of CTTH product
The cloud top temperature and height (CTTH), developed within the SAF NWC
context, aims to support nowcasting applications. This product contributes
to the analysis and early warning of thunderstorm development. Other applications
include the cloud top height assignment for aviation forecast activities.
The product may also serve as input to mesoscale models or to other SAF NWC
product generation elements.
The CTTH product contains information on the cloud top temperature and height
for all pixels identified as cloudy in the satellite scene.
algorithm summary description
The different steps of the processing, applied to cloud-classified image,
are listed below. The exact process applied to each pixel depend on the availability
of NWP and satellite data.
- If all mandatory NWP and satellite data are available (see list of input
The following process is then applied:
- RTTOV radiative transfer model is applied using NWP temperature and humidity
vertical profile to simulate 6.2µm, 7.3µm, 13.4µm, 10.8µm,
and 12.0µm cloud free and overcast (clouds successively on each RTTOV
vertical pressure levels) radiances and brightness temperatures. This process
is performed in each segment of the image (the size of the segment is defined
by the user, the default value being 4*4 satellite IR pixels). The vertical
profiles used are temporally interpolated to the exact slot time using the
two nearest in time NWP fields input by the user.
- The techniques used to retrieve the cloud top pressure depend on the cloud’s
type (as available in CT product):
- For very low, low or medium thick clouds : The cloud top pressure
is retrieved on a pixel basis and corresponds to the best fit between
the simulated and the measured 10.8µm brightness temperatures.
The simulated brightness temperature are available at the segment resolution.
In case of the presence of a low level thermal inversion in the forecast
NWP fields, the very low, low or medium clouds are assumed to be above
the thermal inversion only if their brightness temperatures are colder
than the air temperature below the thermal inversion minus an offset
whose value depends on the nature of the thermal inversion (dry air
above the inversion level or not).
- For high thick clouds: a method called the radiance
ratioing technique (see the next bullet for further explanation of the
method) is first applied to remove any remaining semi-transparency that
could have been undetected by the cloud type scheme. In case of failure,
the method defined for medium opaque clouds is then applied.
- For high semi-transparent clouds: The 10.8µm infrared
brightness temperatures are contaminated by the underlying surfaces
and cannot be used as for opaque cloud. A correction of semi-transparency
is applied, which requires the use of two infrared channels: a window
(10.8µm) and a sounding (13.4µm, 7.3µm or 6.2µm)
channels. The basis is that a cloud has a higher impact in a window
channels than in a sounding channels. The following process is implemented:
- The H2O/IRW intercept method (as described by J.Schmetz), based
on a window (10.8µm) and sounding (13.4µm, 7.3µm
or 6.2µm) radiance bi-dimensional histogram analysis, is first
applied. The histograms are built in boxes of 32x32 satellite IR pixels
centred on each segment of the image (whose size is defined by the
user, the default value being 4*4 satellite IR pixels). It therefore
allows the retrieval of cloud top pressure at the segment horizontal
resolution (i.e., by default 4*4 satellite IR pixels). This method
is successively applied using the 7.3µm, 6.2µm and 13.4µm
radiances, the final retrieved cloud pressure being the minimum
cloud top pressures obtained using single sounding channels.
- If no result can be obtained with the H2O/IRW intercept method,
the radiance ratioing method, as described by Menzel, is then applied
to the 10.8µm and 7.3µm radiances to retrieve the cloud
top pressure at a pixel basis. If no result can be obtained, the
method is applied to 6.2µm and finally to 13.4µm radiances.
- If the radiance ratioing technique leads to cloud top temperatures
warmer than the corresponding 10.8µm brightness temperatures,
the method for thick clouds is used instead.
- For fractional clouds : No technique is proposed in the current
version for low broken clouds. The sounding channels are nearly unaffected
by broken low clouds and are therefore useless; the infrared 10.8µm
and 12.0µm are contaminated by the surface and cannot be used
as for opaque clouds.
- A gap-filling procedure is applied in semi-transparent cloud top pressure
fields: in each box of 32*32 satellite IR pixel, a cloud top pressure is computed
as the average pressure of all pixels containing semi-transparent clouds
inside the current and the eight surrounding boxes. This average cloud top
pressure is then assigned to all pixels of the current box containing semi-transparent
clouds and having no retrieved cloud top pressure.
- Cloud top temperature and height (above sea level) are then computed from
their pressure using general modules. During these processes, the atmospheric
vertical profiles are temporally interpolated to the exact slot time using
the two nearest in time NWP outputs fields.
- Effective cloudiness (defined as the fraction of the field of view covered
by the cloud (the cloud amount) multiplied by the cloud emissivity in the
10.8µm window channel) is also computed during the processing. It
is equal to 1.0 for thick clouds and takes a value between 0.0 and 1.0 for
The forecast's quality may affect the result, especially for the semi-transparent
clouds for which the pressure retrieval relies very much on the humidity vertical
distribution and for the low clouds in case of temperature inversion.
- In case some mandatory NWP or satellite data are missing (see list of
inputs for CTTH):
Cloud top temperature of very low, low, medium and high clouds are then
computed by applying a climatological atmospheric absorption correction
to the 10.8µm brightness temperature using look-up tables. The cloud
top pressure and height are not retrieved.
Details on the methods can be found in the Algorithm Theoretical Basis
Document, that can be downloaded from this web page.
3.- List of inputs for CTTH
Mandatory inputs are flagged, whereas the impact of missing non-mandatory
data on the processing are indicated.
- Satellite imagery:
The following satellite brightness temperatures and radiances are needed at
full IR spatial resolution:
At least one of these channels is mandatory
The CTTH software checks the availability of satellite brightness temperatures
and radiances for each pixel. Full CTTH product is computed only if all
mandatory satellite radiances and brightness temperatures are available. If
T10.8µm brightness temperature is missing, no result is available.
If T10.8µm brightness temperature is available, but mandatory channels
are missing, only the cloud top temperature is computed using the method
based on climatological atmospheric absorption correction.
The satellite channels are input by the user in HRIT format, and extracted
on the processed region by NWC/GEO software package.
- CMA and CT cloud categories
The CMA and CT cloud categories are mandatory. They are computed by the
CMA and CT software.
- Satellite angles associated to satellite imagery
This information is mandatory. It is computed by the CTTH software itself,
using the definition of the region and the satellite characteristics.
- NWP parameters:
The forecast fields of the following parameters, remapped onto satellite
images, are used as input :
- surface temperature
- surface pressure
- air temperature and relative humidity (alternatively dew point temperature)
- air temperature, relative humidity and geopotential on vertical pressure levels
- tropopause temperature, pressure and geopotential
- altitude of the NWP model grid (alternatively surface geopotential on
the NWP model grid). Required if NWP fields are used as input.
Vertical pressure levels on which air temperature and humidity are defined
by the user. All the surface and near-surface NWP informations and at least
NWP informations every 210hPa on the vertical are mandatory to get full
CTTH product. Otherwise only the cloud top temperature is retrieved using
the method based on climatological atmospheric absorption correction.
These remapped fields are elaborated by the SAFNWC software package from
the NWP fields input by the user in GRIB format.
- RTTOV simulations:
The following parameters simulated by RTTOV are used as input :
- Clear sky top of atmosphere radiance
- Transmittance from surface to TOA
- Clear sky downwelling radiance
- Clear+cloudy TOA radiance for given cloud top pressure and fraction (run RTTOV with black cloud at surface level)
- Level to space overcast radiance given black cloud for each vertical level defined by the user
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 mandatory to get full CTTH product. Otherwise only the cloud top temperature is retrieved using the method based on climatological atmospheric absorption correction.
- OSTIA fields:
The following parameters simulated by RTTOV are used as input :
- OSTIA sst and local estimated error
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 will be performed using NWP skin surface temperature.
- Ancillary data sets:
The following ancillary data, remapped onto satellite images, are mandatory
- Land/sea atlas
- Elevation atlas
- Monthly minimum SST climatology
- Monthly thermal emissivity at IR wavelength
- Monthly mean 0.6µm atmospheric-corrected reflectance climatology
These ancillary data are available in the NWC/GEO software package at a global scale; They
are remapped on the satellite disk by NWC/GEO remapping functionality.
One coefficient's file, containing satellite-dependent values and one look-up
table for climatological atmospheric absorption correction, is available
in the SAFNWC software package, and is needed by the CTTH software.
4.- Coverage and resolution
The CTTH software has been designed to allow the processing at satellite 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 CTTH product is commited inside the MSG full disk.
5.-Description of CTTH outputs
CTTH products are coded in NetCdF and includes:
- Cloud top pressure in Pa
- Cloud top temperature in K
- Effective cloudiness in %
6.- Example of CTTH visualisation
It is important to note that the CTTH product is not just images, but numerical
data. At first hand, the CTTH is rather thought to be used digitally (together
with the appended quality flags) as input to mesoscale analysis models, objective
Nowcasting schemes, but also in the extraction of other SAFNWC products.
Colour palettes are included in CTTH NetCdF files, thus allowing an easy visualisation
of cloud top pressure (as illustrated on the SEVIRI example), height, temperature
and effective cloudiness.
The product, if used as an image on the forecaster desk, may be visualized
(together with CT) in an interactive visualisation system, where individual
pixel values (top temperature, height and pressure, cloudiness) may be displayed
while moving the mouse over the image.
Click on thumbnail for
full-sized this image
SEVIRI CTTH cloud top pressure using the colour palette
included in CTTH NetCdF files.