Visualizing tropospheric humidity using differential water vapor transmittance between the 0.91 and 0.85 µm FCI channels

The Flexible Combined Imager (FCI) on the MTG-I satellite includes a near-infrared channel centered at a wavelength of 0.91 µm, where radiation is partially absorbed by atmospheric water vapor. During daytime, in cloud-free regions, the signal detected from the Earth is weakened due to this absorption: first along the path from the sun to the Earth's surface, and then again from the surface to the satellite. By comparing this channel with a nearby one at 0.85 µm, it is possible to accurately estimate the total water vapor content. This method was notably advanced by Hans-Peter Roesli, who pioneered the use of the ratio between these two channels for such analysis.

At ESSL, we display the logarithm of this channel ratio, corrected for sun angle and satellite viewing angle, using an intuitive color map that ranges from bright yellow through green to blue and violet, representing increasing levels of water vapor. This visualization technique allows for the identification of mesoscale features such as sea-breeze fronts and drylines, which mark sharp gradients in low-level moisture that may otherwise go undetected. Furthermore, it enables the tracking of mid-tropospheric structures, such as pockets or filaments of dry or moist air.

These observed mesoscale features can provide valuable insights for forecasters, highlighting processes that may influence the development of convective storms. They also offer a way to evaluate the accuracy of Numerical Weather Prediction (NWP) models. However, a key limitation lies in the difficulty of determining the precise altitude at which moisture variations occur.

A promising future approach involves combining the differential total column water vapor data with information from hyperspectral sounders, such as IASI and especially the Infrared Sounder (IRS) on the MTG platform. Each method compensates for the other's limitations: while the differential water vapor signal does not specify the altitude of the moisture, the IRS can provide vertical resolution, although it is less accurate near the surface. This uncertainty near the ground can be reduced using total column water vapor estimates derived from transmittance measurements.