Table of Contents
By Natasa Strelec Mahovic (EUMETSAT) and Djordje Gencic, Exostaff
Features occurring in the WV6.2μm images indicate the processes going on in the upper troposphere. Usually, the formations in water vapour imagery include dark stripes, which are connected to jet streams. However, sometimes dark circular or spiral formations appear in the imagery. These circular formations are known as water vapour eyes, eddies or vortices.
Figure 1 shows the formation and persistence of at least three distinct water vapour vortices, or eyes, over Europe for almost two days.
Figure 2 shows the development of these vortices over the period of 72 hours. It also features an enhanced colour scale, so the dynamics and development of the whirls in water vapour channel can be more easily detected.
On 23 November, whirls forming in the Norwegian Sea can be seen, along with a long dark stripe stretching west-east across Central Europe. These whirls (or vortices) were very slowly moving (in fact almost stationary) across central Europe throughout much of 24 and 25 November, only to be gradually pushed east/northeast by a strong developing low in the western Mediterranean late on 25 November.
What do these dark spots mean and how do they develop?
These dark spots are often more clearly, and sometimes only, visible in WV 6.2μm images. Often, looking at the other SEVIRI water vapour channel, 7.3μm, dark spots are not visible, or at least not as distinct. However, on this occasion, the dark spots were visible in both the WV6.2 and WV7.3μm images (Figure 3).
A dark spot in the water vapour image means that the radiance of that area is very low. As these are images in the water vapour absorption band, this means that there is very little absorption of water vapour molecules, hence, there is little water vapour — darker shades in the water vapour images mean that the air in these spots is dry.
The weighting functions of SEVIRI channels show that, for these two water vapour channels, the maximum signal comes from different heights (Figure 4). The WV6.2 looks at the upper troposphere, 300-400hPa, whereas the maximal signal in WV7.3 comes from mid troposphere, 500-600hPa.
A cross section through one of the water vapour eyes on 24 November 12:00 UTC (Figure 5) proves that within the area of darker shades in water vapour images relative humidity is very low, below 20% all the way from 300 to 550hPa. That also explains why, in this case, the vortex was visible in both water vapour channels.
Besides low humidity, these dark spots are also characterised by high potential vorticity (PV) values. The cross section in Figure 5 shows that high values of PV are characteristics of the higher troposphere and stratosphere, therefore, the occurrence of high PV values usually signals the lowering of the tropopause and the intrusion of dry stratospheric air into the mid, or even lower, troposphere. In this case, as seen in Figure 5, the value of 1 PVU (Potential vorticity unit), characteristic of the tropopause, was found all the way down to 600hPa in the area of the dark spot.
Additionally, in the Figure 6 the darker shades in the water vapour image in the place of whirls are accompanied by cyclonic circulation in the streamlines field. When these small scale whirls are found inside the larger scale high-pressure fields, as it was in this case, (centre denoted with H above western Czech Republic) they are usually not accompanied by any significant weather phenomena. However, sometimes these water vapour vortices can be found in more unstable atmospheric environments, where they can induce development of a cyclone, or support development of storms.