Documentation of Met Office MSG products

High Resolution Visible Imagery

This image shows the reflected solar radiation from the Earths surface and cloud tops at a spatial resolution equivalent to 1km at the sub-satellite point but increases for higher latitudes and longitudes away from the meridian. The images have been normalised by the solar zenith angle to remove changes in brightness during the day. They are available every 15 minutes. The reflectance is averaged over a broad wavelength range in the visible part of the spectrum. Cloud shadows are clearly discernible at dawn and dusk giving an idea of the cloud top height. Aircraft contrails and ship tracks (in the cloud) can also be seen.

Visible Imagery

This image shows the reflected solar radiation from the Earths surface and cloud tops at a spatial resolution equivalent to 3km at the sub-satellite point but increases for higher latitudes and longitudes away from the meridian. The images have been normalised by the solar zenith angle to remove changes in brightness during the day. They are available every 15 minutes. The reflectance is over a narrower spectral band (centred at 0.8 microns) than the high resolution visible channel and so the land surface will look brighter in this channel over vegetation and land surfaces in general. Cloud should look the same.

Infra-Red Imagery

This image shows emitted radiation from the Earths surface and cloud tops at a spatial resolution equivalent to 3km at the sub-satellite point but increases for higher latitudes and longitudes away from the meridian. They are available every 15 minutes. White areas denote cold temperatures and dark areas denote warm temperatures. Radiation from this channel is averaged over a narrow spectral band centred at 10.8 microns. At mid-latitudes the atmosphere is transparent at these wavelengths and so most of the radiation originates from the surface or cloud tops. The sea surface will not vary in temperature during the day but the land surface temperature can vary by many degrees during the diurnal cycle and this can be seen in a movie loop of these images.

Water Vapour Imagery

This image shows emitted radiation from the Earths atmosphere and cloud tops at a spatial resolution equivalent to 3km at the sub-satellite point but increases for higher latitudes and longitudes away from the meridian. They are available every 15 minutes. White areas denote cold temperatures (cloud tops or moist areas of the upper troposphere) and dark areas denote warm temperatures (no high clouds and drier regions of the upper-mid troposphere). Radiation from this channel is averaged over a narrow spectral band centred at 6.2 microns. Most of the radiation originates from the upper atmospheric water vapour and cloud tops. Movie loops of these images can show areas of potential baroclinic development by showing dry (dark) intrusions appearing in association with areas of ascent (bright). They are also used as a proxy for potential vorticity of the upper atmosphere.
 
Cloud Top Height Product

This product shows the height of the cloud tops in feet, and is calculated using the infrared channels at wavelengths of 10.8, 12.0 and 13.4 microns. Three possible algorithms are available for each pixel. In the first, the  minimum residual method is employed to infer the cloud top height by using data from NWP model profiles to perform radiative transfer calculations, in order to simulate radiances in all three channels for different cloud top heights and effective cloud amounts. The method then attempts to find the solution which minimises the differences between the measured and simulated radiances in all three channels. A check is also made so as not to place the cloud top at the bottom of an unstable layer which would cause vigorous convection if subsequently assimilated into the NWP model. If an acceptable solution is found, i.e. if a well-defined minimisation is obtained, the value of cloud top height is accepted. This is typically the case for most mid- and high-level cloud. If a solution is not found, a single channel approach is used, the so-called stable layers method, which attempts to match the measured brightness temperature at 10.8 microns with the simulated value, reducing the weighting of any solution which would put the cloud top at the base of an unstable layer. The majority of low-level cloud is processed using this method. Finally, if an acceptable solution is still not found, a third method is used, which simply finds the best match between measured and simulated 10.8 microns brightness temperatures, working from the surface upwards, with no account taken of atmospheric stability.

Cloud Top Temperature Product

This product shows the temperature of the cloud tops in degC, and is calculated using the infrared channels at wavelengths of 10.8, 12.0 and 13.4 microns. Three possible algorithms are available for each pixel. In the first, the  minimum residual method is employed to infer the cloud top height by using data from NWP model profiles to perform radiative transfer calculations, in order to simulate radiances in all three channels for different cloud top heights and effective cloud amounts. The method then attempts to find the solution which minimises the differences between the measured and simulated radiances in all three channels. A check is also made so as not to place the cloud top at the bottom of an unstable layer which would cause vigorous convection if subsequently assimilated into the NWP model. If an acceptable solution is found, i.e. if a well-defined minimisation is obtained, the value of cloud top temperature corresponding to this height is accepted. This is typically the case for most mid- and high-level cloud. If a solution is not found, a single channel approach is used, the so-called stable layers method, which attempts to match the measured brightness temperature at 10.8 microns with the simulated value, reducing the weighting of any solution which would put the cloud top at the base of an unstable layer. The majority of low-level cloud is processed using this method. Finally, if an acceptable solution is still not found, a third method is used, which simply finds the best match between measured and simulated 10.8 microns brightness temperatures, working from the surface upwards, with no account taken of atmospheric stability.

Coloured Fog Product (only available at night)

Fog and/or low cloud at night can be detected from the difference in emissivity of the fog/cloud tops and the underlying surface at 3.9 and 11 microns wavelength. This product uses both measured and simulated radiances (the latter using an NWP profile) to determine if fog is present. The magnitude of the difference in radiance between the 2 channels and from the observed minus simulated radiances is a measure of the fog thickness. The fog top temperature can also be inferred from the infrared channel radiance. The intensity of the colour is a measure of the fog thickness and the colour indicates if the fog top is above freezing (red), around freezing (green) and below freezing (blue). Areas with no colour are free from fog or low cloud, and correspond to the 10.8 microns brightness temperature, which is used as a background for the coloured fog mask.
 
Greyscale Fog Product (only available at night)

Fog and/or low cloud at night can be detected from the difference in emissivity of the fog/cloud tops and the underlying surface at 3.9 and 11 microns wavelength. This product uses both measured and simulated radiances (the latter using an NWP profile) to determine if fog is present. The magnitude of the difference in radiance between the 2 channels and from the observed minus simulated radiances is a measure of the fog thickness. Differences larger than around 3.5 K, represented by the white end of the colour scale, correspond to fog or low cloud. Intermediate differences (in the 2.5  3.5 K range) correspond to clear pixels, and differences smaller than around 2.5 K tend to correspond to mid- and upper-level cloud (and appear as black).
 
RGB 'False Colour' 321 Product (only available in the day)

RGB images are composite images generated by combining two or more channels and displaying in colour. The naming convention describes which channel is assigned to the red, green and blue colours. For example RGB 321 means that channel 3 (1.6 micron) is on the red, channel 2 (0.8 micron) is on the green and channel 1 (0.6 micron) is on the blue. This combination can then highlight different physical features through the differing amounts of red, green and blue and hence give a unique colour to that feature. In this case, turquoise clouds contain ice crystals, whilst white clouds are water clouds (inc. fog). Vegetation creates a green signal and sandy areas are pink. Snow covered ground is turquoise.

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