The spatial sampling limitations of surface measurement systems necessitate the use of satellite data for the investigation of large-scale cloud processes. Understanding the information contained in the satellite-observed radiances, however, requires a connection between the remotely sensed cloud properties and those more directly observed within the troposphere. Surface measurements taken during the First ISCCP Regional Experiment (FIRE) Marine Stratocumulus Intensive Field Observations (IFO) are compared here to cloud properties determined from Geostationary Operational Environmental Satellite (GOES) data in order to determine how well the island measurements represent larger areas and to verify some of the satellite-measured parameters

Fairall, C. W.

Minnis, Patrick

Young, David F.

English

NASA Technical Reports Server

N90-28229
INTERCOMPARISONS OF GOES-DERIVED CLOUD PARAMETERS
AND SURFACE OBSERVATIONS OVER SAN NICOLAS ISIAND
P. Minnis
Atmospheric Sciences Division, NASA Langley Research Center
Hampton, Virginia 23665-5225
C. W. Fairall
Department of Meteorology, Pennsylvania State University
University Park, Pennsylvania 16802
D. F. Young
Aerospace Technologies Division, Planning Research Corporation
Hampton, Virginia 23666
i. Introduction
The spatial sampling limitations of surface measurement systems
necessitate the use of satellite data for the investigation of large-scale
cloud processes. Understanding the information contained in the satellite-
observed radiances, however, requires a connection between the remotely
sensed cloud properties and those more directly observed within the
troposphere. Surface measurements taken during the First ISCCP Regional
Experiment (FIRE) Marine Stratocumulus Intensive Field Observations (IFO)
are compared here to cloud properties determined from Geostationary
Operational Environmental Satellite (GOES) data in order to determine how
well the island measurements represent larger areas and to verify some of
the satellite-measured parameters.
2. Data
Total cloud amounts and visible (0.65 #m) top-of-the-atmosphere cloud
albedos were derived with the hybrid bispectral threshold method (HBTM;
Minnis et al., 1987) from hourly GOES-West data (Young et al., 1989) over
two 0.5 ° regions between II9°W and 120°W and 33°N and 33.5°N for July 1-19,
1987. Cloud albedos, cloud fractions, and integrated cloud liquid water
contents were determined from measurements taken nearly continuously over
San Nicolas Island (Fairall et al., 1989) with a variety of instrumentation
during this same time period. San Nicolas Island (SNI) is located near the
center of the east-west boundary of two satellite regions. Results from
both the microwave and solar radiometer cloud liquid water content (LWC)
measurements are compared to the satellite albedos. These preliminary
comparisons match relatively large areal averages to essentially linear
averages of the cloud fields advecting over a fixed point.
3. Results and Discussion
Mean hourly cloud fractions are shown in Fig. I for the SNI and HBTM
results. The SNI cloud amounts, on average, are 0.045 ± 0.076 greater than
the HBTM total cloud amounts. The diurnal variations are similar with early
morning maxima and late afternoon or evening minima. The HBTM diurnal
range, however, is 0.50 compared to 0.30 for the SNI data. Peak cloud cover
occurs - I hour earlier for the HBTM results. The differences in the
cloud amounts may arise for several reasons. Cloud cover over the island
may not be representative of the larger area. Since the clouds generally
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pass over the island, there may be some surface heating during the day which
affects the cloud deck. During the night, the local heating ceases.
Radiative cooling of the island is limited by the large-scale cloud field so
that there should be little island effect at night. Sampling differences
may also affect the comparison. A visual examination of the data using the
video imagery developed by D. Wylie and P. Grimm (University of Wisconsin,
unpublished, 1988) revealed that of the 19 days, the cloud cover over the
island and surrounding regions appeared to vary the same during only 7 days.
During 5 of the days, a V-shaped clearing formed around and downwind of the
island during the afternoon. During another 5 days, the regions surrounding
the island showed considerable clearing while a strip centered on the island
remained overcast. The clouds cleared around the island and over the island
itself during the remaining 2 days, however, the clouds cleared over the
island last. In nearly all cases in the imagery, the cloud cover over the
island and surrounding regions was very similar at the beginning of each day
during the IFO. These visual findings are consistent with the means shown
in Fig. I and with SNI time series of cloud amount. The HBTM and SNI
results show good agreement after midnight until sunrise. Faster and more
extensive clearing occurs around the island than over the island itself.
This apparent island effect may also be responsible for the differences in
satellite and island cloud-top heights observed during the day (Minnis et
al., 1989).
A comparison of the satellite visible and SNI broadband shortwave cloud
albedos is shown in Fig. 2. The narrowband albedo is considerably lower
than the island-derived albedo values. Spectral differences, sampling, and
the atmospheric effects included in the satellite results are primarily
responsible for the differences. To minimize the spectral differences, the
visible albedos were converted to broadband shortwave albedos using an
empirical solar-zenith angle dependent ratio. The resulting albedos were
then corrected for atmospheric effects using the simple model of Chen and
Ohring (1984). Since the clouds are so low, it was assumed that they could
be treated like the surface. The resulting cloud albedos derived from those
in Fig. 2 are shown in Fig. 3. Cloud albedos were averaged without any
weighting by cloud amount for all cases with cloud amounts greater than I0_.
The dashed line represents a mirror image of the morning satellite results.
It is shown to demonstrate the sizable decrease in cloud albedo during the
afternoon. Despite the broadband and atmospheric corrections, the SNI cloud
albedos are still higher by -5_ on average. If it is assumed that the SNI
clouds are more like those over the surrounding regions when there is more
cloudiness, then mean satellite cloud albedos derived by weighting the
albedo by the cloud fraction should be more similar to the SNI results than
those derived using a simple averaging technique. This approach was
implemented with the results shown in Fig. 4. In this instance, the mean
cloud albedo differences are only - 2_ with greater errors near the
terminator as expected (Chen and Ohring, 1984). The remaining discrepancies
may be due to sampling and technique differences, bidirectional reflectance
model biases, and island effects. Though visual examination of albedo is
unreliable, the clouds were distinctly brighter over the island than over
the adjacent areas during at least 2 days, July 5 and 8. An island effect
which results in more cloud cover may also alter the cloud composition
relative to the large scale.
28
The LWC values derived from the SNI microwave and solar measurements are
correlated with the satellite visible albedos in Figs. 5 and 6,
respectively. Two curves are shown in each figure. The lower curve which
levels at an albedo of - 55% is based on the results of Coakley and Snider
(1989). Their regression is
i/p = 1.74 + 48#0/LWC, (i)
where p is the visible reflectance, #0 is the cosine of the solar zenith
-2
angle, and LWC is given in gm The second curve shown in these figures
is a regression fit to the observed data using the relationship,
In(l _) = a + bLWC/ #0. (2)
The cloud albedo is _ = P / X, where X is an anisotropic reflectance
correction factor. The coefficients a and b are -0.44 and -0.0016,
respectively, for the microwave data. The coefficients for the solar data
are a = -0.46 and b = -0.0016. The curve for (2) is tangential to the
knee of the curve for (i). The latter seems to fit the data well for LWC/# 0
-2
< i00 gm , but does not provide for any albedos greater than - 55%. Thus,
the curve is unrealistic for higher values of albedo. Equation (2),
however, shows no skill for matching the lower albedos, but it allows for
additional increases in cloud albedos beyond 50%. It is concluded,
therefore, that a combination of these two functions would provide a more
complete representation of the relationship between albedo and LWC. This
combination is accomplished by matching the curves at _ = 45%.
4. Concluding Remarks
It appears that the SNl-observed clouds may be affected by the island.
Additional support for this thesis will require analysis of satellite data
over a small region centered on the instrument site. Accounting for these
potential effects, the satellite-derived cloud amounts and albedos are very
similar to those observed over SNI. The diurnal variations in cloud amount
and cloud albedo are also comparable to those seen over other areas (e.g.,
Minnis and Harrison, 1984). These preliminary results have also provided a
relationship which may be utilized during the day to determine cloud LWC
over other parts of the IFO area.
REFERENCES
Chen, T. S., and G. Ohring, 1984: On the relationship between planetary and
surface albedos. J. Atmos. Sci., 41, 156-158.
Coakley, J. A., Jr., and J. B. Snider, 1989: Observed reflectivities and
liquid water content for marine stratocumulus. Proc. AMS Symp., Role of
Clouds in Atmospheric Chemistry and Global Climate, Anaheim, CA, Jan. 30-
Feb. 3, 175-177.
Fairall, C. W., J. E. Heve, and J. B. Snider, 1989: An 8-month climatology
of marine stratocumulus cloud fraction, albedo, and integrated liquid
water. Presented at FIRE Annual Meeting/ASTEX Workshop, Monterey, CA,
July 10-14.
29
Minnis, P., and E. F. Harrison, 1984: Diurnal variability of regional cloud
and clear-sky radiative parameters derived from GOES data, Part III:
November 1978 radiation parameters. J. Clim. Appl. Meteor,, 23, 1032-
1051.
Minnls, P., E. F. Harrison, and G. G. Gibson, 1987: Cloud cover over the
eastern equatorial Pacific derived from July 1983 ISCCP data using a
hybrid bispectral threshold method. J, Ceophys. Res., 92, 4051- 4073.
Minnis, P., D. F. Young, R. Davies, M. Blaskovic, and B. A. Albrecht, 1989:
Stratocumulus cloud height variations determined from surface and
satellite measurements. Presented at FIRE Annual Meeting/ASTEX Workshop,
Monterey, CA, July 10-14.
Young, D. F., P. Minnis, and E. F. Harrison, 1989: Satellite-derived cloud
and radiative parameters over the marine stratocumulus IFO. Presented at
FIRE Annual Meeting/ASTEX Workshop, Monterey, CA, July 10-14.
1.00 l I I I
@e_e_ SNI DATA
0.90 la _ _B_D GOES DATA
_'_
-_ 0801 /
C)
_h 0,70 _ /
liD _ I
-A
(-9 O.6O
0,50 m "m
k
\ I
o.,o _ _ ,'_ ,%g 2b 2,
LOCAL SOLAR TIME
i i
Fig. 1. Mean ]FO cloud amounts derived from GOES over a 0.5 x 1.O region
centered on San Nicolas Island and from island observations.
0.90
0.80
0
w O.70
GJ
_1
.,:{
C:, 0,60
:D
0
-J
U 0.50
0.40
0.30
i 1 I i
oo.ooo SNI DATA
GOES DATA
LOCAL SOLAR TIME
Fig. 2. Same as Fig. 1, except for GOES visible cloud albedos at the top of
the atmosphere and island broadband shortwave cloud albedos.
30
OgO
080
0
w_jO,70
M
_J
<{
C2_060
Z)
o
_ 050
040
i i l i i
SNI DATA
B-E_'.=:*O GOES DATA
030 0 '_ 2o_---2 4
I I/1'_
LOCAL SOLAR TIME
Fig. 3. Same as Fig. 2, except for broadband, atmosphere-corrected GOES
cloud albedos.
ogo
OBO
0
LC] 0,70
£D
_J
C3 060
O
L) 0.50
0.40
0,30
i i i i
oe.oeo SNI DATA
cH._,,4J GOES DAIA
--_ _ ,'2 76 2'o 2,
LOCAL SOLAR lIME
Fig. 4. Same as Fig. 3, except for cloud-amount weighted averaging of GOES
cloud albedos.
':I
0
.4 [313 []
._ I I I 1 I
0 I O0 200 300 400 500
LWC/COS(SZA) (g/m 2)
:I
0
r_
0
_J
o
o°O
o
I I I I I
I O0 200 300 400 500
LWC/COS(SZA)(g/m 2)
Fig. 5. Correlation of GOES visible cloud
albedos and LWC derived from
island microwave measurements (see
text for discussion of curves).
Fig. 6. Correlation of GOES visible cloud
albedos and LWC derived from
island solar radiation data (see
text for discussion of curves}.
31



Intercomparisons of GOES-derived cloud parameters and surface observations over San Nicolas Island

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