Differences in convective cloud tops as seen by MSG and proxy MTG imagery.
21 December 2022
20 December 2022
By Djordje Gencic (Exostaff) and Ivan Smiljanic
On two occasions, on 24 and 29 November, cold fronts were traversing the Red Sea, causing severe convection over the area. In this case study the cloud tops of these convective storms are used to demonstrate the potential difference between current MSG SEVIRI and future MTG FCI channels, represented by proxy imagery from current LEO satellites such as Suomi-NPP.
In Figure 1 comparison of the colour-enhanced SEVIRI channel IR108 (left) at nominal 3km resolution and VIIRS M15 (10.8µm) channel at 750m resolution is displayed (FCI will have advanced 1km resolution with IR10.5, and IR3.8 channel). As expected, the fine topology of the cloud top is much more obvious in the VIIRS image than in the SEVIRI one due to much higher horizontal resolution. Additionally a blue X symbol denoted the scene's minimal brightness temperature. This usually coincides with the overshooting tops of cumulonimbus clouds.
Figure 2 is zoomed in on the most active part of the storm (in Figure 1) with the lowest cloud top temperatures. In the annotation above the images shown are the Brightness Temperatures of the coldest pixel for each of the two instruments. The better spatial resolution of VIIRS, compared to SEVIRI, shows that cloud tops were colder by as much as 6.3K than in SEVIRI image. The future IR10.5µm channel on FCI will feature a 1km spatial resolution which is not as high as VIIRS (and also has a different viewing angle), but it is much closer than SEVIRI. Therefore, the VIIRS M15 channel can be considered a decent proxy for FCI's IR10.5 channel.
Severe Convection RGB (Figure 3), is used to locate the most severe parts of the storm (apart from coldest cloud tops). Again, comparison of the SEVIRI and VIIRS-based composites shows a difference in the level of details. Unfortunately, most of the VIIRS bands are prone to striping — elongated artefacts polluting the display and making the visual analysis more difficult. Very bright yellow pixels in this composite mean small ice particles, which implies strong updrafts, which then further give a good idea of where the most severe parts of the storm are. In the SEVIRI image very bright yellow pixels can be seen in the westernmost parts of the cloud, and this is confirmed in the VIIRS image, just with a more striking difference from the rest of the cloud top.
Figure 4 shows another novelty of MTG's FCI instrument, that is devising the Cloud Phase RGB. This composite will, thanks to 2.25µm channel, be able to discern cloud particle phase and cloud particle size. Small ice particles which suggest strong convective processes will show as light blue or cyan colors, which again coincides with the locations that were identified in previous imagery as the location of highest convective activity.
Notice the gradients in colours for single convective cells (for instance, two neighbouring over Egypt), nicely seen due to a strong wind shear that effectively 'smeared' the vertical Cb profile in a horizontal direction. Colours change from white to pink and purple for small to large water particles, and from to dark to light blue for large to small ice particles.
Another event happened on 29 November over a wider Red Sea area.
A moment captured in the Figure 5 again shows a significant convective system affecting areas of Medina and Yanbu, western Saudi Arabia. The difference in the Brightness Temperatures of the channel IR108 in SEVIRI (left, 09:45 UTC) and VIIRS M15 channel (right, 09:48 UTC) is even more significant than in the previous case, and reaches almost 12K (Figure 6).
Apart from the significant difference in brightness temperature, there is also a significant difference in the level of detail in the VIIRS image, consistent with previous examples. The only caveat in this analysis is that the moments of observation by MSG and VIIRS in this figure are not quite the same. Nominally, the VIIRS overpass happens at 09:48 UTC over this area, however, a scan of MSG classed as 09:45 UTC only reaches this part of the globe after seven or eight minutes, i.e. 09:52 or 09:53 UTC, which is an effective difference of just four or five minutes. In most of the cases this fact would not be of significance, however, in case of severe convection, even a few minutes can be enough for overshooting top creation, meaning that comparisons have to be made as close as possible.
Finally, Figure 7 is a comparison showing the same scene from a LEO (Suomi-NPP) and a GEO (MSG) satellite. In that image another effect is visible — significant east/northeastward shift of the high clouds in MSG compared to Suomi-NPP imagery. This is a well-known effect of parallax where at slant viewing angles (a case with MSG) high clouds appear further away from satellite nadir point then they are (or their base is). In comparison, the data from LEO satellites that are performing scanning quite directly above (such as with MODIS or VIIRS) has a smaller parallax effect, except moderately on the edges of the scan swath where the scanning angle is higher.