Two severe MCSs on the same day over Hungary

By Mária Putsay, Ildikó Szenyán, André Simon, Ákos Horváth, Péter Németh and Kornél Kolláth (Hungarian Meteorological Service)

On 29 June 2006 a Mesoscale Convective System (MCS) with a squall line swept across Hungary from the west to the east. The MCS had just left the country when another huge convective system developed over the Julian Alps and Dinaric Mountains, and soon arrived over Hungary from the south-west. The two MCSs caused heavy precipitation, hail and strong winds on the same day. Extreme weather conditions were experienced: at Lake Balaton more than 100km/hour wind speed was measured; trees fell across roads and railways, and flash floods occurred on some creeks.

The synoptic situation of that day. The evolution of the MCSs can be followed on radar (03:00–23:45 UTC) and in airmass RGB satellite (00:00–23:45 UTC) animations.

The first MCS formed over the Alps the day before and reached the northwest boundary of Hungary in the early morning as a strong instability line. After 10:30 UTC an interesting asymmetric bow echo can be seen on the radar images. The northern part of the line curves more and more with time (see time sequence of radar images).

The asymmetric bow shape of the squall line echo is due to the mid-level rear inflow jet and the Coriolis effect. The Coriolis effect significantly impacted the system evolution, since this MCS was a long living, huge system (see conceptual model .

Cloud top features like overshooting tops, gravity waves, and plumes are easily seen in the MODIS visible image (PDF) and in some cases in the MSG HRV images. The MSG IR10.8 channel satellite image shows the cloud top temperature structure.

In some images one can observe cold rings and cold U shapes which are characteristic of severe systems (see example (08:15 UTC) and conceptual model). Combining the HRV and IR10.8 channels we get the HRV cloud composite image, a good tool for investigating convection (see example).

To investigate the relative locations of the main updrafts and downdrafts (overshooting tops and highest radar Zmax values), we marked the locations of the overshooting tops on the HRV images by circles, and overlaid them on a combined radar-satellite image. The shape of the line connecting the overshooting tops and the high radar reflectivities were similar and close to each other. They are shifted a little due to the effect of parallax.

Lightning activity was extremely high in the first MCS. Ten-minute lightning data was also added to the simultaneous radar and satellite images (see time sequence of radar, satellite and lightning data). The majority of flashes detected were close to the locations of the overshooting tops and the most intense precipitation.

The 3.9 micron reflectivity contains information about the average size of the cloud top ice crystals. This reflectivity can be calculated from the daytime IR3.9 channel data by subtracting the thermal radiation, or it can be approximated by the brightness temperature difference (BTD) of IR3.9–-IR10.8. The storm RGB shows this difference in green.

We visualised both the IR3.9refl and the storm RGB together with the HRV+radar and IR10.8 images. In the cloud top of the first MCS we see many small ice particles almost everywhere in the anvil (see time sequence, PDF). These small ice particles are caused by cells with strong updrafts. In a strong updraft the small water particles at the cloud base reach the cloud top very quickly and have no time to interact or grow.

The system was already quite old when it arrived over Hungary so we see many old, small ice crystals which were already widespread over the whole anvil. In some cases one can also see that the IR3.9–IR10.8 brightness temperature difference depends on the IR10.8 brightness temperature and not merely on the particle size.

Investigating the much younger system over North Italy (see time sequence), one can see near the overshooting top a concentrated, growing area of small ice particles. The second MCS was formed from this system and the cells developed over the Dinaric Mountains after 13:10 UTC.

The HRV cloud animation (12:15–17:15 UTC) shows the dissipating phase of the first MCS. This MCS transformed to a MCV (Mesoscale Convective Vortex) due to the significant Coriolis effect. A MCV consists of mid-level convergent cyclonic flow and high-level divergent anticyclonic outflow within the anvil, which can be seen in the animation with some still active cells.