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GACP Projects

Global Aerosol Climatology Project: Year 2 Progress Report

I. Field Experiments

A. In June and July, 2000, we participated in the Puerto Rico Dust Experiment (PRIDE), a multi-agency field study of the radiative, microphysical, and transport properties of Saharan dust. There were two primary objectives:

  1. Determine the extent to which the properties of dust particles and the spectral surface reflectance of the ocean surface need to be known before remote sensing systems can accurately determine optical depth and flux.
  2. Evaluate/validate the skill in which the Naval Research Laboratory's Aerosol Analysis and Prediction System (NAAPS) predicts the long-range transport and vertical distribution of African dust.

The results of these efforts will support Navy and NASA applied science objectives on satellite validation and the prediction of dust-induced visibility degradation. In addition, secondary thrusts of PRIDE will address in situ issues of coarse mode particles and basic research issues on climate forcing, geochemical cycles, and meteorology.

Our specific contributions to PRIDE were to provide measurements and analyses of solar spectral fluxes. The Ames Solar Spectral Flux Radiometer was deployed on the SPAWAR Navajo, measuring upwelling and downwelling spectral irradiance between 300 and 1700 nm. A similar SSFR was deployed at a ground site to obtain downwelling irradiance at the surface. The data will be used to determine the net solar radiative forcing of dust (and other) aerosol, to quantify the solar spectral radiative energy budget in the presence of elevated aerosol loading, to support satellite algorithm validation, and to provide tests of closure with in situ measurements.

B. In August/September 2000 we will participate in the Southern African Regional Science Initiative (SAFARI 2000), an international science initiative aimed at developing a better understanding of the southern African earth-atmosphere-human system. The goal of SAFARI 2000 is to identify and understand the relationships between the physical, chemical, biological and anthropogenic processes that underlie the biogeophysical and biogeochemical systems of southern Africa. Particular emphasis will be placed upon biogenic, pyrogenic and anthropogenic emissions, their characterization and quantification, their transport and transformations in the atmosphere, their influence on regional climate and meteorology, their eventual deposition, and the effects of this deposition on ecosystems

During SAFAR-2000 the Ames Radiation Group will deploy SSFRs on the ER-2, the University of Washington CV-580, and at a ground site. Data will be used to characterize the spectrally dependent cloud and aerosol radiative forcing.

II. Publications:

A paper titled "The Discrepancy Between Measured and Modeled Downwelling Solar Irradiance at the Ground: Dependence on Water Vapor" was published in the journal of Geophysical Research Letters. The results presented in this paper show that a bias between measured and modeled downwelling irradiance at the surface is highly correlated with water vapor and increases at a rate of 8 Wm-2 per cm of water vapor and is concentrated between 400 and 800 nm.

Accepted for publication by the Journal of Geophysical Research was a paper titled "Multivariate analysis of solar spectral irradiance measurements." Principal component analysis was used to characterize approximately 7000 downwelling solar irradiance spectra retrieved at the Southern Great Plains site during an Atmospheric Radiation Measurement (ARM) Short Wave Intensive Operating Period. This analysis technique has proven to be very effective in reducing a large set of variables into a much smaller set of independent variables while retaining the information content. It is used to determine the minimum number of parameters necessary to characterize atmospheric spectral irradiance or the dimensionality of atmospheric variability. It was found that well over 99% of the spectral information was contained in the first six mutually orthogonal linear combinations of the observed variables (flux at various wavelengths). Rotation of the Principal Components was effective in separating various components by their independent physical influences. The majority of the variability in the downwelling solar irradiance was explained by the following fundamental atmospheric parameters, in order of importance they are cloud cover, water vapor, molecular scattering, and ozone. None of the primary components could be assigned to aerosol extinction, implying that for this data set aerosol radiative effects were highly correlated with other variables.

A related paper titled Rabbette "Principal component analysis of Arctic solar irradiance spectra" was submitted to the Journal of Geophysical Research, Oceans.

III. Radiative Transfer Modeling

A new radiative transfer model has been developed for analyzing SSFR data. The model uses the SSFR slit functions and the correlated-k method of deriving absorption coefficients in bands that match the SSFR spectral response. This model will improve our ability to assess various aspects of solar radiative transfer and improve our efficiency in reducing large data sets from field experiments.

We have completed work on the following aspects of the k distribution model:

  1. Generation of the k’s for H2O, CO2, O2 and O3 from the latest LBLRTM and the latest HITRAN database (including the Giver corrections to water vapor)
  2. Construction of the algorithm for computing the k’s for any arbitrary pressure and temperature (an interpolation in log pressure and temperature from a standard atmosphere) for the 140 spectral bands (16 sub intervals per band). The bands range from 300 nm to 1700 nm in 10 nm increments.
  3. Generation of correlation of the k’s and the instrument specific filter function for all of the bands. This allows the direct comparison of the predictions with the data.
  4. Integration of the DISORT scattering code to the program.
  5. Successful test runs of 140 bands, 32-layer atmosphere, prescribed aerosol properties, 16 scattering angle DISORT computations. Time required was about 5 minutes on a Pentium III, 500 mghz Linix machine per spectra.
  6. Generation of the solar spectra (from Kurucz calculations) for all 140 bands accounting for the correlation of the water lines and the solar emission lines.

Publications:

Pilewskie, P., M. Rabbette, R. Bergstrom, J. Marquez, B. Schmid, and P.B. Russell, The discrepancy between measured and modeled downwelling solar irradiance at the ground: Dependence on water vapor. . Geophys. Res. Lett. 25, 137(2000).

Rabbette , M. and P. Pilewskie, Multivariate analysis of solar spectral irradiance measurements. Accepted for publication, J. Geophys. Res. (2000).

Bergstrom, et al., A Radiative Transfer Model for Climate Calculations, Submitted, J. Geophys. Res. (2000).

Rabbette , M. and P. Pilewskie, Principal component analysis of Arctic solar irradiance spectra. Submitted, J. Geophys. Res. (2000).

Conference Proceedings:

Bergstrom, RW, P. Pilewskie, and E. Mlawer, Radiative Transfer Modeling of Hyperspectral Solar Irradiance and Comparison to Measurements, Poster given at the International Radiative Symposium – IRS 2000, St. Petersburg, Russia, July 24-28, 2000

Pilewskie, P., J.S. Reid, and S. Tsay, Measurements of the sea-surface spectral reflectance in a coastal region and estimates of solar spectral radiative forcing of a marine boundary layer aerosol. Proceedings of the American Geophysical Union, 1999 Fall Meeting, San Francisco. (1999)

Rabbette, M., P. Pilewskie, P.V. Hobbs, and J. Petti, The solar radiative energy budget in the Arctic. Proceedings of the American Geophysical Union, 1999 Fall Meeting, San Francisco. (1999)