Thunderstorms with above-anvil plumes over Italy

By André Simon, Maria Putsay (Hungarian Meteorological Service) and Jochen Kerkmann (EUMETSAT)

Two thunderstorm clouds with above-anvil plumes were observed in satellite imagery over northern Italy in the morning and afternoon of 6 July 2010. The synoptic situation was favorable for the development of deep convection. A cold front was passing over the Gulf of Genoa, northern Italy and Austria toward the east, southeast.

A frontal wave and an area of low mean sea level pressure developed over the Alps. At the same time, there was a significant upper air trough with its axis over Germany and Austria, which moved toward east. The air was unstable over northern Italy (K-index over 35, CAPE over 800 J/kg), the low-level shear was weak but there was a veering of the wind with height from southeasterly to westerly direction in the sounding of Udine at 12 UTC .

The morning thunderstorm with above-anvil plume

We can follow the development of the convective clouds on the Met-8 rapid scan (5-min) imagery (see animation, 03:55–08:00 UTC, animated ). The frames were created by overlaying semi-transparent IR10.8 infrared images on HRV images, a method introduced by Martin Setvák (see e.g. case study backbuilding storms between Tunisia and Sicily, 23 September 2009).

On such images one can see at the same time the cloud top temperature distribution, including features like cold rings (Setvak et al., 2010), and the cloud top topography. To illustrate this, have a look at the images at 04:30 UTC, which show an overshooting top, gravity waves and an above-anvil plume. The overshooting top shows a very low brightness temperature minimum (well below -60°C, see IR10.8 image).

As regards the cloud top temperature distribution, at 04:10 UTC a small warm area developed on the lee side of the overshooting top. At 05:50 UTC (see also the animation) the embedded warm area is clearly visible, together with an above-anvil plume and 'ship trails'-like gravity waves. It lasted until about 06:45 UTC, i.e. for about two and a half hours. So we can classify this cloud as a cold ring shaped thunderstorm.

Above-anvil plumes may form over some thunderstorms. Its source is usually close to the overshooting top. The development of above-anvil plumes was successfully simulated by Pao Wang (2007). Rise and decay of intense overshooting tops trigger gravity waves and in some conditions plumes may form due to the gravity wave breaking mechanism.

In the IR3.9 imagery one can recognise the plume after 05:35 UTC, when the Sun elevation is high enough (see the red colours in the middle of the anvil in the HRV/IR3.9 blended image animation). Increased IR3.9 brightness temperatures (BT) are visible around the plume in a V shape (see the IR3.9 image at 06:20 UTC, upper left panel). The IR3.9 brightness temperature depends on both top temperature and reflectivity. The latter is very sensitive to particle size, smaller ice particles have higher IR3.9 reflectivity causing higher IR3.9 brightness temperatures. Thus, the plume must consist of very small size ice particles.

The afternoon thunderstorm with above-anvil plume

The afternoon thunderstorm showing an above-anvil plume developed around 14:00 UTC in the Veneto region. It grew fast and lasted several hours. It is possible that the observed cloudiness was related to a multicellular or a hybrid supercellular-multicellular system, but this hypothesis could only be confirmed or rejected using radar imagery. The IR10.8 brightness temperature of the coldest top was below -65°C.

In the HRV/IR10.8 blended animation (14:00–18:55 UTC) one can see intense overshooting tops (some of them very huge and long lasting), a long-lived plume trailing from the thunderstorm top toward the east-northeast and intense gravity waves, which formed in the plume area and then diverged. The plume casts shadows on the anvil. It is even more evident at the late afternoon, e.g. in the 18:35 UTC image (RGB HRV-HRV-IR10.8), when the edge of the plume was still sunlit, in contrast to the thunderstorm anvil.

Looking at the temperature distribution of the cloud, the thunderstorm top sometimes shows a cold-ring, sometimes a U-shape. The embedded warm area was present from 15:00 UTC to 18:55 UTC. Interestingly, the warm area is also visible in the WV6.2 images, see e.g. 17:00 UTC image (upper right image). The plume has a higher IR temperature compared to the temperature of the anvil cloud in its environment, which is a typical feature. However the warm area might not have been caused only by the warmer above-anvil plume, because in some images e.g. in the 17:30 UTC image the area of the warm spot close to the overshooting top is broader than the plume.

In the animation one can see that the plume significantly deflects to the left of the direction in which the penetrating thunderstorm tops move (mainly southward). The propagation of the plume (toward the east-northeast) is also different from the propagation of the thunderstorm anvil (mainly toward southeast). The plume orientation is clearly visible at low solar elevation e.g. in the 18:30 UTC image . We overlaid the 18:10 UTC HRV image with 100 and 150hPa streamlines (cyan and green lines) and 100 and 250hPa streamlines (cyan and red lines) calculated from the ECMWF wind field forecast for 18:00 UTC.

The 100hPa level (nearly 16km high) streamline best fits the direction of plume propagation, while the propagation of the thunderstorm anvil better fits the streamlines of the 250hPa (nearly at the tropopause) or 150hPa level. It is possible that the plume propagation was somehow related to the flow at 100hPa (the motion of the air parcels in the thunderstorm environment is affected by many factors, e.g. by divergent outflow of the storm, however, at some distance from the thunderstorm top it is supposed to follow the direction of the environmental wind). In the figures, both 150 and 250hPa streamlines are plotted to show that a significant wind shear was forecasted between the 100, 150 and 250hPa pressure levels.

Note that a low-level outflow boundary is detectable on the west side of the convective system between 17:00 and 18:20 UTC, best visible in the RGB HRV-HRV-IR10.8 animation (see interpretation of the 17:55 UTC image).

Finally, above-anvil plumes are often an indicator of possible severe weather on the ground. According to available information from newspapers, severe weather and damage caused by wind occurred in the provinces of Rovigo, Vicenza and Padova.

Additional content

Met-8 HRV animation (03:55–08:00 UTC)
Met-8 RGB HRV-HRV-IR10.8 animation (03:55–08:00 UTC )
Met-8 HRV/IR10.8 and HRV/IR3.9 blended animation (03:55–08:00 UTC)
Met-8 HRV animation (14:00–18:55 UTC)
Met-8 RGB HRV-HRV-IR10.8 animation (14:00–18:55 UTC)