Severe convective storms over the Czech Republic

By Martin Setvak (CHMI)

The storms shown below were quite severe in some parts of the Czech Republic (downbursts with gusty winds, hail and intense rain causing local flash floods), though there were no reports of tornadoes. The main reason why we show these images here is the form of their cloud top brightness temperature (BT) field, resembling a cold doughnut.

This form of BT field is by no means exceptional since many storms exhibit it to some degree during parts of their life cycle. However, this one can be considered as a textbook case because of the feature's 'purity', magnitude and persistence.

A general rule is that the colder the cloud top, the higher its altitude. However, when the storm penetrates the coldest part of the troposphere, called the tropopause (in mid-latitudes this generally lies somewhere between 8 and 12km above sea level), the highest points of such a storm begin to penetrate the warmer lower stratosphere. Things then begin to happen:

1) The most violent updrafts create overshooting tops (often called penetrating towers) which keep on cooling, because of their rapid ascent, down to temperatures which can be by about 20K (or even more) lower than the lowest temperature at the tropopause level. As these towers lose their buoyant energy, they stop ascending and begin to descend back to the cloud top equilibrium level (CTEL), spreading out and creating the familiar anvil cloud formation. As they descend they warm up adiabatically, and mix to some extent with the warmer environment of the lower stratosphere. This process also generates gravity waves which occasionally can be seen around the anvil tops, especially at low solar elevations. In general, we can find the lowest cloud top temperatures detected in our atmosphere at the very tops of these high storm cloud towers.

2) The elevated dome of the CTEL which resides in the warmer, lower stratosphere partially balances its temperature with that of the warmer environment. Therefore, the warm cores (or warm embedded areas) of 'cold doughnut' shaped storms should not be interpreted as 'depressions' or 'holes' within the storm, but just the opposite, that is, as quasi-steady elevated regions. And the cold doughnut shape is really only a manifestation of the outer, lower part of the anvil, situated close to the cold tropopause. As the elevated dome gradually disappears (or flattens), the storm top begins to appear cooler, which is occasionally interpreted incorrectly as a manifestation of storm intensification which, in reality, is just the opposite. A similar feature, referred to as a cold-U shape (or alternatively, a cold-V, or enhanced-V shape) seems to be related very closely to the 'cold doughnut' feature. However, it should be noted here that several alternative explanations of the warm embedded areas exist, the one described above is only one of the possibilities, but favoured by the author. Other phenomena may look similar, eg above-anvil cirrus or plumes.

Additionally, this case records a different notion (concept) of convective storm cell when observed by satellites and radar. What appears as one large storm cell in satellite imagery in fact comprises several storm cells when detected by radar observations (11:30–17:00 UTC, animated source: CHMI).

These individual storm cells form one common 'umbrella' storm anvil, spreading far downwind from the cluster of storm cores. This can be nicely seen in the schematic cross-section of the storm (author: M. Setvak).

This image is based on a real radar cross-section; however since the operational processing of CHMI's radar measurements ends at an altitude of 15km, the storm morphology above this level can only be inferred, partially based on satellite images. The anvil on the downwind side (northern part) of the storm spreads about 150km away from the core of the storm, without any apparent precipitation falling from it at the ground.

Therefore, one has to be very cautious when inferring storm size based merely upon satellite observations. The actual size of the storm (where the cloud is close to the surface or where there is precipitation) is nearly always much smaller than what is seen in satellite imagery.

Amendement (Martin Setvak, 24 September 2007)

Based on the latest studies, it appears that the central warm area (CWA) of the cold-ring shaped storm of 25 June 2006 (see muli-image panel ) resulted from a different mechanism than the one described above. There are at least two arguments for this:

1) The overshooting tops area as seen in the HRV image (top left frame of the panel) is obviously at the upwind side (south-southwest part) of the CWA. The CWA itself appears in the HRV image as more or less uniform, flat region. This suggests that most of the CWA can NOT be an elevated dome, as interpreted above.

2) Important are the radar cross-sections of the storm, as well as satellite-radar composite images of CAPPI 15.5km (corrected for the parallax shift) superimposed over the IR10.8 BT image — see central part of the panel. While the cross-section A-B crosses the overshooting tops region and the cold part of the cold-ring (the cold-ring here being still high above the tropopause level), the cross-section C-D cuts the storm in the 'downwind axis' of the CWA. A small part of the CWA corresponds to the downwind side (right side in the cross-section) of the overshooting top, however its majority is located at the flat stratiform part of the anvil, only slightly higher than the tropopause.

Since the uncertainty of the radar echo top height is about +/- 1.5km, the explanation of the warm spot given in the original text can't be fully excluded. However, it seems much more likely (at least in this case) that the CWA is a manifestation of some form of a 'wake effect' or 'gravitation breaking' mechanism rather than the 'warm elevated dome'.

The bottom image of the panel is a crop of the Earth disc, showing how small and common the storms discussed above (located here north-east of Alps) may appear without proper image processing.

This case shows nicely the necessity of complex studies, utilising all available data and their combinations, in order to achieve better insight on phenomena observed by the weather satellites. Also, it documents that the present conceptual models of tops of convective storms are far from being conclusive and unambiguous, they still evolve, depending on most recently available observational means and data processing techniques.

Additional content

Weather radar composite (Zmax) (14:00 UTC, source: CHMI)