Météo-France Total Water Vapour composite: precipitable water imagery

99% of atmospheric water is in gaseous form, namely water vapour, mainly concentrated in the lower layers of the atmosphere. This amount of water vapour defines the amount of precipitation that a given area can receive: this is referred to as precipitable water.

Water vapour can be transported over long distances and releases energy as it condenses. It can therefore be considered as the fuel that powers meteorological processes. In situations of intense convection in particular, the severity of storms will depend on this moisture supply.

The water vapour channels typically available with geostationary satellite imagers allow us to assess the amount of water vapour in the upper and even middle troposphere. However, it is precisely in the lower layers that the concentration of water vapour is highest and most relevant to assess for the analysis of moist inflow of storm cells.

A new channel is available with (and only with) the MTG FCI imager: VIS0.9. This channel provides information on vertical water vapour content, particularly in the lower third of the troposphere. Although a signal may also be visible at higher levels, this is the first time that water vapour content can be observed and tracked near the ground, and not just in the upper and middle troposphere. Satellite products have already been developed within the international community, notably the ESSL total moisture composite.

It was during an ESSL testbed that a senior national forecaster at Météo-France discovered this possibility of assessing water vapour in the lower layers and highlighted the need for it. In addition to its obvious contribution to convection, this product may also prove useful in addressing the issue of forest fires by identifying dry air advections.

The Meteorological satellite centre of Météo-France has therefore worked on developing a product that meets these needs, with additional objectives:

• to offer a contrasting colour palette to obtain maximum detail in water vapour content gradients;
• easily identify cloud masses in order to avoid them without using a cloud mask. This product can only be used in clear skies.

This poster presents a preliminary version of the “Météo-France Total Water Vapour Composite” product.