Cloud Mask
PGE01 SAFNWC

 
  Table of contents

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


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

1.- Goal of CMA product

The cloud mask (CMA), developed within the SAF NWC context, aims to support nowcasting applications, and additionally the remote-sensing of continental and oceanic surfaces. The CMA allows identifying cloud free areas where other products (total or layer precipitable water, land or sea surface temperatures, snow/ice cover delineation) may be computed. It also allows identifying cloudy areas where other products (cloud types and cloud top temperature/height) may be derived.

The central aim of the CMA is therefore to delineate all cloud-free pixels in a satellite scene with a high confidence. In addition, the product provides information on the presence of snow/sea ice, dust clouds and volcanic plumes.

 

2.- CMA algorithm summary description

The algorithm is based on multispectral threshold technique applied to each pixel of the image.

A first process allows the identification of pixels contaminated by clouds or snow/ice. It consists in a series of tests applied to various channels combination for each pixels of the current slot. This is complemented by an analysis of the temporal variation (on a short period of time: around 15 minutes depending on the satellite) of some spectral combination of channels (to detect rapidly moving clouds), a specific treatment combining temporal coherency analysis and region growing technique (to improve the detection of low clouds), a temporal analysis of the high resolution visible channel (on SEVIRI: HRV) to detect sub-pixel cumulus clouds and finally a second limited set of multispectral tests with thresholds computed from RTTOV applied on-line to NWP vertical profiles to allow a more accurate detection of low or thin high clouds that remained undetected. The characteristics of the first set of tests are summed up below:

This first process allows to determine the cloud cover category of each pixel (cloud-free, cloud contaminated, cloud filled, snow/ice contaminated or undefined/non processed) and compute a quality flag on the processing itself. Moreover, the tests that have allowed the cloud detection (more that one test are possible, if some tests were not really successful) are stored.

A second process, allowing the identification of dust clouds and volcanic ash clouds, is applied to all pixels (even already classified as cloud-free or contaminated by clouds). The result is stored in the dust cloud and volcanic ash cloud flags.

Details on the algorith are available in the Algorithm Theoretical Basis Document, that can be downloaded from this web page.
 

Nighttime

Twilight

Daytime

Sunglint

Solar elevation < -3°

-3°<Solar elevation<10°

10° < Solar elevation

Cox & Munck > 10%
Solar elevation > 15°

Table 1: Definition of illumination conditions

  Cox & Munck stands for the reflectance computed using Cox & Munck theory ; the solar elevation is expressed in degrees.
 

 

Daytime

Twilight

Nighttime

Snow detection

Snow detection

T10.8µm

T10.8µm

T10.8µm

T10.8µm-T12.0µm

R0.6µm

R0.6µm

T8.7µm-T10.8µm

T10.8µm-T12.0µm

T10.8µm-T12.0µm

T10.8µm-T8.7µm

T8.7µm-T10.8µm

T8.7µm-T10.8µm

T10.8µm-T3.9µm

T10.8µm-T3.9µm

T10.8µm-T8.7µm

T3.9µm-T10.8µm

T3.9µm-T10.8µm

T10.8µm-T3.9µm

Local Spatial Texture

Local Spatial Texture

T3.9µm-T10.8µm

T8.7µm-T3.9µm

Local Spatial Texture

 

 

T8.7µm-T3.9µm

 

Table 2: Test sequence over land

 

Daytime

Sunglint

Twilight

Nighttime

Ice detection

Ice detection

Ice detection

SST

SST

SST

SST

T10.8µm-T12.0µm

R0.8µm (R0.6µm)

T10.8µm-T12.0µm

R0.8µm (R0.6µm)

T8.7µm-T10.8µm

R1.6µm

T8.7µm-T10.8µm

R1.6µm

T10.8µm-T3.9µm

T10.8µm-T12.0µm

Local spatial texture

T10.8µm-T12.0µm

T12.0µm-T3.9µm

T8.7µm-T10.8µm

R0.8µm (R0.6µm)

T8.7µm-T10.8µm

T3.9µm-T10.8µm

T10.8µm-T3.9µm

T10.8µm-T3.9µm

T10.8µm-T3.9µm

Local spatial texture

T3.9µm-T10.8µm

Low clouds in sunglint

T12.0µm-T10.8µm

 

Local spatial texture

 

T3.9µm-T10.8µm

 

 

 

Local Spatial Texture

 

Table 3: Test sequence over sea.

[T3.9µm, T8.7µm, T10.8µm, T12.0µm stand for brightness temperatures at 3.9, 8.7, 10.8 and 12.0 micrometer; R0.6µm, R0.8µm, R1.6µm stand for VIS/NIR bi-directional top of atmosphere reflectances at 0.6, 0.8 and 1.6 micrometer normalised for solar illumination; SST is the split-window (used for SST calculation) computed from T10.8µm and T12.0µm measurements. Low Clouds in Sunglint is a specific module for low clouds identification in sunglint areas.]

 


3.- List of inputs for CMA

Mandatory inputs are flagged, whereas the impact of missing non-mandatory data on the processing are indicated.
 

HRV R0.6µm R0.8µm R1.6µm T3.9µm T8.7µm T10.8µm T12.0µm T13.4µm
  Mandatory     Mandatory   Mandatory Mandatory  

The CMA software checks the availability of channels for each pixel. If non mandatory channels are missing for one pixel, the tests using these channels are not applied, or applied differently (for example, snow detection uses either R1.6µm or T3.9µm; visible channel test over the ocean uses either R0.8µm or R0.6µm) and a result is available for this pixel. No results are provided for pixels where at least one mandatory channel is missing.

The following bi-directional reflectances or brightness temperatures or CMA or CT of the scene analysed one hour sooner are optionnaly needed to improve the cloud detection in day-night transition. If one is missing this improvement is not performed.

R0.6µm (1h) T8.7µm (1h) T10.8µm (1h) T12.0µm (1h) CMA (1h) CT (1h)
           

The following brightness temperatures or CMA or CT of the scene analysed 15 minutes sooner are optionnaly needed to improve the cloud detection of fast moving clouds. If one is missing this improvement is not performed.

T8.7µm (15mn) T10.8µm (15mn) T12.0µm (15mn) CMA (15mn) CT (15mn)
         

The HRV bi-directional reflectances of the scene analysed 15 minutes sooner are optionnaly needed to improve the sub-pixel cumulus cloud detection. If not available, this improvement is not performed.

The satellite channels are input by the user in HRIT format (or netcdf format for foreign satellites), and extracted on the processed region by NWC/GEO software package.
 

These remapped fields are elaborated by the NWC/GEO software package from the NWP fields input by the user in GRIB format.

The NWP fields are not mandatory: the CMA software replaces missing NWP surface temperatures or total water vapour content of the atmosphere by climatological values extracted from ancillary dataset, but the quality of CMA is then lower.
 

These remapped fields are elaborated by the NWC/GEO software package by applying RTTOV to the NWP fields input by the user in GRIB format.

The RTTOV simulations are not mandatory: if not available, the GEO-CMA software does not apply corresponding tests, the GEO-CMA quality being then slightly lower (especially in nightime conditions).
 

High resolution global daily bulk SST fields (OSTIA) are input by the user who can obtain them from MyOcean service desk (see http://www.myocean.eu.org). They are used in conjunction with RTTOV simulations.

These OSTIA fields are not mandatory: if not available the RTTOV simulations are not used over ocean and the CMA software does not apply corresponding tests, the GEO-CMA quality being then slightly lower (especially in nightime conditions).
 

Rttov bias files are used as input. They can be downloaded from AEMET ftp server. They are valid only for ECMWF model.

These files are not mandatory. If not available, the bias can be computed by GEO-CMA (the processed region needs to contain large enough area covered by oceanic surfaces. If this computation is not possible, the GEO-CMA does not apply test using RTTOV simulation and the GEO-CMA quality being then slightly lower (especially in nightime conditions)).
 

The following ancillary data, remapped onto satellite images, are mandatory :

These ancillary data are available in the NWC/GEO software package at a global scale ; They are remapped on the satellite full disk by NWC/GEO remapping functionality.

Coefficients's file (also called threshold tables), containing satellite-dependent values and look-up tables for IR thresholds and for solar channels' thresholds, are available in the NWC/GEO software package, and are needed by the CMA software.
 

4.- Coverage and resolution

The CMA 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 CMA product is commited inside the MSG full disk.
 

5.-Description of CMA outputs

CMA product are coded in NetCdF format and include :


6.- Example of CMA visualisation

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

Colour palettes are included in CMA NetCdF files, allowing an easy visualisation of CMA main categories, dust and volcanic ash clouds flags.

No examples of CMA main categories are given, as it is thought that the user will be more interested to visualize the CT product which can be seen as a refinement.

Examples of visualisation of the dust cloud and the volcanic ash clouds flags superimposed on an infrared image are presented, using SEVIRI and MODIS imagery.