Backbuilding storms between Tunisia and Sicily

By Martin Setvak (CHMI ), Tomas Pucik (CHMI and Estofex), Lukas Ronge (Amateur Stormchasing Society of the Czech Republic), Jan Kanak (SHMI), Jochen Kerkmann and HansPeter Roesli (EUMETSAT)

Besides their accompanying weather described below, these storms have shown several nice satellite-observed cloud-top features, documenting their severe convective activity. Here we focus on two types of these features: the overshooting tops and the warm spots (or larger warm areas) developing on the downwind side of the overshooting tops.

The overshooting tops are usually detectable as 'bubbles' of several image pixels across, seen either in visible and near infrared spectral bands due to their texture and shadows they cast, or in thermal bands as local brightness temperature minima. Under certain favourable conditions these may be accompanied by smaller-scale warm spots or larger and longer lived embedded warm areas, forming downwind of the overshooting tops, and detected in color-enhanced thermal IR imagery. Usually these two types of features are observed independently, in the two stand-alone bands. Here we present a new image product, which merges together the two bands, giving the images almost a 3D appearance.

The product consists of two layers: the base one, which is the High Resolution Visible (HRV) image, and the upper layer, containing the color-enhanced IR10.8 image, showing the details of the brightness temperature field. Next, there are several options how to blend these two layers together. The simplest way is to use just the partial transparency of the upper layer, setting the layer opacity somewhere between 40 to 75%. Somewhat better results can be obtained by using some other type of blending of the two layers together — for example in Adobe Photoshop you can try either the 'Multiply' or 'Linear Burn' functions, again in combination with the layer opacity set as above. Of course, the bottom layer, the HRV image, can be enhanced by some of the standard procedures, for example using the 'Unsharp Mask' filter.

We show here the individual bands: the HRV imagery, the colour-enhanced IR10.8 imagery, and the two bands blended together (see image and animation below). The new product, also called the HRV_IR10.8 'sandwich' product, shows all the details seen in the HRV image, 'colourised' by the IR10.8 image above it, making it much easier to study the individual cloud-top features and their links. Similar products have been used for images from polar orbiting satellites; here we apply the procedure to a series of images from the MSG satellite, obtained in its Rapid Scan Service. All the images were created using the MSG Processing Tool for Windows (Kanák, Ronge).

Another interesting view of the thunderstorm clouds can be obtained using both MSG satellites (Meteosat-8 and Meteosat-9) to display these storms as stereo images [1]. It is possible due to the orbit distance of both satellites, 9.5° in longitude, and scanning the same region approximately at the same times. To see the vertical dimension of clouds users have to use special eyeglasses with colour filters. Red filter is for left and cyan for right eye. Overshooting tops and plumes above anvils can be seen clearly on the 15:45 UTC image.

Weather aspects of the storms

The convective storms of 23 September developed close to the eastern coast of Tunisia, on the forward flank of the mid-level cut-off low centered over eastern Algeria (see GFS analysis 500 hPa, source: P. Groenemeijer). Strong flow at mid and upper levels of the troposphere led to the significant wind shear and storms attained a high level of organisation*, forming a large complex, possibly having supercellular elements (see GFS analysis of MLCAPE and 0-6 km bulk shear, source: P. Groenemeijer). The complex of storms slowly advanced to the north-northeast, affecting Sicily by evening hours. As it was fed by very moist and unstable airmass from the south, the whole system kept 'backbuilding' — that means storms were repetitevely reformed upstream of the mean flow, so that thunderstorms lasted several hours over certain areas (a typical flash flood situation).

As regards SYNOP reports, several stations received excessive rainfall, in the belt from the eastern coast of Tunisia to Sicily. This resulted from the intensity of the storms, their long duration and very high moisture content over the sea. The European Severe Weather Database (ESWD) even includes an unconfirmed report of a tornado over Palermo. The worst hit was the island of Pantelleria, receiving an amount of 201mm in 12 hours and 160mm in just six hours. Sfax El Maou station in Tunisia reported 70mm of rain in six hours and the island Lampedusa 162mm between 12:00 and 18:00 UTC.

As the most severe stages of thunderstorm complex occurred over the vast extents of the sea, it is highly probable that many phenomena associated with it were not observed at all. It should be noted as well, that several rounds of such storms have been spotted in this area ( Algiers-Tunisia-Sicily-Southern Italy) this year.

*Note: The overlap of both, high instability and wind shear (0–6km bulk shear over 25m/s — values that have been found to be conducive for supercells), suggest potential for well organised severe storms.

Additional content

Met-9 Airmass RGB animation (23 September 09:00–24 September 09:00 UTC)