Cloud Type:
Description and examples

Contents:

CloudType
  1. Goal of the Cloud Type product
  2. Cloud Type algorithm description
  3. List of inputs for Cloud Type
  4. Coverage and resolution
  5. Description of Cloud Type outputs
  6. Example of Cloud Type visualisation

Goal of the Cloud Type product

The main objective of the Cloud Type product is to distinguish between thin and opaque clouds, and provide a rough estimate of the cloud top height, and also try to distinguish between water particle clouds and ice particle clouds. The highest priority is given to the reliable identification of the major cloud categories: low, medium, high, and semi-transparent cirrus.

Cloud Type algorithm description

The Cloud Type algorithm takes as input the Cloud Mask output, and utilise all 5/6 spectral channels of the avhrr/2 or avhrr/3 sensor (or the corresponding channels from the viirs sensor), nwp short range forecast data, 1 km gis (digital elevation model and landuse) data and emissivity data. The algorithm distinguish different cloud types using thresholds defined by off-line radiative transfer calculations and a database of interactively collected training targets.

List of inputs for Cloud Type

pps products as input: Cloud Mask

Satellite data: 0.6 microns, 0.9 microns, 11 microns, 12 microns and (1.6 or 3.7 microns), from avhrr or from viirs. From viirs also channel 8.5 microns.

Auxillary data:

Coverage and resolution

This product is available in full imager resolution and for the whole field of view.

Description of Cloud Type outputs

The Cloud Type output consists of:

Main output

Cloud Type Classes
Class number Class name
0 Non-processed
1 Cloud free land
2 Cloud free sea
3 Land contaminated by snow
4 Sea contaminated by snow/ice
5 Very low cumuliform clouds
6 Very low stratiform clouds
7 Low cumuliform clouds
8 Low stratiform clouds
9 Medium level cumuliform clouds
10 Medium level stratiform clouds
11 High cumuliform clouds
12 High stratiform clouds
13 Very high cumuliform clouds
14 Very high stratiform clouds
15 High semi-transparent very thin cirrus
16 High semi-transparent thin cirrus
17 High semi-transparent thick cirrus
18 High semi-transparent cirrus above low or medium level clouds
19 Fractional clouds (sub-pixel water clouds
20 Unclassified (due to known separability problems)

Quality flag

The quality flag, which occupies 16 bits, is partly the same as for Cloud Mask. It provides the user with valuable information on the conditions under which the cloud mask and cloud type processing was performed, and on the quality of the thresholding. The exact outline of the quality flag is given below.

Seven bits to describe the illumination and environmental conditions under which the cloud mask was derived - numbers refer to the individual bits:

Illumination & Environmental conditions
Bit number Meaning of the bit
0 Land/Not land
1 Coast/Not coast
2 Night/Not night
3 Twilight/Not twilight
4 Sunglint/No sunglint
5 High terrain/low terrain
6 Inversion/No inversion: Low level inversion present or not.

Two bits to describe the use/availability of nwp data and the availability of the avhrr channels:

Missing data
Bit number Meaning of the bit
7 nwp data has been used/nwp data not used.
8 One or more avhrr channels missing.

Two bits to describe the quality of the thresholding - if the measurement was close to one of the thresholds of the active test the result will be assigned a low confidence:

Thresholding quality
Bit number Meaning of the bit
9 Low quality/High quality
10 Very low quality - Pixel has been reclassified after spatial smoothing

Two bits to inform on the performance of the cumuliform-stratiform separation, for Cloud Type. This bit will be set to one if separation between cumuliform and stratiform clouds has been performed:

Cumuliform-stratiform distinction
Bit number Meaning of the bit
11 Distinction make/No distinction made
12 Spare. To be used in the future

Three bits for describing the sea ice:

Sea ice data
Bit number Meaning of the bit
13 External sea-ice information used (e.g. osisaf ice maps).
14 Ice information derived from nwp model.
15 Sea-ice present according to sea ice maps (osisaf or nwp).

Example of Cloud Type visualisation

noaa19_20120514_1125_16826.mesanX.cloudtype.thumbnail.jpg (425000 bytes)
clfree_land.gif (101 bytes) Cloud free very_hiopaque_st.gif (178 bytes) Very high
clfree_sea.gif (99 bytes) Cloud free verythincirrus.gif (178 bytes) Very thin cirrus
snow_land.gif (178 bytes) Snow thincirrus.gif (178 bytes) Thin cirrus
snow_sea.gif (178 bytes) Snow/Ice thickcirrus.gif (178 bytes) Thick cirrus
verylow_st.gif (178 bytes) Very low cirrus_above.gif (178 bytes) Cirrus above
low_st.gif (178 bytes) Low fractional.gif (178 bytes) Fractional
medium_st.gif (178 bytes) Medium level unclass.gif (178 bytes) Unclassified
hiopaque_st.gif (178 bytes) High unproc.gif (96 bytes) Unprocessed

Here an example image of the pps Cloud Type product over northern Europe is displayed. The data are from a noaa19 overpass (orbit 16826) May 14, 2012, 11:25 UTC, as received and processed at smhi, Norrköping, Sweden. The corresponding rgb images using avhrr channel 0.6µm,0.9µm,11µm and 3.7µm,11µm,12µm are shown below for comparison with the Cloud Type.

The choice of colours is based first of all on the idea that the meteorological interesting features (clouds and snow) should be easily detected against the background (clear). Secondly it shall be possible to identify the individual cloud classes (in terms of hight, opaqueness, and dominating particle phase) from each other, and from cloud free snow or ice on the ground or sea. Therefore dark colours (black and green) has been chosen for the (clear) background. The semi-transparent cirrus clouds are kept in blue-cyan. Opaque clouds go from orange over yellow, and grey-greenish to white for increasing hight (and increasing amount of ice-particles in the cloud). Fractional cloud is dark purple, whereas ice/snow is bright violet.

The cloud type image shown here has been generated with the nwcsaf/pps software. It displays only part of the information available in the product. The pps Cloud products are most appropriately visualised using a dedicated Graphical User Interface (gui) tool allowing the display of all relevant information available in the hdf5 file, including flags, geographical location, etc. smhi, has developed such a tool running under Unix and freely available upon request. Otherwise the hdf-viewer (hdfview) available from ncsa is recommended.

noaa19_20120514_1125_16826.mesanX.avhrr.ch_06_09_11.thumbnail.jpg (476000 bytes) noaa19_20120514_1125_16826.mesanX.cloudtype.thumbnail.jpg (425000 bytes) noaa19_20120514_1125_16826.mesanX.avhrr.ch_37_11_12.thumbnail.jpg (476000 bytes)