Cloud Type
PGE02 SAFNWC
 

 Table of contents

1.- Goal of CT product 
2.- CT algorithm summary description 
3.- List of inputs for CT 
4.- Coverage and resolution
5.- Description of CT outputs
6.- Example of CT visualisation


Access to "Algorithm Theroretical Basis Document for Cloud Products Processors of the NWC/GEO" for a more detailed description.


1.- Goal of CT product

The cloud type (CT), developed within the SAF NWC context, mainly aims to support nowcasting applications. The main objective of this product is to provide a detailed cloud analysis.
It may be used as input to an objective meso-scale analysis (which in turn may feed a simple nowcasting scheme), as an intermediate product input to other products, or as a final image product for display at a forecaster 's desk. The CT product is essential for the generation of the cloud top temperature and height product and for the identification of precipitation clouds. Finally, it is also essential for the computation of radiative fluxes over sea or land, which are SAF Ocean & Sea Ice products.

The CT product therefore contains information on the major cloud classes : fractional clouds, semitransparent clouds, high, medium and low clouds (including fog) for all the pixels identified as cloudy in a scene. A second priority (not implemented in the current version) is the distinction between convective and stratiform clouds.

 

2.- CT algorithm summary description

The CT algorithm is a threshold algorithm applied at the pixel scale, based on the use of CMA and spectral & textural features computed from the multispectral satellite images and compared with a set of thresholds.

The set of thresholds to be applied depends mainly on the illumination conditions, whereas the values of the thresholds themselves may depend on the illumination, the viewing geometry, the geographical location and NWP data describing the water vapour content and a coarse vertical structure of the atmosphere.

The CT classification algorithm is based on the following approach:

Some details on the tests are given in the Algorithm Theoretical Basis Document, that can be downloaded from this web page.

 

3.- List of inputs for CT

 

4.- Coverage and resolution

The CT software has been designed to allow the processing at SEVIRI IR full spatial resolution of any rectangular areas defined by the user inside the MSG full disk (the processing of the MSG full disk is also possible). The validity of the CT product is commited on the MSG full disk.

 

5.-Description of CT outputs

6.- Example of CT visualisation

It is important to note that the CT product is not just images, but numerical data. At first hand, the CT is rather thought to be used digitally (together with the appended flags (quality) as input to mesoscale analysis models, objective Nowcasting schemes, but also in the extraction of other SAFNWC products (CTTH for example).

Colour palettes are included in CT NetCdF files, thus allowing an easy visualisation of CT categories as illustrated on the SEVIRI CT example.

The user may be interested in visualising all the available classes as displayed on the SEVIRI CT example, or highlight one or a few categories suitable for the application of interest. Product's animation will be a help for the user to interpret the visualized CT, and to identify artefacts (for example, the replacement of a snowy area by a low cloud between two successive pictures may be due only to the transition from day to night, as the snow detection is not possible at nighttime).


Click on thumbnail for full-size this image

Example of SEVIRI CT using the colour palette included in CT NetCdF files.