AVHRR/APOLLO - Cloud physical products of latest AVHRR-scenes received in Oberpfaffenhofen


The AVHRR (Advanced Very High Resolution Radiometer) sensor onboard NOAA-16 measures in six spectral bands at 0.65 µm, 0.9 µm, 1.6 µm (day only), 3.7 µm (night only), 11.3 µm and 12.3 µm. These measurements are used to determine cloud parameters and ocean temperature. The APOLLO (AVHRR Processing scheme Over cLouds, Land and Ocean) software-package is used to routinely derive cloud physical parameters from AVHRR measurements. For each pass of the satellite the program puts each AVHRR pixel into boxes labeled either cloud-free, partially cloudy, fully cloudy, or snow-covered. This is achieved by applying several threshold tests to each pixel. The result is stored in a so-called cloud-mask (a reference source), together with a mask identifying whether the pixels in the image are land or sea, and information about sun glint, solar zenith, and satellite observation angles. From these data, cloud cover can be computed for each pixel, the categories being total, thick (low, medium, high) or thin cover. In addition, cloud optical depth, cloud liquid water or ice path (vertical column content), cloud top temperature, and cloud thermal emittance can also be determined. Cloud-mask, coverage and cloud-top-temperature are shown on this page together with a color-composite and are updated daily using the latest passes as soon as a precise or automatic georeferencing is available. If the automatic georeferencing is used, this is indicated in the pass-id line together with date and time and by an empty file "....._auto_nav" in the according archives directory. Automatic georeferencing is nearly as good as precise (interactive) georeferencing in almost all passes, but sometimes failes and leads to one or even a few pixels deviation. The reason for that can be for example too much clouds in the scene so that not enough coastlines can be found.
Normally, the APOLLO products are available here within three hours at the maximum from reception of the data, i.e. in near-real time (NRT).

Note that discrimination between snow/ice and clouds is only possible in daytime-passes. In nighttime-passes most of the snow is classified as clouds and also more clouds are classified as partially cloudy than totally.
The APOLLO program can be applied to all AVHRR scenes except in polar regions in presence of the persistent temperature inversions there. APOLLO requires navigated and calibrated input data such as that achieved by applying either ESA/ESRIN's SeaSHARK program, or

TeraScan. The input AVHRR-HRPT data for Europe can be obtained at DFD. AVHRR-LAC and -GAC data input is also possible. This data can be ordered for example from NOAA. APOLLO is additionally able to process the data of the ATSR-2 instrument onboard the ERS-2 satellite and will be able to process AATSR data onboard ENVISAT.