The total amount of ozone in a vertical column is being measured by Nimbus 4 and 7 observations of the intensity of ultraviolet sunlight scattered from the earth. The algorithm for deriving the amount of ozone from the observations uses the assumption that the surface reflects the light isotropically and the albedo is independent of wavelength. The effects of anisotropic surfaces and clouds on the estimate of total ozone are computed for models of the earth-atmosphere system

Ahmad, Z.

Fraser, R. S.

English

NASA Technical Reports Server

Paper No. 41
THE EFFECT OF SURFACE REFLECTION AND CLOUDS ON THE ESTIMA-
TION OF TOTAL OZONE FROM SATELLITE MEASUREMENTS
R. S. Fraser, Goddard Space Flight Center, Greenbelt, Maryland
Z. Ahmad, Systems and Applied Sciences Corporation, Riverdale,
Maryland
ABSTRACT
The total amount of ozone in a vertical column is being
measured by Nimbus 4 and 7 observations of the intensity
of ultraviolet sunlight scattered from the earth. The
algorithm for deriving the amount of ozone from the ob-
servations uses the assumption that the surface reflects
the light isotropically and the albedo is independent of
wavelength. The effects of anisotropic surfaces and
clouds on the estimate of total ozone are computed for
models of the earth-atmosphere system.
INTRODUCTION
Nimbus 4 and 7 are making an important series of measure-
ments of the total amount of ozone for most of the world over
a period of many years. The amount of ozone is estimated from
the intensity of the sunlight scattered from the earth in several
bands within the 300 to 400 nm band. Since ozone absorbs light
within this band, the intensity of the scattered light decreases
as the amount of ozone increases. The algorithm for deriving
the amount of ozone is based on a model of the earth-atmosphere
system utilizing two assumptions: a) the surface reflects light
isotropically according to Lambert's law; and, b) the surface
reflectance, or albedo, is independent of wavelength.
Generally, these assumptions are not true, but methods have
not been developed for improving the physical reality of the
algorithm model. Nevertheless, advanced models of the earth-
atmosphere system can be developed for computing the errors
caused by these assumptions in estimations of the total amount
of ozone. Results of computing the effects for strongly aniso-
tropic surfaces will be discussed first, and then for clouds.
Anisotropic Reflection
The method for estimating the ozone error is to compute
the intensity of ultraviolet light scattered by models of the
earth-atmosphere system. The total amount of ozone in a vertical
column is derived from the intensity by means of the same algo-
rithm used for processing the Backscattered TTltraviolet Instru-
ment measurements.
247
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The effect of rough ocean surfaces is investigated first.
The surface is assumed to be an ensemble of randomly oriented,
small, reflecting facets. The reflection characteristics are
computed according to the Cox-Munk theory (1954). Figure 1
shows how well the computed and measured values compare. The
dots represent the intensities measured by Dr. Curran from the
NASA Convair 990 aircraft at a height of 10.5 km. The abscissa
gives the nadir angle of observation. The intensities are com-
puted for 3 model atmospheres, each with a different aerosol
model. The model containing haze L (index of refraction
m=1.341-0.0i) agrees best with the measured values over an in-
tensity range of an order of magnitude. The absolute accuracy
of the computed intensity is not critical, but good precision is
needed for computing changes in intensity associated with model
perturbations. The precision of such computations are as good as
the measured values -- namely, a few percent.
Table 1 gives the total ozone errors associated with wind
speeds of 2 and I0 ms -I and three solar zenith angles. The
third column gives the albedo, or hemispherical reflectance for
the model. The fourth column gives the albedo that is derived
from the ozone algorithm on the assumption that the surface
reflects light according to Lambert's law. The albedo is strong-
ly overestimated when the nadir field-of-view is directed towards
the bright solar glint. As the solar zenith angle increases and
the glint pattern moves away from the nadir field-of-view, the
derived albedo approaches the true value.
The total ozone errors in Dobson Units (DU) are given in the
last column. They are -2% to -3% -- slightly less than the er-
rors in the satellite values. The errors are small, because the
algorithm utilizes the ratio of intensities in moderately and
weakly absorbing bands. The effects of surface anisotropy on
the reflected intensities is about the same in both bands.
TABLE 1
Model and derived surface albedos for %380 rim. The last
column gives the deviation in the derived amount of ozone from
the model value of 240 DU.
Solar Total
Wind Zenith Ozone
Speed Angle Surface Albedo Deviation
(m/s) (D_grees) Model Derived (DU)
0 0.04 0.23 -5 (-2%)
28 0.04 0.08 -7 (-3%)
58 0.08 0.08 -4 (-2%)
i0 0 0.040 0.13 -8 (-3%)
28 0.045 0.09 -6 (-2%)
58 0.080 0.07 -4 (-2%)
248
Effects of deviations in the physical state of the atmosphere
from the algorithm model tend to cancel in the ratio.
The effect of a snow-ice surface is discussed next. The
measured and computed intensities of light reflected from a
snow surface are shown for the principal plane containing the
sun, the observer, and his zenith in Figure 2. The radial lines
give the direction of travel of a photon. The parallel source
of light oft units of flux is incident at an angle of 150 ° from
the surface. The circular curves give the intensity of reflected
light. The value of isotropic reflection is 1/2, independent of
the angle. The values computed for a snow-ice surface are given
by the continuous curve labeled as anisotropic. The dashed curve
gives measured values. The magnitudes are not important, since
they can be adjusted by varying the albedo within a large allow-
able range (O'Brien, 1977). The measured and anisotropic values
are nearly isotropic for backward reflection; but show the strong
forward scattering, similar to the solar glint pattern on water.
Table 2 gives the error in the derived amount of ozone,
which is 16 DU out of a total of 342 DU. The error is the same
when the surface reflects light isotropically, showing that the
anisotropic effect is not the cause of the error. The error
arises because the Antarctic region is modeled, and the surface
is 3.6 km above sea level. When the surface reflectance exceeds
0.8, the algorithm calculates the total amount of ozone for a
surface that is 7 km above sea level. The difference between the
model and algorithm heights is the reason for the error.
TABLE 2
Surface albedos and total ozone deviations using the inten-
sities at the top of the atmospheres with two surface characteris-
tics. The total amount of ozone in the model is 342 DU. The
solar zenith angle eo=58 °. The-surface is 3.6 km above sea level.
Surface Albedo
Surface (%380 nm)
Reflectance Model Derived (DU)
Isotropic 0.96 0.96 16 (5%)
Anisotropic 0.95 0.94 16
Clouds
There are no reliable observations of cloud albedo for the
%300 - 400 nm band. We have calculated the reflection and trans-
mission characteristics of a cloud of water drops with an optical
thicknessTc20 at %380 nm. Its albedo increases slightly from
0.66 to 0.67 as the wavelength increases from 312 to 380 nm (the
solar zenith angle 0o=48°). Addition of 2 DU of ozone, appro-
priate for a cloud 1.5 km thick, does not change the albedo
appreciably (see Figure3).
249
The effect of the atmosphere and a ground surface at sea level on
the albedo at the top of cloud is shown in Figure 3. The top of
the cloud is placed at the pressure level of 0.4 atm and the base
at 0.5 atm. The albedo at the cloud top is weakly dependent on
the wavelength, when the surface albedo is small, as for oceans,
bare and vegetated lands. On the other hand, when the surface al-
bedo is high, as for snow and ice , the albedo at the cloud top
increases significantly with wavelength (Figure 3).
Table 3 shows the deviation in the total amount of ozone
in a vertical column caused by clouds. When the surface albedo
is small, the error is small. When the surface albedo is large,
the ozone errors become significant. The errors decrease with
solar zenith angle, because the cloud reflectance increases,
and less light is transmitted to interact with the lower atmos-
phere and ground. These results indicate that high clouds cause
a small error in the derived amounts of ozone over surfaces of
low reflectivity, but a moderate positive bias over snow surfaces.
TABLE 3
Deviation in the total amount of ozone (DU) caused by a
layer of clouds whose upper surface is at pressure of 0.4 atm.
The total amount of ozone is 350 DU.
surface_ solar zenith
albedo _ngle 0 ° 68 °
0.07 5 (1.47%) 1 (0.3%)
0.90 27 (7.7%) ii (3.1%)
Summary
Surface anisotropy causes small errors in the amount of
ozone estimated from satellite measurements of the intensity of
scattered ultraviolet sunlight. The anisotropic effects for
water and snow-ice are nearly independent of wavelength. The
algorithm for estimating the total amount of ozone computes the
ratio of intensities in moderate and weakly absorbing bands and
compares this ratio with values computed for a model with Lambert
reflectance. The difference in intensities between the Lambert
and anisotropic models tends to cancel in the ratio, resulting in
small ozone error.
Since the surface albedo of the algorithm model is indepen-
dent of wavelength, variations from such a model cause ozone
errors. A high cloud above a bright surface causes moderate
ozone errors, but small errors above a dark surface.
250
References
Cox, C. and W. Munk, 1954: The measurement of roughness of the
sea surface from photographs of the sun glitter. Jo. Opt.
Soc. Amer., 44, 838-850.
Middleton, W.E.K. and A.G. Mungall, 1952: The luminous direc-
tional reflectance of snow. Jo. Opt. Soc. Amer., 42, 572-579.
O'Brien, H.W., 1977: Observations of the Ultraviolet Reflectance
of Snow. Cold Regions Research and Engineering Laboratory,
Hanover, New Hampshire, Report No. 77-27, 19p.
0,20
0.i0
)..
I-
0.05
I,U
l-
z
h,
>
iI-
'_ 0.02
_,1
0,01
I? +i ,.,_,Y" "r_ HAZE L (m= 1.5-oi)
\. .4 _ l
Fo _rr _
-rm = 0,15 (.T_) HAZE Um=1.341-oi)
V _ 8 m/s II ' I
i i i i ,
60 30 0 30 60
NADIR ANGLE IN DE6REEB
_" ,e° _= m2o
Fig. I. Upwelling intensities at a height of 10.5 km.
The dots give the measured values, and the lines
give the values computed for model atmospheres.
251
Fig. 2. The computed intensity of light reflected from
model snow surfaces compared with values measured
by Middleton and Mungall (1952) as a function of
the angle from the horizon in the vertical plane
containing the source of parallel light, which is
located at a zenith angle of eo=60 °. The source
flux is _units. The model isotropic, model aniso-
tropic, and measured intensities are represented
respectively by the dot-dashed, continuous and
dashed lines; their respective albedos are 1.0,
0.94, and 0.67.
1.00
.90-
.80"
0
bJ
m
.70"
.60-
.50
300.0
Fig. 3.
Ag-I.O
Ag'O.O
NO atmos.
3"0.02 I , l34G0 360.0 380.0
X(em)
The albedo at the upper cloud surface as a
function of wavelength. The upper two curves are
for ground albedos of 0.0 and 1.0. The lowest
curve gives the cloud albedo in the absence of
the earth's atmosphere and surface. The solar
zenith angle is 0 °.
252


The effect of surface reflection and clouds on the estimation of total ozone from satellite measurements

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