2001b), Ground-based infrared remote sensing of cloud properties over the Antarctic Plateau. part II: Cloud optical depths and particle sizes

• Main Authors: Ashwin Mahesh, Von P. Walden, Stephen G. Warren
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Antarctic; The Antarctic; South Pole; Antarc*; South pole
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.498.2886; http://www.atmos.washington.edu/~sgw/PAPERS/2001_MaheshII.pdf

One full year of twice-daily longwave atmospheric emission spectra measured from the surface at 1-cm 21 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-mm wavelength and the cloud’s trans-mittance of stratospheric ozone emission in the 9.6-mm 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 mm 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, 5, and the effective particle radii reff are distinguished only for reff, 25 mm, 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, 1, in contrast to coastal clouds, which usually have tg. 20. Most have reff in the range of 5–25 mm, with a mode at 15 mm. 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 mm, 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