Diabatic heating of the ground, combined with cold air advection in lower layers of the troposphere resulted in perfect conditions for a vertical wind vortex to intensify in June 2021.
30 September 2022
22 July 2021
By Ivan Smiljanic (CGI)
Intense vertical rotation of the air column, stretching from the ground to the cloud base, was spotted over heated, flat terrain in the Rhine valley, south of Frankfurt, on 15 June (Figure 1). The rotation was even more vivid because it happened in a rural area over sandy soil, namely asparagus fields covered with the elongated black plastic sheets.
Although, similar to a tornado in appearance, due to the fact that there was an absence of vertically developed cumulonimbus clouds, this feature was actually a dust devil.
During warm and calm days, with no considerable cloud developments, dust devils, relatively short lived, well formed whirls on the ground, can form. They can occur in different sizes, from a metre in width and height, to more than 10 m wide and 1km high. This dust devil was wider than 10 m and higher than 1 km.
So, what caused such a big dust devil?
It was a combination of a very well heated surface and cold air advection in lower layers of the atmosphere. Dust devils form when a heated volume of air, being positively buoyant, starts to rise and stretch. The intense vertical stretch of the air column is the main driver for the increase of initial rotation of that column, producing a tornado-like rotation throughout the vertical extent. As seen from Figure 1, this rotation is wider and more apparent closer to the ground, due to the friction and ingested (sand/dust) particles and objects. The intensity of the stretch is dependent on the temperature difference between the air at the surface and the lowest layer of the troposphere (1-2 km).
In this particular case, almost cloud-free skies allowed for plenty of insolation earlier that day, and the ground was already quite warm due to an extended warm period over the previous days. Added to that, the passage of the weak Baroclinic Boundary was responsible for a cold advection aloft, in the lower layers of the troposphere (at around 2 km) which contributed to the air temperature gradient between the surface adjacent and the atmosphere aloft (Figure 2).
A vertical cross section confirms the convergence-divergence dipole very close to the ground, associated with the superadiabatic stratification only at lowest levels of the troposphere (Figure 2). Otherwise, the troposphere was quite stable in the mid and higher levels, not favourable for considerable vertical cloud development. This is also apparent from the HRV animation in Figure 2, which shows that bigger clouds did not develop during the course of the day.
The comparison between radiosounding measurements in nearby Meiningen, at 00:00 and 12:00 UTC that day (Figure 4), reveals the cooling of the lowest parts of the troposphere (around 800 hPa) during the day, and very strong diabatic heating close to the ground. The steep slope of the temperature readings between ground and 800 hPa can be seen in the 12:00 UTC sounding (right).
On the list of hazardous weather events, dust devils do not usually peak very high, normally bringing only minor destruction to the local infrastructure. In this particular case, however, maintenance teams spent many hours clearing the black plastic sheets which had been thrown on to the power lines hundreds of metres away from the asparagus fields (see header image).
Tornado in Pfungstadt? (hr fernsehen)