Cloud Top Temperature, Pressure and Height:
Description and examples
- Goal of the ctth product
- ctth algorithm description
- List of inputs for ctth
- Coverage and resolution
- Description of ctth outputs
- Example of ctth visualisation
The output provides information on the temperature and height (in meters or hPa) of opaque, semi-transparent and sub-pixel cloud tops.
The main use of this product is in the analysis and early warning of thunderstorm development and the height assignment for aviation forecasting. The product may also serve as input to mesoscale models for use in Nowcasting in general, or as input to other satellite retrievals used for Nowcasting. The nwcsaf/pps ctth retrieval, based on imager data from polar orbiter, will also be used to build up cloud climatologies within the cmsaf.
The ctth product is derived using two algorithms, one for opaque and one for fractional and semitransparent clouds, and is applied to all cloudy pixels as given by the ct product.
The ctth takes as input the Cloud Type, and utilises the 11 and 12 micron channels of the avhrr or viirs sensor, nwp short range forecast data, and 1 km gis (digital elevation model and landuse) data. To compensate for the atmospheric attenuation above the opaque clouds, and for the semi-transparency correction, radiative transfer calculations (rttov) using nwp short range forecast output of the vertical temperature and humidity profiles are used.
For pixels classified as semi-transparent cirrus or fractional water clouds there is a histogram technique used handling all semi-transparent pixels in a segment (eg. 32x32 pixels) at the same time.
pps products as input: Cloud Type
Satellite data: 0.6 microns, 11 microns, 12 microns and (1.6 or 3.7 microns), from avhrr or from viirs.
- nwp temperature at several vertical pressure levels
- nwp relative humidity at several vertical perssure levels
- Sun zenith, satellite zenith and sun-satellite azimuth difference angle
- 1 km landuse data (including land/sea mask)
- 1 km digital elevation map
This product is available in full imager resolution and for the whole field of view.
For the calculation at semi-transparent clouds is used as segment resolution, where a segment is eg. 32x32 pixels. This is then overlaid with no-data and opaque cloud data, both in full resolution.
The ctth produces three parameters for the cloud top:
- temperature in kelvin
- height in meters
- height in pressure units (hPa)
There is a quality flag for ctth. It has 16 bits, though some of them are spare bits for possible future use. The flag provides information on the product quality and on processing conditions.
|Class number||Meaning of the bit|
|0||Non-processed (containing no data or corrupt data)|
|1||Cloudy (either opaque, semi-transparent or fractional)|
|3||rttov derivation available|
|4||No nwp data available|
|5||Low level temperature inversion present|
|6||Some (one or several channels) avhrr data missing|
|7||rttov simulations applied in the derivation|
|8||Window/histogram technique applied (for semi-transparent and fractional clouds)|
|9-13||Bit 9-13 are spare bits, for possible other algorithms to come|
|14||Quality assessment performed|
Here an example image of the pps Cloud Top Temperature, Pressure and Height (ctth) product over northern Europ 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.
The ctth 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.