The South Pole Atmospheric Radiation and Cloud Lidar Experiment (SPARCLE)

During a full-year field experiment at South Pole Station, longwave radiation emitted by clouds, greenhouse gases, and the snow surface is being measured at a spectral resolution of 1 cm-1 from 3 to 25 micrometers wavelength using a Polar Atmospheric Emitted Radiance Interferometer (PAERI). These spectra are being used to improve atmospheric radiation models of the water-vapor greenhouse effect, and to quantify the contributions of water vapor, carbon dioxide, ozone, and clouds to the longwave energy budget of the Antarctic Plateau. The spectra are also being used for ground-based remote sensing of cloud base heights, cloud optical depths, and cloud ice crystal sizes. The vertical distribution of backscattering by ice crystals in clouds is measured by a micropulse lidar. Ice crystals in clouds, as well in diamond dust and blowing snow, are sampled by a hydrometeor videosonde (HYVIS) flying on a tethered balloon, and are also collected at the surface as they fall, for photomicroscopy. The balloon also carries a frost-point hygrometer to measure humidity in the lowest 2 km of the atmosphere throughout the year, for use in explaining the ice crystal shapes observed and to constrain the radiation modeling.

PAERI observations of the snow surface are used to infer simultaneously the spectral longwave emissivity and the surface temperature of snow and surface frost. The emissivity experiment is designed to resolve a conflict between theoretical predictions and laboratory measurements of the dependence of emissivity on snow grain size. Snow surface temperature, and temperature in the lowest 2 meters of the atmosphere, are measured continuously by a vertical string of closely-spaced thermistors to support the emissivity experiment and to investigate the surface-to-2-meter temperature difference and its dependence on wind speed and cloud cover.

The horizontal uniformity of the surface of the Antarctic Plateau makes it suitable as a natural laboratory for measurement of the continuum absorption spectrum of water vapor and its temperature dependence at low temperatures. The continuum absorption coefficients are needed for climate modeling to compute cooling to space by upper tropospheric water vapor worldwide. For this experiment, the PAERI is reconfigured as a transmissometer during two intensive operating periods of one month each. An infrared beam from a hot source is sent via a telescope to a retroreflector 500 m away, to obtain a 1-km horizontal path over which spectral transmission is measured; the water vapor density is measured by frost-point hygrometers spaced along the path.

The one-year experiment is underway from November 2000 to November 2001; results from the first few months will be presented at the meeting.