GACP Projects
Global Aerosol Climatology Project (GACP) First Year Accomplishment Report
Title: "A Longterm Testbed and Analysis of Radiative Forcing by Aerosols at Two Sites"
http://www-cave.larc.nasa.gov/gacp/ is the project web page
PI: Thomas P. Charlock, NASA Langley Reaearch Center, MS 420, Hampton, VA 23681-0001 ( mailto:t.p.charlock@larc.nasa.gov)
Co-I: Timothy L. Alberta (t.l.alberta@larc.nasa.gov)
Co-I: Fred G. Rose (f.g.rose@larc.nasa.gov)
Assistance this year from: David A. Rutan (d.a.rutan@larc.nasa.gov) , Ken Rutledge (c.k.rutledge@larc.nasa.gov) , Yongxiang Hu (y.hu@larc.nasa.gov)
Original Abstract of Proposal:
The radiative forcing of aerosols to the atmospheric column is the difference of flux with aerosol and flux without aerosol. As the entire column always contains some aerosols, there is strictly speaking no single direct measurement of radiative forcing by aerosols. Aerosol forcing must rather be inferred from theoretical radiative transfer calculations of fluxes under different conditions of aerosol loading. The theoretical aerosol forcing may then be validated by an analysis of separate measurements of fluxes and aerosol loadings. Such an analysis is proposed here. A rigorous testbed will be developed for the radiative transfer calculations, for the measurements of the fluxes, and for the measurements of aerosols and other atmospheric and surface properties. The testbed will be available to the Science Team on-line. Successive versions of the testbed will re-visit, as appropriate, the original time series with improvements. The testbed is intended as a guidepost for improving retrievals of aerosol radiative forcing over the globe with satellite and surface data.
The testbed will begin at the ARM Southern Great Plains (SGP) Central Facility with a nearly continuous record of:
- measured surface broadband radiative fluxes under full-sky (total-sky, all-sky) conditions,
- atmospheric soundings,
- measured spectral optical depths of aerosol under clear-sky conditions,
- computed vertical profiles of broadband SW and LW fluxes for clear skies, and
- the height profile of aerosol optical properties used to calculate the fluxes.
For a subset of this domain, retrievals of cloud optical properties and measurements of TOA fluxes will be included; these will be obtained from:
- GOES data processed for ARM during Intensive Observing Periods (IOPs),
- CERES overpasses of TRMM, EOS-AM, and EOS-PM, and
- those ARM ground-based MWR, MPL, and cloud radar measurements which coincide with (a) and (b).
For the subset with cloud measurements, computed fluxes for full-sky will be included. For the subset with TOA measurements, an experimental objective analysis algorithm will produce an alternate set of aerosol optical properties that are constrained with radiative transfer calculations.
Radiative fluxes will be computed with a delta-4-stream code, testbed inputs, and assumed aerosol optical properties. Aerosol radiative forcing will be analyzed. New models of aerosol optical properties will be developed to produce fluxes more consistent with the measurements. Aerosol retrievals and optical models by other groups (i.e., MODIS, MISR, and the Science Team supported by this NRA) will be included in the sequence of tests. Data from a second site, an ocean platform at the Chesapeake Lighthouse, will eventually be included.
Goals:
Develop an aerosol and radiation testbed to serve as a guidepost for improving retrievals of aerosol and radiative forcing over the globe with satellite data. Also, develop a package for ocean spectral BDRF that can be used to improve the accuracy of GACP global processing.
Objectives:
Issue a web-based set of radiative transfer calculations with inputs, including aerosol optical properties, and measurements to validate the same, at two sites: the ARM Southern Great Plains (SGP) Central Facility and the Chesapeake Light Tower. The testbed will contain a nearly continuous record of:
- measured surface broadband radiative fluxes under full-sky (total-sky, all-sky) conditions,
- atmospheric soundings,
- measured spectral optical depths of aerosol under clear-sky conditions,
- computed vertical profiles of broadband SW and LW fluxes for clear skies, and
- the height profile of aerosol optical properties used to calculate the fluxes.
(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 transfer 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 measurements, 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 measurements. 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.
