Work that has been completed is described in reprints and preprints, and summaries in terms of broad categories are given as follows: (1) The Relationship between Surface and Satellite Shortwave Radiative Fluxes; (2) Cloud-Climate Interactions in Atmospheric General Circulation Models; (3) Absorption of Shortwave radiation by clouds; (4) Clear-sky atmospheres shortwave radiation; and (5) Surface shortwave radiation measurements

Cess, Robert D.

English

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FINAL REPORT
The Surface Radiation Budget and Cloud Climate
Interactions as a Part of CERES
NASA Grant Number NAG11264
May 1, 1991 to October 31, 1998
Project Director: Robert D. Cess
Marine Sciences Research Center
State University of New York
Stony Brook, NY 11794-5000
https://ntrs.nasa.gov/search.jsp?R=19990034154 2020-06-15T22:28:46+00:00Z
Work that has been completed under NASA Grant number NAG11264 is
described in the attached reprints and preprints, and summaries in terms of broad
categories are given as follows.
The Relationship between Surface and Satellite Shortwave Radiative Fluxes
Considerable effort was devoted towards understanding the relationship between
shortwave (SW) radiative fluxes measured at the surface by pyranometers, and at the top-
of-the-atmosphere (TOA) by satellites as part of the Earth Radiation Budget Experiment
(ERBE). The initial focus was upon understanding how CERES measurements could be
used to infer the surface SW radiation budget. This was later expanded to investigate
satellite cloud scene-identification algorithms for various underlying surfaces.
Cloud-Climate Interactions in Atmospheric General Circulation Models
To better understand cloud-climate interactions in atmospheric general circulation
models, with the goal towards addressing cloud feedback, we compared seasonal changes
in cloud-radiative forcing (CRF) at the top of the atmosphere from 18 atmospheric general
circulation models and observations from the Earth Radiation Budget Experiment (ERBE).
To enhance the CRF signal and suppress interannual variability, we consider only zonal-
mean quantities for which the extreme months (January and July) have been differenced
as well as differencing the Northern and Southern Hemispheres. Since seasonal variations
of the shortwave component of cloud-radiative forcing are caused by both seasonal
changes in cloudiness and in solar irradiance, the latter was removed. In the Earth
Radiation Budget Experiment data, seasonal changes in cloud-radiative forcing are
primarily driven by changes in cloud amount. The same conclusion applies to the general
circulation models. The shortwave component of seasonal cloud-radiative forcing is a
measure of changes in cloud amount at all altitudes, while the longwave component is
more a measure of upper-level clouds. Thus, important insights into seasonal cloud-
amount variations of the models have been obtained by comparing both components, as
generatedbythemodels,to thesatellitedata. Forexample,in tenof the eighteen models
the seasonal oscillations of zonal cloud patterns extend too far poleward by one
latitudinal grid.
Absorption of SW Radiation by Clouds
To investigate the absorption of shortwave radiation by clouds, we collocated
satellite and surface measurements of shortwave radiation at several locations.
Considerable effort was directed towards understanding and minimizing sampling errors
caused by the satellite measurements being instantaneous and over a grid that is much
larger than the field of view of an upward facing surface pyranometer. The collocated
data indicate that clouds absorb considerably more shortwave radiation than is predicted
by theoretical models. This is consistent with the finding, from both satellite and aircraft
measurements, that observed clouds are darker than model clouds. In the limit of thick
clouds, observed top-of-the-atmosphere albedos do not exceed a value of 0.7, whereas in
models the maximum albedo can be 0.8. Interpretations were further made of aircraft
measurements of shortwave radiation measurements that were obtained as part of the
Atmospheric Radiation Measurements (ARM) Enhanced Shortwave Experiments
(ARESE). These interpretations use the 500 nm (I 0 nm bandwidth) measurements to
minimize sampling errors in the broadband measurements. It is indicated that the clouds
present during this experiment absorb more shortwave radiation than predicted by clear
skies and thus by theoretical models, that at least some (< 20%) of this enhanced cloud
absorption occurs at wavelengths less than 680 nm, and that the observed cloud
absorption does not appear to be an artifact of sampling errors nor of ins_,'ument
calibration errors.
Clear-Sky Atmospheres Shortwave Radiation
To address a recent issue of whether contemporary clear-sky shortwave radiative
transfer models do or do not portray reality, we have collocated satellite measurements of
reflectedshortwaveradiationat thetop-of-the-atmosphere,madeaspartof theEarth
RadiationBudgetExperiment,with measurementsof surface insolation made at 24
stations located throughout southern Canada. From this a clear-sky data set has been
constructed by using both the satellite and the surface measurements to identify clear
days, and the data set extends over a 4-year period (1985 through 1988). Two quite
different types of shortwave radiative transfer models were employed, one a model-
derived algorithm that converts the top-of the-atmosphere measurements to surface
insolation, and the other a stand-alone column model which computes the surface
insolation independently of the satellite measurements. Both models incorporate
prescribed aerosols, and they were compared to the clear-sky data by using satellite-
measured atmospheric water vapor as model input. The models are in excellent agreement
with the clear-sky measurements when averaging is performed over all stations and over
all seasons. The relative bias error in surface insolation is roughly 1% for both models,
despite the fact that one model is constrained to the top-of-the-atmosphere measurements
and the other is not. Thus the models' incorporation of tropospheric aerose!s seems
quite realistic when compared to the averaged data. For daily means and individual
stations, however, the models' relative bias errors in surface insolation are observed to
range from -6% to 10%, and it is demonstrated that this is caused by temporal and
geographical variability in tropospheric aerosol loading. Because the models incorporate
prescribed aerosols, they produce a relative bias error of roughly -6% for aerosol-free
conditions. The opposite limit of 10% corresponds to aerosol loading in excess of that
prescribed in the models. There is no evidence that the models incorrectly portray any
impooant physical processes, and we find no support for a suggestion that there" is an
unknown and substantial source of shortwave absorption by water vapor. Our
comparisons do not, however, rule out recently suggested minor absorbers of shortwave
radiation. Also, since a very stringent clear-sky identification procedure was used with
the goal of removing forest fire smoke as well as clouds, our study does not rule out
possible atmospheric absorbers resulting from urban pollution.
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Surface Shortwave Radiation Measurements
Motivated by the appearance of several recent studies that suggest significant
deficiencies in our knowledge of atmospheric clear-sky shortwave radiation, we have
performed some rather straightforward consistency tests on the types of data used to
arrive at these conclusions. Specifically, the data consist of broadband measurements of
shortwave irradiances to the surface (total, direct and diffuse) taken in Oklahoma. To
place these data in perspective, we have also examined similar data taken at 8 stations
located throughout Canada. With the exception of a single station, the Canadian data
show no apparent inconsistencies. The Oklahoma data, on the other hand, exhibit some
very troubling inconsistencies, suggesting that these data not be used for the purpose of
testing atmospheric radiation models until these inconsistencies are either understood or
rectified.
There are no inventions to report
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The Surface Radiation Budget and Cloud Climate Interactions as a Part of CERES

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