Severe convective storms over Romania

By Florinela Popa (INMH) and Jochen Kerkmann (EUMETSAT)

On the day following the severe storms over Austria, Czech Republic and Poland (see case study), Romania was affected by a severe storm that appeared to cross the Romanian Plain from north-west to south-east.

The images below show the situation at 14:30 UTC when the severe storm had already reached the southern parts of the country, see the storm that has a marked U or V shape and which appears quite yellow in the RGB composite (Figure 2).

From the animation it can be seen that the storm started at around 11:30 UTC over the Carpathian Mountains. It then moved into the Plain where it found sufficient warm, unstable air to trigger further convection.

At low levels cold air was penetrating from the west into the Romanian Plan, while at high levels winds were from easterly directions (see absolute topography at 300hPa, source: Deutscher Wetterdienst). This vertical wind shear helped to separate updrafts from the downdrafts, which is essential for persistent severe storms. At the same time, the storm was following a low-level convergence line, which explains its straight path from north-west to south-east.

More information on the specific situation of the Romanian Plain is given below the images.

Meteosat-8, with its 12 channels, provides essential information for the forecasters allowing him/her to decide whether convective storms are severe or not. Not only does it give detail on the spatial structures of the storms (see the U shape in the IR10.8 and WV6.2 images) but it also provides information on the size of the ice particles (e.g. in the NIR1.6 and IR3.9 images, or in the IR3.9–IR10.8 brightness temperature difference). Storm tops with small ice particles (high reflectivity in channels NIR1.6 and IR3.9) appear with a marked yellow colour in the RGB composite, which indicates that there are very strong updrafts/downdrafts with possible hail and wind gusts.

In situations like the one shown in this case study, when cold air is advected above Romanian territory, the cold air tends to follow the orography and penetrates into the Romanian Plain:

• Eastward from the Hungarian Plain
• Southwest to westward, from the Ukraine/Russian Plain

Thus, the airflow converges over the central part of the Romanian Plain (southern Romania), enhancing the convective activity. Usually, the convergence line appears along the river Olt (see geographic map of Romania, source: INMH). The case shown here has, however, some special features to note:

• The eastward cold advection from Central Europe (behind the cold front) was very strong (see surface chart at 12:00 UTC source: INMH).
• There was no westward cold advection, the eastern part of the country being in a warmer airmass.
• The convergence line along the Olt river was however evident from the surface analysis (see source: INMH) due to the cold air advection from the central part of Romania along the Prahova Valley (north–south oriented).
• There was a temperature difference of few degrees across the convergence line: 28–29 ° C on the western side, 31–32 ° C on the eastern side.

In the convergence area, the amounts of precipitation did not reach very high values (maximum 29mm/m² in three hours, between 12–15 UTC at Curtea de Arges and respectively, 18.4mm/m² at Pitesti) but frequent thunderstorms, hail and wind gusts were reported. In the western and central part of Romania significant higher precipitation was recorded: the precipitation amounts exceeded frequently 20 -30 mm in a short time (maximum 105mm/m² in the Mures Department -Mica village).

Additional content

RGB Composite VIS0.8, IR3.9r, IR10.8 31 May 2005, 14:30 UTC,
RGB Composite VIS0.8, IR3.9–IR10.8, IR10.8 31 May 2005, 14:30 UTC