In April 2020, pockmarked DIBS cloud, produced by a cyclone over the Mediterranean Sea, entered an elongated deformation zone - stretching from France to Turkey.
By HansPeter Roesli (Switzerland), Mária Putsay (Hungarian Meteorological Service) and Ivan Smiljanic (CGI)
A Mediterranean low ingested Saharan dust and produced elongated DIBS (dust‐infused baroclinic storm) cloud, spreading over France and Turkey on 20 and 21 April. The pockmarked high cloud could be seen on infrared (IR10.8) and High Resolution Visible (HRV) imagery from Meteosat-11 (Figure 1).
The pockmarks are better defined on the zoomed-in imagery in Figure 2.
A similar view over the semi-transparent, dust-induced, cirrus clouds was provided by the Metop-A AVHRR instrument, through the Clouds RGB and Day Microphysics RGB at 1 km spatial resolution (Figure 3), and through the 300 m resolution of Sentinel-3 OLCI instrument (Figure 4).
The Meteosat 11 Natural Colour RGB annotated image (Figure 5) highlights the elongated deformation zone that was delineated by this cloud system. DIBS clouds normally take longer to dissipate, compared to similar clouds that lack a dust load, so have time to stretch extensively along the deformation zone. That leads to cloud formations in areas where cloudiness has not been anticipated, i.e. forecast by weather models. In fact, for this particular event Austrian weather forecasters reported that clouds were not predicted in the southern and western parts of Austria.
The origin of the dust load and low-pressure system dynamics can be seen on the animated Dust RGB imagery (Figure 6). Note: more frames (from 1 hour to 15 min frequency) were added to animation at the time when DIBS cloud was stretching inside the Deformation Zone, to better capture cloud dynamics.
Figure 6: Meteosat-11 Dust RGB animation, 18 April 00:00 UTC–22 April 23:00 UTC
A long-lived, cumuliform texture, with high reflection in products sensitive to particle size, typically appear with orange shades in Day Microphysics and yellow shades in Convection RGB products. The set of VIIRS images (created with NIR1.6 instead ofthe usual IR3.9) are examples of how the ‘next generation’ RGBs (also available with the future MTG FCI instrument) could see DIBS — Cloud Phase RGB and Cloud Type RGB, matched with the more standard Day Microphysics RGB.
The images are compared at two consecutive overpasses, primarily to show the scanning angle dependency. For example, with 11:34 UTC overpass of the VIIRS instrument, the cloud system over France is seen at a much higher viewing angle than the one over the Adriatic Sea. The opposite is true for next overpass at 12:23 UTC. At higher viewing angles, due to viewing geometry, thin clouds appear to be optically thicker. This is obvious in all of the RGB products where, for example in the Cloud Phase RGB, bright blue clouds (small ice particles) become even brighter, also more apparent over the land surfaces or underlying clouds.
The VIIRS Cloud Phase RGB (Figure 7) is greenish (green/cyan) for cirrus. If it were thick than it would appear as a lighter blue for small particles. However, for thin cirrus the colour contrast between cirrus with small or large ice crystals is not big. The VIIRS Cloud Type RGB (Figure 8) is red for thin cirrus over land/sea and yellow if cirrus it is over a thick cloud. The VIIRS Day Microphysics RGB with NIR1.6 (Figure 9) confirms small ice crystals (red-orange shade).