Climate Forcing by Clouds and Aerosols: Two Years of Field Studies


Peter Pilewskie, Warren Gore, and Larry Pezzolo

The NASA Ames Radiation Group contributed to four major multi-agency field campaigns conducted during 2000 and 2001. These experiments were geared towards improving our understanding of how aerosols and clouds affect global climate through their influence on the radiative energy budget. In the case of atmospheric aerosols, for example, it is considered that their impact may mitigate to some degree the expected warming due to the increasing concentrations of carbon dioxide and other greenhouse gases. The extent to which aerosols may reduce global warming depends on how they absorb and scatter radiation and on the spatial and temporal distribution.

The Radiation group has measured the spectral distribution of solar radiation from various airborne platforms during the DOE ARM ARESEII mission in 2000; the ONR/NASA Puerto Rico Dust Experiment in 2000; the NASA SAFARI 2000 mission in 2000; and the ONR/NSF ACE-Asia experiment in 2001. During 2001 the group made significant strides in interpreting the data collected from these experiments.

_ Puerto Rico Dust Experiment (PRIDE)

We have derived for the first time the absorption spectrum of Saharan dust aerosol and determined its spectral radiative forcing. This was achieved by measuring solar spectral radiative flux above and below layers of dust in the atmosphere over the Western Atlantic Ocean. The origin of the dust aerosol can be traced to the Sahara Desert. Thus the influence of Saharan dust is of global scale and our radiometric observations will enable us to determine how this dust affects the global energy balance. Figure 1 shows results from PRIDE.

[Measured and Modeled Dust Absorption Plot]

Figure 1. Measure (blue curve) and modeled (red symbols) fractional dust absorption for a Saharan air layer during the PRIDE flight on 15 July 2000. In the near-infrared water vapor is a strong absorber in several bands; at wavelengths less than 600 nm the absorption is due primarily to dust. The calculation is from a correlated-k distribution model developed by R. Bergstrom custom designed to match SSFR spectral range and slit function.

_ Atmospheric Radiation Measurement Enhanced Shortwave Experiment II (ARESEII)

The ARESEII mission was conducted to determine the amount of solar radiation absorbed by cloud layers. This topic has remained a large source of concern in the radiation science community because its uncertainty is considered to be the largest in the entire global radiation budget and perhaps four times as large as the expected forcing due to a doubling of carbon dioxide. The Ames Radiation Group has provided the first simultaneous measurements of a complete spectrum of solar radiation reflected from and transmitted through clouds. This enables us to assess the performance of models used to predict the amount of solar radiation absorbed by clouds. We have determined that our most detailed radiative transfer models can quantitatively match the measured cloud absorption over much of the solar spectrum. However, in some bands discrepancies between measurement and model remain and need to be resolved in order to better understand the role of clouds in the climate system. For example, a comparison between measured and modeled cloud irradiance and albedo during ARESEII in Figure 2 shows a discrepancy between measured and modeled upwelling irradiance as high as 5% in the mid-visible. However, this may be due to errors in the exoatmospheric solar source function rather than the cloud model.

[SSFR meaured and modeled Spectra]

Figure 2. Upper Panel: Comparison between SSFR measured (blue spectra) and modeled (red spectra) upwelling and downwelling spectral irradiance for the ARESEII case on 29 March 2000. The retrieved cloud optical depth was 45 and effective droplet radius was 9.5 microns. Lower Panel: Measure and modeled albedo (ratio of upwelling to downwelling) for the same case as above.

_ Southern African Regional Science Initiative (SAFARI)-2000

In a manner similar to our analysis of PRIDE spectral irradiance, we have determined the spectral absorption properties of dust and smoke aerosol in the atmosphere over the southern African continent and found them to be quite varied. Aerosol radiative properties depend on composition, size, and shape and thus are linked to their origin. During the south Africa dry season there are various sources of smoke and dust ranging from anthropogenic sources such as industrial and agricultural burning and from wind-borne dust linked to agricultural practices. Our efforts will aid in determining how these various types of aerosol can affect the radiative energy balance over this region.

_ Aerosol Characterization Experiment-Asia (ACE-Asia)

ACE-Asia conducted in 2001 was the third in a series of experiments focused on aerosol radiation and chemistry. This latest ACE mission was focused primarily on Asian Dust. The Ames Radiation Group contributed by measuring solar spectral radiative flux throughout the lower troposphere. Twenty flight missions took place over a six week period in March and April during which time we collected nearly 200,000 spectra. Data are being analyzed much in the same manner as that which we collected during PRIDE and SAFARI-2000. Preliminary analysis indicated that the Asian dust absorption is similar in its spectral signature to the Saharan dust we observed over the Western Atlantic.


Collaborators: Maura Rabbette, John Pommier, Steve Howard, Robert Bergstrom, BAERI

Point of Contact: Peter Pilewskie, 650/604-0746,