Tuesday, 11 January 2005
Cloud cover climatology for the South Pole from surface-based infrared radiation measurements.
Fractional cloud cover (FCC) is poorly known at high latitudes during wintertime . Nighttime visual observations of FCC (FCCvis)are unreliable because observers are not typically able to distinguish between thin cirrus, which are prevalent during the polar night, and clear skies. Satellite retrievals of FCC are also uncertain during night predominantly due to lack of reliable ground-based measurements from which to validate the retrievals. During the polar day, satellite retrievals suffer from the challenge of radiatively distinguishing clouds and snow pack of similar temperatures and albedos. A cloud cover climatology for the South Pole derived from ground-based spectral and broadband infrared radiation measurements is presented here. The spectral infrared measurements were made as part of the South Pole Atmospheric Radiation and Cloud Lidar Experiment (SPARCLE) during 2001. Ground-based pyrgeometer measurements of infrared flux from 1994-2003 made by NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL) are presented here. The spectral infrared data are used to accurately estimate FCC for a single year and compared with FCC estimated from the pyrgeometer data (FCCpyr). The multi-year FCCpyr climatology is then derived and compared to three independent measures of FCC: (1) FCCvis, (2) FCC derived from the International Satellite Cloud Climatology Project D-series data set (FCCISCCP), and 3) FCC derived from Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder data (FCCAPP-x). Summertime FCCpyr agree fairly well with the FCCvis. The wintertime FCCpyr is much higher (by about 0.3-0.5) than the FCCvis. FCCISCCP and FCCAPP-x only agree with with FCCpyr on long-term averages during winter.