On 13 May 2018 storms formed along a convergence line, causing extreme rainfall and flooding in Germany.
By Mária Putsay, Ildikó Szenyán and Zsófia Kocsis (Hungarian Meteorological Service)
This study discusses satellite images of 13 May. We concentrate on Suomi-NPP VIIRS images, firstly looking at those RGB images which cannot be created from MSG SEVIRI data, but will be available with MTG FCI data. We will compare such VIIRS RGBs with SEVIRI RGBs.
On 13 May a pronounced moisture boundary was seen in the satellite images. At the time of the daytime VIIRS overpass (12:17 UTC) it crossed eastern Germany, see the SEVIRI 24-hour Microphysics RGB image in Figure 1 (top right, click to expand). The moisture boundary is seen in cloud-free areas as colour gradient: darker blue colour indicates higher moisture, while the lighter blue areas are drier. Poland was very dry. Radiosondes measured 13–15 mm total precipitable water (TPW) in eastern Germany and Poland, while 25–29 mm was recorded in central and western Germany. Figure 1 shows that convective clouds formed in the moist area over Germany, eastern Austria, western Hungary and western Balkan.
Figure 2 shows the SEVIRI 24-hour Microphysics and Day Microphysics RGBs, together with the VIIRS Cloud Phase and Cloud Type RGBs. These two VIIRS RGBs use channels which are not available on MSG SEVIRI, but will be available on MTG FCI.
- The Cloud Phase RGB is used to separate ice and water clouds, and provide information on cloud top particle size.
- The main aim of the Cloud Type RGB is to detect thin cirrus clouds.
Both Cloud Phase and Cloud Type RGBs are daytime RGBs.
Cloud Phase RGB
The Cloud Phase RGB was devised by Jochen Kerkmann (EUMETSAT). It uses NIR1.61, NIR 2.25 and VIS0.49 channels (M10, M11, M3 in case of VIIRS), (see the Cloud Phase recipe and typical colours). NIR2.25 channel will be a new channel on FCI.
The Cloud Phase RGB separates the ice clouds from water clouds better than the SEVIRI RGBs. The combined NIR1.61 and NIR2.25 is an excellent channel pairing for this purpose. VIS0.49 adds the cloud optical thickness information. The Cloud Phase RGB provides very good cloud phase separation (except very thin clouds, in some cases) and also particle size information. Unfortunately, it does not provide temperature information.
In the next few figures the VIIRS Cloud Phase RGB is compared with the SEVIRI Day Microphysics RGB, as this RGB provides the best microphysics information from SEVIRI data.
Looking at the cloudiness over the Atlantic Ocean, north of Spain and west of Ireland (Figure 3), it can be seen that the colour contrast between ice and water clouds with large droplets is higher at the Cloud Phase RGB (blue against violet) than it is in the SEVIRI Day Microphysics RGB (red against pink).
When studying the frontal cloudiness over France (Figure 4), the colour contrast between cloud-free land and clouds is better in case of the VIIRS Cloud Phase RGB, because of the darker background (black sea and brown land), although the clouds are also better seen due to the higher spatial resolution.
There is an elongated ice cloud band (indicated by black arrow) consisting of small ice crystals (depicted in light blue). Its colour has some orange shades in the SEVIRI Day Microphysics RGB, also indicating small ice crystals. The cirrus clouds over the North Sea (indicated by the yellow arrow) are better seen on the SEVIRI Day Microphysics RGB, as it uses an infrared channel as well as the solar channels.
The convective ice clouds over the West Balkans are depicted in darker and lighter blue colours in the VIIRS Cloud Phase RGB (left panel of Figure 5). Lighter blue indicates smaller ice crystals, while darker blue indicates larger ice crystals on the cloud top. The cell indicated by an arrow is the lightest blue, likely the more intense cell.
This cell is bright yellow in the SEVIRI Convection RGB (right panel of Figure 5) also indicating small ice crystals on the cloud top. The cell has an orange shade in the SEVIRI Day Microphysics RGB as well, but this is the only cell in the SEVIRI image which looks more orange than the other ice clouds.
The colour shades, referring to different ice particle sizes, are more eye-catching in the Cloud Phase RGB than in the SEVIRI Day Microphysics RGB image. It is due to the different spectral features and, also, partly due to the higher spatial resolution. Note: in the SEVIRI image we see colour shades referring to different cloud top temperature — warmer ice clouds are magenta — information which is missing from the Cloud Phase RGB as it does not use the infrared channel.
Inside the storm top over Italy (black arrow in Figure 6) there is a lighter blue patch indicating smaller ice crystals coming out from the overshooting top. The presence of the smaller particles was confirmed by the NWC SAF effective radius product (not shown here) and by the yellowish pixels in the SEVIRI Convection RGB.
Benefits of Cloud Phase RGB:
- Very good separation of ice and water clouds (In case they are not too thin)
- Good colour contrast between:
- ice and water clouds;
- water clouds with small and big water droplets on their tops;
- ice clouds with small and big ice crystals on their tops.
Limitations of Cloud Phase RGB:
- It works only daytime.
- No temperature information included.
- Thin cirrus clouds are less seen as it uses only solar channels.
- The phase separation of thin cirrus clouds might be problematic.
Cloud Type RGB
To see the thin cirrus clouds better there is another new RGB, the so-called Cloud Type RGB, see the Cloud Type recipe and typical colours. This RGB was devised by Andrew Heidinger (CIMSS).
The VIIRS Cloud Type RGB uses the NIR1.38 channel together with VIS0.67 and NIR1.61 channels (M9, M5, M10).
- The NIR1.38 channel is in a weak water vapour absorption band. As a consequence, the surface is normally not seen. The clouds are seen in front of the dark background and so the very thin cirrus clouds are seen much better than for example in case of a lighter land surface background.
- VIS0.67 is used to separate thin and thick clouds.
- NIR1.61 is used to separate ice and water clouds.
For thin cirrus detection in SEVIRI data we use RGBs that use brightness temperature difference (BTD) in the red colour beam (IR12.0-IR10.8). This difference is negative for thin clouds. In the other two colour beams we put phase and temperature information. The Cloud Type RGB is a daytime RGB, while the BTD of (IR2.0-IR10.8) can be used for both night and daytime.
Figure 7 shows cirrus clouds over Spain and the nearby sea. For comparison, other VIIRS and SEVIRI RGBs are visualised. The thin cirrus clouds are best seen in the Cloud Type, Dust and 24-hour Microphysics RGBs, which use either NIR1.38 or the BTD of (IR12.0-IR10.8). The thin cirrus clouds are least seen in the Cloud Phase RGB, which only uses solar channels. The thin cirrus clouds are seen slightly better in the HRV Cloud and Day Microphysics RGBs as they use infrared channels as well as solar channels.
Figure 8 shows the same RGBs over a different area. A thin cirrus band elongates from Sweden to Finland. There are cirrus clouds west of Norway and Denmark, and contrails over the Baltic countries. The thin cirrus clouds are best seen in the Cloud Type RGB and in the RGBs using BTD of (IR12.0-IR10.8), and least seen in the Cloud Phase RGB. For example, the thin cirrus southwest of Poland is not seen by the Cloud Phase, the Day Microphysics or the HRV Cloud RGBs. The cirrus clouds west of Norway seem thicker in the Dust and 24-hour Microphysics RGBs, than in the HRV Cloud, Day Microphysics and Cloud Phase RGBs. The optical thickness depends on the wavelength.
The colour contrast between the thin and thick parts of the anvil is very ‘eye-catching’ on the Cloud Type RGB (red against yellow, see Figure 9). They are also separated in the Dust RGB (black against reddish brown).
Benefits of Cloud Type RGB:
- It detects well thin cirrus clouds over cloud-free areas.
- It provides information on cloud optical thickness and cloud phase.
- It detects thin mid-level clouds as well (unless the atmosphere above the mid-level cloud tops is very moist).
- Good colour contrast between:
- clouds and cloud-free and snow-free surface;
- thin and thick high clouds;
- ice and water clouds.
- It only works during the daytime.
- The interpretation of the colours depends on the humidity situation.
- It does not detect thin cirrus over opaque high cloud.
- Mid-level thin clouds might be not detected in case of high moisture content above them.
- It does not contain temperature information.