(1-5 are here repeated from the Abstract above)
Take sufficient observations of upwelling spectral shortwave
radiances at the Chesapeake Light Tower to adjust and validate
a model of sea BRDF in AVHRR channels that can be used for
global processing by GACP. The new sea BRDF model will include
the effects of waves driven by local wind, large scale swell,
and white caps; it should surpass models that include local
wind (from NWP) but not large scale swell (which is available
operationally from NCEP's Ocean Branch). This is explained in
more detail on the web page http://www-cave.larc.nasa.gov/gacp/
Relevance to the NASA GACP Program:
GACP aims to develop a 20-year record of aerosols. A principal
driver of this task is the need to determine the radiative forcing
by the aerosols. Significant problems confronting this task include
(1) uncertainties in aerosol optical properties, (2) imperfect
knowledge of ocean optical properties (a boundary condition
for the retrievals based on AVHRR), and (3) the need to validate
inferred radiative forcing with ground-based and other satellite
measurements which may be superior to AVHRR. Our development of
an on-line radiation and aerosol testbed at SGP, which is arguably
the best instrumented and most studied facility in the world
for this purpose, will aid the GACP Science Team in (1) and (3).
Our program to observe and parameterize ocean spectral BRDF
addresses (2); it will improve the GACP ocean BRDF by including
the effects of large scale swell (i.e., the impact of non-local
winds). Our testbed at the Chesapeake Light Tower will address
(1), (2), and (3). The testbed web page is
http://www-cave.larc.nasa.gov/gacp/
Activities in First Year:
[summary of results]
Our initial estimate for aerosol forcings to clear-sky surface
shortwave at the ARM Southern Great Plains (SGP) for April 1998:
AEROSOL FORCING = total insolation -19.3 Wm-1
direct horizontal -80.7 Wm-2
diffuse 61.3 Wm-2
Discrepancies with measurements (i.e., the "clear sky insolation
anomaly") have been substantially reduced with both a newly
modified Fu-Liou code and our adjustments to observed values
of diffuse flux; the adjustments account for the thermal
balance at the shortwave detector surface. For April 1998:
(COMPUTED-OBSERVED) = total insolation 9 Wm-2
direct horizontal -4 Wm-2
diffuse 13 Wm-2
[testbed now on-line at http://www-cave.larc.nasa.gov/gacp/ ]
The web page covers most of the interesting work. We have have
dubbed it the "Langley GACP Home Page" for now, but note that it
does not include the independent work of NASA Langley GACP
investigations led by Richard Ferrare, Norman Loeb, Paul Stackhouse,
and David Winkler. The GACP testbed has a good start for April 1998,
providing graphics with theoretical radiative forcing (clear skies)
and validation of the computed fluxes with SGP measurements. The testbed
provides links to the inputs for the calculations, which have been
taken from the CERES ARM Validation Experiment (CAVE) web site;
users may download measured aerosol optical depths, temperature and
humidity soundings, and observed CERES TOA fluxes, for most of 1998
at SGP.
[flux measurements]
One vexing problem in the radiation community is the accuracy
of radiative flux measurements at the ground, especially
for the diffuse component. Reports that under clear skies,
computed broadband shortwave (SW) insolation generally exceeds
measurements have diminished the credibility of theoretically
inferred radiative forcing by aerosols. Tim Alberta has
made an advance on that problem by carefully correcting
at least part of the error in the Eppley diffuse record;
the adjustment involves data from the night offset of the
shaded Eppley PSP and both the day and night data streams
from the PIR. The testbed supplies the corrected fluxes,
which are readily downloadable.
[radiative transfer]
Fred Rose has inserted Ming-Dah Chou's parameterizations for
SW absorption by CO2, O2, and a weak H2O band into the
Fu-Liou radiative tranfer code. By incorporating other
changes to the Fu-Liou code that better account for
solar radiation outside the 0.2-4.0 micron band, we have
produced a more accurate code. We can release the new version
of the Fu-Liou code to the full GACP team, but ask that those
seeking it obtain the prior concurrence of Qiang Fu
(qfu@atm.dal.ca ).
[equipment purchase for ocean BRDF]
We have spent most of our GACP funds for this year on a
critical purchase: A spectral photometer and sea-worthy
tracker that will scan DOWNWARDS to measure upwelling
radiances from the ocean in AVHRR channels 1 and 2.
As noted in the Objectives and Relevances sections above
(and on our web page at http://www-cave.larc.nasa.gov/gacp/ ),
this will be deployed at the Chesapeake Light Tower, where
the CERES Ocean Validation Experiment (COVE) runs a set of
broadband radiometers and aerosol photometers that will enter
the Baseline Surface Radiation Network (BSRN) archive.
We have purchased the expensive Schulz Spectralphotometer
SP1A (now in hand and partly tested by Ken Rutledge) and a
special tracker (still awaiting the tracker). The SP1A is sealed
for filter stability and has temperature compensation. We have been
firmly advised that the SP1A is a better instrument than say
a Cimel, for our application of extended sea measurements.
The Chesapeake Light Tower has NOAA measurements of wind and
waves, differentiating large scale sea swell (which is
predicted in operational ocean models worldwide but is not in
the current GACP ocean BRDF algorithm) and local wind driven
waves.
Future Plans:
A postdoc will be hired to support the on-line testbed.
No major purchases of equipment (beyond the SP1A now in hand)
are needed. The SGP testbed will be extended, first by including
assessments of the accuracy of computed fluxes using CERES TOA
measuremets, and spanning January-August 1998. The second step
at SGP will be an improvement of the aerosols at the SGP testbed,
giving more realistic vertical profiles from Raman lidar and
alternative formulations for optical properties. There will
be a focus on data taken by the DOE Gulfstream aircraft (in situ
aerosol samples) and NASA Langley helicopter (surface spectral
albedo and BRDF) during August 1998 at SGP. During the next year,
we will also take the several months of ocean upwelling radiance
measurements at the Chesapeake Light Tower; these are needed to
fully develop Yongxiang Hu's ocean BRDF model in AVHRR channels.
Shorter period measurements from aircraft (supported by CERES) at the
Tower will be used. The GACP-supported measurements of upwelling
spectral SW radiances (SP1A) will be coordinated with existing
broadband measurements at the Tower, which are supported by CERES.
During the third year, we will deliver the ocean BRDF algorithm
in AVHRR channels (with dependence on local wind and sea swell
from operational NCEP ocean model output) to GACP. Testbeds
for aerosol and radiation will be extended to span 1998-2000
at both SGP and at the Chesapeake Light Tower.
Bibliography:
Only partial support to the following paper and presentations:
Kato, S., M. H. Bergin, T. P. Ackerman, T. P. Charlock,
E. G. Clothiaux, R. A. Ferrare, R. N. Halthore, N. Laulainen,
G. C. Mace, J. M. Michalsky, and D. M. Turner, 1999:
A comparison of the aerosol optical thickness derived from
ground-based and airborne measuremnts. Submitted to
the Journal of Geophysical Research.
Alberta, T., and T. P. Charlock, 1999: A comprehensive
resource for the investigation of shortwave fluxes in
clear conditions: CAGEX Version 3. Proceedings of the
Tenth Conference on Atmospheric Radiation. 28 June -
2 July 1999, Madison, WI. pp. 279-282.
Charlock, T. P., T. Yamanouchi, and F. G. Rose, 1999:
Polar clouds and the budgets of heat and moisture at
high latitudes: A review of recent observational studies.
Invited presentation at IAMAS Symposium on Radiation and
Clouds in Polar Regions, IUGG, Birmingham UK, July 21, 1999.
