Contrail shadows over the stratus
15 November 2018 07:00–15:00 UTC
Unusually shaped shadow contours of high contrail clouds, casted over the wide stratus field, were explored through various VIIRS (i.e. FCI proxy) and SEVIRI products.
21 December 2020
15 November 2018
A wide ridge over Central Europe was responsible for stable weather with low convective activities, i.e. weak vertical cloud development. In such conditions big patches of stratiform clouds and fog formed in the area. Shadows of the high ice clouds formed after the passage of a plane in the high altitudes (contrails) are well seen over these stratus fields — in many instances better than the contrails themselves.
Looking over the Central South Germany with the VIIRS instrument (Figure 1) one can easily spot the unusual curvy lines in the stratus field. By using only one satellite product it would be difficult to understand the source of this shape, perhaps even misleading, for instance they could be considered to be distrails (distrail, i.e. dissipation trail = void in the airplane wake caused either by water evaporation or precipitation of merged ice crystals from disturbed airflow). A synoptic view over different RGB products often offers an unambiguous solution.
Two RGB products relying on the ‘solar channels’ (namely the Natural Colours RGB or Day Microphysics RGB, Figure 1b and 1c) offered two reasonable solutions — the dark shade feature was either a void in the stratus field or a shadow. The Dust RGB (Figure 1a and Figure 2), relying on IR channels, suggested it was a high, thin ice cloud — a contrail.
By carefully observing the difference between IR- and solar-based RGB products it can be seen that the position of the feature does not overlap — there is a certain shift towards the north east in the visible spectrum.
More complete information is provided only by the less standard Cloud Type RGB (Figure 1d and 3), which incorporates the 1.3 µm channel — expected with the future FCI imager on board MTG. This channel is limited to the upper troposphere due to a strong water vapour absorption. Hence, incorporated in the RGB product this channel offers a good discrimination between high and low tropospheric features. In this case contrail clouds are shown as white stripes and their shades seen as dark green/brown stripes. For confirmation, yellow arrows in Figure 1 showing high cirrus cloud and blue arrows pointing at its shadow.
Figure 3 is a zoomed-in Cloud Phase RGB view over the scene. The 1.3 µm channel is placed on the red beam of this RGB — showing a strong red signal for contrails over cloud free areas and white over the stratus (where the signal is supplemented by other two RGB components, which together give a cyan colour for stratus cloud). Individual components of this RGB are seen in Figure 4.
In general, the position of the cloud shadow depends on the relative position of the Sun and the height of the cloud. It also depends on the relative position of the observing instrument. This is the reason for different shadow positions seen between two consecutive overpasses of the Suomi-NPP satellite (Figures 5 and 6).
At 10:51 UTC the scanned area was sensed at the very edge of the swath (Figure 5), hence the bigger separation of the shadow from the cloud. A slant view through the cirrus clouds made it appear optically thicker and horizontally narrower. At 12:31 UTC the airplane track shadow is still seen, however, it is more blurred because the viewing geometry is different.
The table below shows the Sun and Suomi-NPP satellite angles for the observed area (namely lat/lon 48.4N/10.18E) and two different time steps.
|Overpass Time||Solar Azimuth Angle [°]||Solar Zenith Angle [°]||Satellite Azimuth Angle [°]||Satellite Zenith Angle [°]|
|10:53 UTC||177.08 (from North)||67.01||63.46||66.96|
It seems that also the colour shades/brightness of the Cloud Type RGB very much depend on the viewing geometry, at least with the observed conditions. The colour shades are there to provide visual information, for example on the cloud thickness. However, the tone of the colours also depends on the satellite and solar angle at the observed point.
Final analysis comes from the SEVIRI instrument and associated products. Figure 7 reveals that nominal SEVIRI resolution (top panels) is, perhaps, not sufficient to resolve contrail clouds. Nominal resolution of 1 km of HRVIS channel proves to be good enough for contrail (shadow) detection (bottom panels).
Animation offered, perhaps, the most comprehensive understanding of the observed scene. The combined view of the HRVIS and IR10.8 channel in Figure 8 highlighted the movement of high cirrus clouds over the wide stratus field, again mostly through the cloud shadow detection. High clouds were barely detectable — seen as slightly brighter shapes. The relative difference of the clouds and shadows were, again, determined by the solar and satellite angles.
Towards the end of the sunset shadows become very much separated from the clouds above, due to a high solar zenith angle. Note that in case of SEVIRI imagery the satellite angle remains constant in time for certain area.
Aforementioned ‘distrails’ were observed during same day, not far from the region of interest. The MODIS instrument managed to capture the scene after the airplane ascended through the stratiform altocumulous clouds, leaving the void in the airplane wake area somewhere above the border of Croatia and Bosnia and Herzegovina. (Figure 9).
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