One of the goals of the First ISCCP Regional Experiment (FIRE) is the quantification of the uncertainties in the cloud parameter products derived by the International Satellite Cloud Climatology Project (ISCCP). This validation effort has many facets including sensitivity analyses and comparisons to similar data or theoretical results with known accuracies. The FIRE provides cloud-truth data at particular points or along particular lines from surface and aircraft measurement systems. Relating these data to the larger, area-averaged ISCCP results requires intermediate steps using higher resolution satellite data analyses. Errors in the cloud products derived with a particular method can be determined by performing analyses of high resolution satellite data over the area surrounding the point or line measurement. This same analysis technique may then be used to derive cloud parameters over a larger area containing similar cloud fields. It is assumed that the uncertainties found for the small scale analyses are the same for the large scale so that the method has been calibrated for the particular cloud type; i.e., its accuracy is known. Differences between the large scale results using the ISCCP technique and the calibrated method can be computed and used to determine if any significant biases or rms errors occur in the ISCCP results. Selected ISCCP results are compared to cloud parameters derived using the hybrid bispectral threshold method over the FIRE IFO and extended observation areas

Gibson, Gary G.

Harrison, Edwin F.

Heck, Patrick W.

Minnis, Patrick

Young, David F.

English

NASA Technical Reports Server

N90-28267
,_° -
A COMPARISON OF ISCCPAND FIRE SATELLITE CLOUD PARAMETERS
Gary G. Gibson, Patrick W. Heck, David. F. Young
Aerospace Technologies Division, Planning Research Corporation
Hampton, Virginia 23666
Patrick Minnls and Edwin F. Harrison
Atmospheric Sciences Division, NASA Langley Research Center
Hampton, Virginia 23665-5225
i. Introduction
One of the goals of the First ISCCP Regional Experiment (FIRE) is the
quantification of the uncertainties in the cloud parameter products derived
by the International Satellite Cloud Climatology Project (ISCCP). This
validation effort has many facets including sensitivity analyses (Rossow et
al., 1989) and comparisons to similar data or theoretical results with known
accuracies. The FIRE provides cloud-truth data at particular points or
along particular lines from surface and aircraft measurement systems.
Relating these data to the larger, area-averaged ISCCP results requires
intermediate steps using higher resolution satellite data analyses. Errors
in the cloud products derived with a particular method can be determined by
performing analyses of high-resolution satellite data over the area
surrounding the point or llne measurement. This same analysis technique may
then be used to derive cloud parameters over a larger area containing
similar cloud fields. It is assumed that the uncertainties found for the
small-scale analyses are the same for the large scale so that the method has
been "calibrated _ for the particular cloud type; i.e., its accuracy is
known. Differences between the large-scale results using the ISCCP
technique and the "calibrated" method can be computed and used to determine
if any significant biases or rms errors occur in the ISCCP results. In this
paper, selected ISCCP results are compared to cloud parameters derived using
the hybrid bispectral threshold method HBTH (Minnis et al., 1987) over the
FIRE IFO and extended observation areas.
2. Stratocumulus
GOES-West ISCCP B3 data taken every 3 hours during July 17-31, 1983
analyzed with the HBTM on a 2.5 ° latitude-longltude grid between 40°N and
IO°N and 145°W and II0°W (Minnis et al., 1988) are compared to the
corresponding CI (Rossow et al., 1988) results. The cloud data have been
stratified as total, low, midlevel, and high clouds. The ISCCP low, middle,
and high clouds are those with cloud-top pressures p > 800 mb, 800 mb _ p >
440 mb, and p ! 440 mb, respectively. HBTM low, middle, and high clouds
are those with cloud-top altitudes, z < 2 km, 2 km _ z < 6 km, and z _ 6
km. There are two primary types of ISCCP cloud cover, VIR, determined with
visible and infrared data, and IR, determined with infrared-data alone. The
two cloud amounts are the same at night. As noted by Minnis et al. (1988),
the cloud amounts, diurnal cloud variations, and cloud-top heights do not
vary dramatically on an interannual basis over this area. Also, the cloud
properties derived from the satellite near the coast are very much like
those determined over the open ocean within this grid. Thus, the large-
scale average properties derived for this region are similar to those
determined over smaller areas. Initial validations of the HBTM are
.... -,:,._,(.i'_OT F:LMED
257
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described elsewhere (e.g., Minnis and Harrison, 1984; Minnis and Wielicki,
1988; and Minnis et al. (1989a,b).
Figure i shows the mean HBTM-derived total, low, and sum of middle and
high cloud amounts. Total and low cloud amounts increase from the
California coast to a maximum of 91% near 20°N, 130°W with a relative
maximum in low cloudiness within the IFO region. This extensive cover of
low clouds is referred to as the main stratocumulus region. Significant
amounts of upper-level clouds occur in the southeastern quadrant of the
grid. Differences between the HBTM and VlR results are shown in Fig. 2,
while the HBTM-IR differences are plotted in Fig. 3. Neglecting the land
areas, the mean VIR total cloud amounts (Fig. 2a) are 2 ± 6% greater than
the HBTM values. Most of the ISCCP clouds, however, are placed in the
middle layer as seen in the differences in Figs. 2b and 2c. More clouds are
found with the HBTM over the main stratocumulus region than with the IR
results. The IR underestimates total cloudiness by 7 ± 11%.
The differences in the 3-hourly means are examined in Fig. 4 for two
large regions outlined in heavy lines in Fig. ic. The western box is
designated the PAC region, while the other is the IFO region. Over the PAC
region (Fig. 4a), there is generally good agreement between the results for
all three analysis techniques. The HBTM cloudiness is very close to the IR
results during the day but greater at night. Addition of the visible data
increases the ISCCP cloud amounts so the VIR cloud cover exceeds the HBTM
amounts during the day. On average, the HBTM and IR cloud amounts are the
same, while the VIR cloudiness is greater than the HBTM's. This tendency
for slight daytime overestimation by the VlR technique (relative to the
HBTM) and underestimation with the IR method is accentuated near the coast
over the IFO region (Fig. 4b). There, the IR diurnal range is much smaller
than the HBTM's with a 20% underestimate in total cloudiness at night and
more than 10% during the day. The VlR data only underestimate the cloud
cover during the night leading to an overall cloud amount deficit of 10%.
The overestimation of ISCCP cloud-top heights over the stratocumulus
region is probably due to the use of low-resolution NMC soundings over areas
with strong boundary-layer inversions (Minnis et al., 1989b). The VIR cloud
amounts agree quite well with the HBTM results primarily because of the
effects of underestimation at night and overestimation during the day. This
result is consistent with the Landsat analyses of Parker and Wielicki
(1989). The differences between the results over the PAC and FIRE regions
are attributable to the variations in contrast between the clear-sky and
cloudy temperatures. Near the coast, the clouds are lower than those
further west so fewer pixels are cold enough to pass the 3-K threshold.
Diurnal variations in cloud amount determined with either the VIR or IR
techniques should be used carefully. While both techniques appear to give
the correct times for maximum and minimum cloudiness, there may be
significant discrepancies in the diurnal ranges and the variations in
cloudiness between the extrema.
3. Cirrus
Another method for validating an algorithm is to apply it directly to a
high-resolutlon satellite data set corresponding to a cloud-truth set. The
complete ISCCP algorithm was not available for this study so an attempt is
made here to simulate its relevant characteristics. The adjustment of cloud-
top temperature to account for the infrared semi-transparency relies on a
258
relationship between the visible and infrared optical depths, _v and re,
respectively. In the ISCCP algorithm, Tv - 2_ e (Rossow et al., 1988). The
value of T is determined from the observed reflectance by first isolatingv
the cloud reflectance by accounting for the surface and atmospheric
contributions to the reflectance. The cloud reflectance is related to
v
using the results of a radiative transfer model of clouds based on a
scattering phase function determined from Mie theory using a droplet size
distribution with an equivalent radius of I0 _m. Once the value of _ is
v
determined, the observed cloud temperature for a given pixel is adjusted in
the same manner used by Heck et al. (1989). The corrected temperature is
then compared to the tropopause temperature and, if lower, set to the
tropopause value. The temperatures or corresponding pressures for each
pixel are then averaged for the area of interest to obtain an average cloud-
top temperature or altitude.
The approach of Heck et al. (1989) is used here with some modifications
to simulate the ISCCP cloud-height adjustment scheme. Instead of an
empirical cloud bidirectional reflectance model, a T -dependent model is
v
used here which was constructed from the results of an adding-doubllng model
of radiative transfer (Takano and Liou, 1989) using a Mie-scattering phase
function determined for a droplet distribution with an effective radius of
I0 _m. The temperature of each cloudy pixel is adjusted individually using
_v - 2_e" Averages are constructed from the adjusted pixel temperatures.
Otherwise, all other steps are the same as those used by Heck et al. (1989).
This simulated ISCCP algorithm was then applied to the lidar-satellite data
used by Heck et al. (1989). The lldar-derlved cloud-top heights are used as
a reference set in the same manner used by Heck et al. (1989) to determine
uncertainties in the results from the empirical method.
Comparisons of the simulated ISCCP cloud-top heights and the lidar
cloud-center and cloud-top altitudes are shown as crosses in Figs. 5a and
5b, respectively. On average, the simulated ISCCP cloud-top heights are 2.8
km lower than the lidar cloud-center heights and 4.7 km lower than the lidar
cloud-top altitudes. The range of differences leads to a large rms error of
3.4 km in the cloud-center height comparison. Average cloud heights for the
2.5 ° region bounded by 42.5°N and 45°N and 87.5°W and 90°W are also shown in
Fig. 5 as circles. The ISCCP adjusted cloud-top heights are taken from the
GOES CI data for October 27 and 28, 1986. Averages from the 0.5 ° regional
results of Heck et al. (1989) are used as the reference heights. The two
lower cloud heights were observed on the 27th. The other three cases fall
within the envelope of simulated data. Without the two low-level clouds,
the observed ISCCP cloud-top heights are 1.7 km lower than the reference
cloud-center heights and 2.5 km lower than the cloud-top heights.
The results shown in Fig. 5 demonstrate some consistency between the
simulated and actual ISCCP cloud-top height results for semitransparent
cirrus. Other cirrus IFO studies have indicated that the Mie scattering
phase function is not a good representation of scattering in cirrus clouds.
These preliminary findings support those conclusions.
259
REFERENCES
Heck, P. W., G. G. Gibson, P. Minnls, and E. F. Harrison, 1989: Satellite
derived cloud fields during the FIRE IFO case study. Presented at FIRE
Annual Meeting/ASTEX Workshop, Monterey, CA, July 10-14.
Minnis, P., and E. F. Harrison, 1984: Diurnal variability of regional cloud
and clear-sky radiative parameters derived from GOES data, Part I:
Analysis method. J, Clim, Appl, Meteor,, 23, 993-1011.
Minnis, 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.Geophys, Res,, 92, 4051- 4073.
Minnis, P., and B. A. Wielicki, 1988: Comparison of cloud amounts derived
using GOES and Landsat data. J, Geophys. Res., 93, 9385-9403.
Minnis, P., E. F. Harrison, and D. F. Young, 1988: Extended time
observations of California marine stratocumulus from GOES for July 1983-
1987. FIRE Science Team Workshop, Vail, CO, July 11-15, 195-199.
Minnls, P., C. W. Falrall, and D. F. Young, 1989a: Intercomparisons of GOES-
derived cloud parameters and surface observations over San Nicolas
Island. Presented at FIRE Annual Meeting/ASTEX Workshop, Monterey, CA,
July 10-14.
Minnis, P., D. F. Young, R. Davies, M. Blaskovic, and B. A. Albrecht, 1989b:
Stratocumulus cloud height variations determined from surface and
satellite measurements. Presented at FIRE Annual Meeting/ASTEX
Workshop, Monterey, CA, July 10-14.
Parker, L., and B. A. Wiellcki, 1989: Comparison of satellite based cloud
retrieval methods. Presented at FIRE Annual Meeting/ASTEX Workshop,
Monterey, CA, July 10-14.
Rossow, W. B., L. C. Garder, P. Lu, and A. Walker, 1988: International
Satellite Cloud Climatology Project (ISCCP), Documentation of cloud
data. WCRP Report, August, 78 pp.
Rossow, W. B., L. C. Garder, and A. A. Lacis, 1989: Global, seasonal cloud
variations from satellite radiance measurements. Part I: Sensitivity of
analysis. J. Clim,, 2, 419-418.
Takano, Y. and K. N. Liou, 1989: Radiative transfer in cirrus clouds: I.
Single scattering and optical properties of oriented hexagonal ice
crystals. J. Atmos, Sci,, 46, 3-20.
260
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262
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A comparison of ISCCP and FIRE satellite cloud parameters

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