where Q(q_) is the solar insolation at latitude q_, a(_) the albedo, I(_b) is the outgo-
ing terrestrial infrared radiation. The presence of a stratospheric aerosol layer mod-
ifies ct through increased reflectivity and I through an increased greenhouse effect
acting in the opposite direction. The reduction in solar energy absorbed, QAa,
caused by a uniform stratospheric layer of rvi s = 0.1 is plotted in Figure 2 which
follows from Figure 1 and the distribution of Q(¢) for each month.
The net loss in a column of the earth-atmosphere system, however, is QAc_
+ A1 (note that AI is a gain, hence negative), which is the change in the radiation
balance and it is necessary to compute the infrared greenhouse sensitivity of the
aerosol layer. This is done for five model atmospheres using an atmospheric radi-
ation model similar to the one by Harshvardhan and Cess (1978) in which an emis-
sivity formulation has been used to calculate the infrared flux. When the infrared
effect is combined with the solar perturbation, the net reduction in zonal radiation
balance is obtained and is plotted in Figure 3.
The perturbation in the radiation balance can also be expressed as a change
in the effective black-body radiative temperature of the atmosphere. This change
is given by
Q_ + AI
AT - (3)
4aT 3
where o is the Stefan-Boltzmann constant, T the effective black-body radiative
temperature of the atmospheric column and (QAt_ + AI) the decrease in the radia-
tion balance. Figure 4 is a plot of AT in °K for rvi s = 0.1. It can be seen that
the major radiative effect of the stratospheric aerosol layer occurs in the spring
and fall when there is a pronounced change in the equator to pole radiative energy
260
gradient. As the results pertain to the entire atmospheric column, it is not possible
to estimate the change in the diabatic heating at various levels of the atmosphere
or estimate surface temperature changes. However, it appears from this analysis
that a uniform layer of stratospheric aerosols would have only a small effect on
the long term radiative regime equatorwards of 50 ° - 60 °, if the change in turbid-
ity corresponds to that caused by the Agung eruption.
REFERENCES
Ellis, J. and T. H. Vonder Haar, Atmos. Sci. Pap. 240, Colorado State Univ., Fort
Collins, Co. (1976).
Harshvardhan and R. D. Cess, Tellus 28, 1 (1976).
Herman, B. M., S. R. Browning, and R. Rabinoff, J. Appl. Meteor. 15, 1057 (1976).
Pollack, J. B., O. B. Toon, C. Sagan, A. Summers, B. Baldwin and W. van Camp,
J. Geophys. Res., 81, 1071 (1976).
261
_i,_-o. _ --,,,_ "__i "\ "\ " ..... /---'\ "\ I o/ol;r_
#oZ?_ , , , ? , t, \ , ,\, _, l( ,_,
HJ.EION _ORJ.IJ.V-I Hznos
._o
_o = .
_"_ il
e. t--t o
a_
_,,, o )1tl/ , =
_;_ _-;</ ../..'../ / /"
_s ...i"," / . ,. / "
. ;./. -" . ")1. - . ./
. -- ..-//- ;.;- / i.-.- .
• _ . . "'.P_ / //./ _/JJ-4
•- ... , , - / I I" / ... / / // /'/,//,_
I/e_ i / / . /i, /
• .... -. , , . . - // //_
" _\" " _ / I "" ' ' i o / fl.l'/_'_.¢_; I""
" _-:\_ ' ,, , , . o 'e o' / ;<<-?J
HJ.EION H_.rlOS
:IGf'I/IIV'I
O_
"u,_
,...,
©
I
262
I _ I I_ , l l I: , , , ,:r i =.i._i
HIUON H.LflOS
30nlliVl
_o=,,
_o
_o
k2
_E
i i i i ; i i i I i i I , [ 4 /
_ o - z
/:'17_ ..o
_. --, ////'/ -
"(" / I1_ I \\ \ L/ 0_.
I I I I I I I I I I I I I I I I ]
HIUON ]onllzVl HJ-nos
..
_c5
II
I
263


Perturbation of the zonal radiation balance by a stratosphere aerosol layer

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server