The cloud analysis tool APOLLO:: improvements and validations

During its more than 12 years of existence, the AVHRR Processing scheme Over cLouds, Land and Ocean (APOLLO) has been modified and extended. The intention of this paper is to document the present state of APOLLO and to summarize the experiences gained. The scope of the processing scheme is reviewed and the actual state of the daytime and night-time algorithms are presented. The reason for the modifications are discussed. Validations of the modifications are made by comparisons with surface observations. They show agreement to +/-1/8 of the cloud coverage in 70% of the approximate to90 000 single comparisons. A consistency check with the International Satellite Cloud Climatology Project (ISCCP) shows the influence of coarse sampling on the ISCCP DX data reliability. APOLLO is now a robust and versatile cloud analysis tool. It is presently used to establish a cloud climatology in Central Europe, based on processing of 15 years of daily Advanced Very High Resolution Radiometer (AVHRR) data.