Cloud properties derived from spectral radiance measurements made at South Pole Station in 1992/93. --------------------------------------------------------------- File description: column 1: Year of measurement column 2: Month of measurement column 3: Day of measurement column 4: Hour of measurement column 5: Minute of measurement column 6: Retrieved value of cloud particle radius (in micrometers) column 7: Retrieved value of infrared optical depth column 8: Retrieved value of cloud-base height (in meters above the surface; surface elevation is 2835 meters above sea level) Notes ----- 1) Particle radii values of '-888' indicate that the radii are greater than 25 um. 2) Optical depth values of '-999' indicate that the optical depth is greater than 5. --------------------------------------------------------------- The retrievals of the cloud-base heights are described in: Mahesh, A., V.P. Walden, and S.G. Warren, 2000. Ground-based infrared remote sensing of cloud properties over the Antarctic Plateau. Part I: Cloud-base heights, J. Appl. Meteor., 40, 1265-1278. Abstract: A Fourier-transform interferometer, operated throughout 1992 at South Pole Station, measured downward spectral longwave radiance from 550 to 1500 cm 21 (7-18 microns) at a resolution of 1 cm-1 . Radiance measurements were usually made twice daily, coincident with routine launches of radiosondes made by the South Pole Weather Office; 223 radiance measurements (40% of the observations) were of cloudy-sky conditions. Cloud-base heights are retrieved from these data using a ground-based version of the radiance-ratioing method, which was originally developed to retrieve cloud-top heights from satellite data. Frequencies in the R branch of the 15-micron carbon dioxide band are used, exploiting the variation of atmospheric opacity with wavenumber. The annual cycle of cloud-base heights shows a bimodal distribution in all seasons except during the brief summer (December-January). Cloud-base heights are typically higher in the summer than in winter. Although retrieved cloud-base heights are uncorrelated with heights estimated by visual observers, both the retrieved and observed data indicate that base heights are bimodal. Most clouds have bases in the lowest few hundred meters, within the surface-based temperature inversion. The other mode is of higher clouds with base heights 1.5-3 km above the surface. Even the highest tropospheric clouds are within 6 km of the surface. Radiance ratioing can be used to detect polar stratospheric clouds, but their base heights are not reliably determined by the method. --------------------------------------------------------------- The retrievals of particle radii and optical depths are described in : Mahesh, A., V.P. Walden, and S.G. Warren, 2000. Ground-based infrared remote sensing of cloud properties over the Antarctic Plateau. Part II: Cloud optical depths and particle sizes, J. Appl. Meteor., 40, 1279-1294. Abstract: One full year of twice-daily longwave atmospheric emission spectra measured from the surface at 1-cm-1 resolution are used to infer optical thicknesses and ice crystal sizes in tropospheric clouds over the Antarctic Plateau. The method makes use of the cloud’s emissivity at 10- and 11-micron wavelength and the cloud’s transmittance of stratospheric ozone emission in the 9.6-micron band. Knowledge of the cloud-base temperature and the vertical distributions of ozone and temperature is required; these are available at South Pole Station from radiosondes and ozonesondes. The difference in emissivity between 10 and 11 microns is sensitive to ice particle size because the absorption coefficient of ice varies greatly between these two wavelengths. The retrieval of optical depth (expressed as its value in the geometric-optics limit tg ) is limited to tg less than 5, and the effective particle radii reff are distinguished only for reff less than 25 microns, but 80% of the clouds observed have tg and reff in the retrievable range. These clouds over the Antarctic interior are found to be optically thin, usually with tg less than 1, in contrast to coastal clouds, which usually have tg greater than 20. Most have reff in the range of 5-25 microns, with a mode at 15 microns. The retrieved reff is larger in summer than in winter, in agreement with in situ measurements. From November to April, reff was usually at least 10 microns, whereas, for a 3-month period in winter (July-September), no reff values greater than 25 mm were retrieved. The particle sizes retrieved from the infrared spectra are compared with dimensions of ice crystals falling to the surface and measured on photomicrographs. Effective spherical radii are computed from the photographs in three ways: equal area, equal volume, and equal volume-to-area ratio (V/A). Agreement with the reff derived from radiation measurements is best for equal-V/A spheres. The optical thicknesses and base heights inferred from the emission spectra agree qualitatively with the visual reports of the weather observers, in that the optically thicker clouds are usually reported as nimbostratus and clouds with the highest retrieved bases are reported as cirrus or cirrostratus. Stratus clouds tend to be reported as low; altostratus and altocumulus are intermediate in height. --------------------------------------------------------------- The spectral radiance measurements are described in: Walden, V.P., S.G. Warren, and F.J. Murcray, 1998. Measurements of the downward longwave radiation spectrum over the Antarctic Plateau and comparisons with a line-by-line radiative transfer model for clear skies. J. Geophys. Res., 103, 3825-3846. Walden, V.P., 1995. The downward longwave radiation spectrum over the Antarctic Plateau, Ph.D. thesis, Univ. of Washington, Seattle.