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.
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 sensor projection is the generic projection of the data. It represents the view of the sensor with optimum pixel resolution at nadir-view and decreasing resolution with increasing view-to-nadir-angle. The spatial resolution of the AVHRR instrument is 1.1 km at nadir up to about 5 km to the edges of the pass. To compare different passes or mosaic them together into one multitemporal image, they before must be remapped into a map-projection, e.g. a stereographic projection. The images shown below are a color-composite and three thematic maps of APOLLO-products together with scales explaining the color-coding.
NOAA-19 AVHRR, 2019 Jun 24, 12:12 UT
The stereographic projection used here has its reference point at 60° north and 0° meridian. The center of the image is shifted to 48° north and 11° east for the Central Europe region and to 51° north and 10° east for the Germany region. These remapped images are not shown on this page but can be downloaded from the archives (see download paragraph).
Note: All images and content shown on this page or on sub-pages of this page are allowed to be copied for private use only.
Any use for non-private or commercial purposes must be permitted by DLR under contractual rights.
The image-filenames are in the format "RRSSSYYMMDDHHMM_key1_key2" where RR is the receiving site (OP=Oberpfaffenhofen, BE=Berlin),
SSS is the Satellite (N16=NOAA-16), YY is the year, MM is the month, DD is the day, HH is the hour and
MM is the minute of reception of the first line in UT. key1 stands for the product where "comp" is the
color-composite, "cov" is the coverage, "ctt" is the cloud-top-temperature and "ext" is the color_extract of the
cloud-snow-ice-mask. A product-legend is given as color-scales above on this page. key2 stands for the projection
and region where "sat" resp. "sat_full" means satellite(=sensor)-projection in reduced resp. full resolution and
"ster" resp. "germany_ster" means stereographic projection of Central Europe region resp. Germany. The images
all have a coast-line overlay and the stereographic images additionally have political border-lines overlayed.
Last changes: March 2nd, 2011