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Scientists have only a small data set from which to characterize the austral marine radiation environment. Yet, this atmosphere/sea-ice/ocean system is a primary component in the annual global energy budget. The University of Alaska at Fairbanks in collaboration with the University of Denver conducted a small atmospheric radiation campaign in the Ross, Amundsen, and Bellingshausen Seas aboard the R/V Nathaniel B. Palmer in August and September 1995. During a variety of meteorological conditions, middle infrared emissions of the winter atmosphere were measured with a Fourier transform infrared (FTIR) spectrometer. The instrument measures zenith radiance from 550 inverse centimeters (cm-1) to 1,650 cm-1 [18 to 6 microns (µm)] with 1-cm-1 resolution. This wavelength interval is particularly useful because it contains not only much of the blackbody emission for the cold polar atmosphere but also major emission bands for such trace gases as carbon dioxide, ozone, and water vapor.
The instrument was mounted in a portable, insulated house on the back deck of the Palmer. Each 15-minute measurement sequence viewed the zenith sky emission and two calibration blackbodies. Weather permitting, the instrument was operational several hours per day along the cruise track shown in figure 1. Radiosondes were also launched from the ship, providing vertical profiles of atmospheric pressure, temperature, and relative humidity to accompany the radiometric measurements. The upper deck and mast of the ship were equipped with a standard meteorological equipment package and a ground-based ultraviolet radiometer (GUV).
Figure 2 is an example of clear-sky emission in the Ross Sea. Carbon dioxide (667 cm-1), ozone (1,042 cm-1), and water vapor (1,595 cm-1) are major contributors to the infrared radiance observed at the sea-ice surface (Lenoble 1993). The region between the large carbon dioxide band and the water vapor band is known as an "atmospheric window" (770-1,250 cm-1, 8-13 µm). It is relatively transparent but still contains ozone emission, water-vapor continuum emission, and weak lines of other trace gases. A second window (360-625 cm-1), partially captured by the FTIR, is often found in the polar regions where water vapor is minimal, as seen in figure 2.
Many cloud types, ranging from thick, low stratus to altocumulus rolls, were observed along our cruise track. Atmospheric emission received at the surface is greatly increased with the presence of clouds, which close the atmospheric windows. Two cloudy emission spectra, recorded in the Bellingshausen Sea, are shown in figure 3. Curve A represents emission from a developing stratus cloud deck. Although many of the trace-gas emission lines in the 8-13-µm atmospheric window are now obscured, the ozone emission feature is still present. Future radiative transfer calculations will be used to determine if this contribution comes from stratospheric ozone above the cloud or tropospheric ozone below the cloud base (Lubin and Gautier 1992).
Curve B, observed 1.5 hours after curve A in the same location, is emission from a snowing cloud. The snow cloud radiates nearly like a blackbody and doubles the surface radiance measured under a clear sky. These two spectra highlight the dramatic effect of clouds on longwave surface radiation. Clouds must be characterized correctly in modeling exercises in order to obtain accurate values of surface radiance. As an example, models often treat clouds as black or gray emitters. Preliminary analysis of our data and the observations of Lubin (1994) at Palmer Station suggest that precipitating maritime antarctic clouds have emissivities very close to unity, but that cloud types, such as the stratus shown in curve A, figure 3, have emissivities less than unity.
Currently, we are performing error analysis on the spectra. When this task is complete, we will begin a cloud radiative properties study, which involves determining cloud emissivity and optical depth from the observed emission spectra and radiative transfer computations. Further, we hope to extract cloud microphysical properties following the methods developed by Lubin and other collaborating universities. In addition to quantifying these cloud parameters, we will perform trace-gas emission and abundance studies of the late winter, marine atmosphere.
Small field campaigns and cooperative investigations between many universities and geophysical disciplines will benefit the advancement of antarctic science. We hope our FTIR and supplementary data will complement radiometric measurements made in the interior plateau at Amundsen-Scott South Pole Station (Walden 1995) and the coastal Palmer Station (Lubin 1994) and assist in quantifying the longwave radiation budget across the entire antarctic region.
We would like to thank Martin Jeffries, sea-ice researcher at the University of Alaska at Fairbanks, for inviting us to participate in his dedicated cruise on the R/V Nathaniel Palmer. We would like to thank the Atmospheric Technology Division at the National Center for Atmospheric Research for providing us with the radiosonde system and support and the Antarctic Support technicians for their assistance during the cruise.
This work was supported by National Science Foundation grant OPP 95-23260 to the University of Alaska at Fairbanks.
Lenoble, J. 1993. Atmospheric radiative transfer. Hampton, Virginia: A. Deepak.
Lubin, D. 1994. Infrared radiative properties of the maritime antarctic atmosphere. Journal of Climate, 7, 121-140.
Lubin, D., and C. Gautier. 1992. Fourier Transform Infrared spectroradiometer measurements of atmospheric longwave emission over Palmer Station, spring 1991. Antarctic Journal of the U.S., 27(5), 276-278.
Walden, V.P. 1995. The downward longwave radiation spectrum over the antarctic plateau. (Doctoral thesis, University of Washington, Seattle, Washington.)