Dependence of frequency of convective cloud occurrence on the orbital drift of satellites.

Deriving accurate time-series of cloud cover from satellite sensor data still remains a challenging task. The instruments onboard polar orbiting NOAA satellites offer the opportunity to prepare cloud climatology on a global scale; however, the orbital drifts of these satellites can introduce uncertainty when deriving such cloud climatology. The aim of this letter is to point out the importance and to estimate the impact of orbital drift on long-term time-series of the observation of convective cloud frequency of occurrence. The 20 years of daytime AVHRR data from over the Indian subcontinent for the summer monsoon season is used in this study. All four AVHRRs onboard NOAA-7, -9, -11, and -14 satellites show positive correlation between increased cloud frequency and the delay in equator crossing-times during their lifetime. This increase is significant over land, but over the ocean, there is no discernible effect. This effect should be considered to avoid spurious trends in cloud cover. Further in-depth investigations are needed to make possible corrections. [ABSTRACT FROM AUTHOR]