Fog from Po Valley penetrates south-alpine valleys

By Ivan Smiljanic and HansPeter Roesli

Fog is common weather feature in the Po valley during stable, cold days. However, on 6 January 2023, the fog was so widespread it filled most of the Po Valley and penetrated further into the small valleys of the southern Alps. The spread of the fog was essentially blocked by the Verzasca dam in the Swiss Canton of Ticino — captured by the higher resolution satellite imagers (dam crown at about 470m) (Figure 1).

Figure 2 shows the very high resolution view from the Sentinel-2 MSI imager, with the zoomed-in segment at full resolution of 10m (annotated square). While the fog thinned out gradually as it moved along the valleys, it abruptly stopped in the narrow Verzasca valley — exactly where James Bond jumped the dam in the movie Goldeneye.

The NOAA-20 VIIRS imager provided a comparable view with the FCI imager on board MTG satellite (operational later this year), both in terms of spatial resolution (500–1000m, depending on the channel used) and spectral signature (looking through novel Cloud Phase RGB). At the VIIRS resolution of 750m the dam blockage of the fog is still very much apparent (Figure 3).

Closer examination of the colour shades of the fog/stratus layer over the region, highlights the gradient from light purple (thick stratus, bigger water droplets) to very light purple and white/light grey shades (thinner stratus, smaller water droplets). Figure 4 reveals the sampling (via the RGB tool, credit EUMeTrain) of these two distinct cloud areas, showing individual RGB components.

Thick fog v thin fog

Thick fog Thin fog

 

Going from the thick fog, with bigger water particles, to the area of thin fog (perlucidus/translucidus), the ‘microphysical’ RGB beams from near-IR spectral region (red and green) show more reflection (from smaller water particles), while the visible beam (blue) shows less reflection (cloud layer more transparent). With thinner water clouds near-IR solar channels are also expected to show less reflection (again, more transparent cloud layer), however, the microphysical signature of smaller droplets seems to have more contribution here (enhancing the reflection). This effect is more apparent with the NIR2.25 channel, suggesting it can resolve particle size better than NIR1.6.

The neighbouring Novara Cameri radiosounding confirms that the particles on top of the extended cloud sheet were liquid (ca. 4°C at the inversion level).

Ice clouds normally show in blue shades in the Cloud Phase RGB, similar to snow in this case, however, none were present over the observed domain. It is worth noting that particle size of ice crystals can be also distinguished with this RGB product. A slight gradient from lighter to darker blue is apparent when looking from high peaks to lower lying snow, however, blue gradients are not very evident in this case. They may also vary due to different slope inclination/orientation (different reflection geometry).

For a reference, an MSG SEVIRI view is provided in Figure 5. SEVIRI provides imagery in nominal resolution of 3km (every five minutes in this case), but, because it does not have the NIR2.25 band in its suite, a Cloud Phase RGB can't be constructed. The RGB product closest to the capabilities of the Cloud Phase RGB would be the Day Microphysics RGB. As the name suggests, it can also resolve the particle size of water/ice, the darker the pink/purple colour the bigger the stratus droplets.

The SEVIRI product that can be considered a predecessor of the Cloud Phase RGB is a Natural Colour RGB (Figure 6). However, this product does not reveal the particle size (due to the lack of NIR2.25 channel), only the phase of cloud particles can be resolved (white for water, cyan for ice). Many cases show that even the phase separation, especially in the regions of phase transition, is better resolved with the Cloud Phase RGB (hence the name).