Wall cloud over Catalonia

Squall line crossing Catalonia

21 January 2021 21:00 UTC-22 January 12:00 UTC

Photo credit: Carles Rabadà Garcés de Marcilla

Wall cloud over Catalonia
Wall cloud over Catalonia

Overnight on 21 to 22 January 2021 a 400 km squall line formed over the Iberian peninsula, with a speed of around 100 km/h, which caused severe wind, rain and hail over Catalonia.

Last Updated

14 June 2021

Published on

14 June 2021

By Xavier Soler, Manuel Álvarez, Tomeu Rigo, Montse Aran, Clara Brucet, Carme Farnell and Sergio Gallego (Meteorological Service of Catalonia), Natasa Strelec Mahovic (EUMETSAT) and Ivan Smiljanic (CGI)

A squall line is defined as a shallow linear system of convective cells, organised along, or ahead of, a cold front or convergence line, with a gust front at the leading edge. The squall lines in mid latitudes are usually observed during spring and summer, therefore, a convective line that developed on 21-22 January 2021 in southwest Europe had quite different characteristics compared to the convective lines developing during the warm and moist period of the year (Figure 1).

Figure 1: Morning view of squall line advecting eastwards over the Mediterranean Sea. Meteosat-11 HRV, 22 January 07:00–12:00 UTC

A loop of SEVIRI Water Vapour (WV) 6.2 μm images (Figure 2) provides information about the differences in humidity at upper and mid-levels of the troposphere. The evolution of the squall line from west to east, crossing the north of Iberian Peninsula on 22 January, can be followed.

Figure 2: Meteosat-11 SEVIRI WV6.2 μm loop, 22 January 00:00-09:00 UTC.

At the beginning of the loop the dark area north of Portugal corresponds to the dry air related to a tropopause dynamic anomaly on the polar side of the jet. Conversely, the light area coincides with the upper-level jet moisture boundary. The early stage of development begins at 00:00 UTC in the transition zone of dark (dry) and light (moist) shades in the WV image. This upper-level forcing is persistent in time, causing increasing convection over north-east Spain, and resulting in the main cloud band becoming more organised.

From the loop it can be assumed the squall line was displacing very fast. Specifically, from Zaragoza (north-east Spain) to Lleida (Catalonia) where the system speed was about 100 km/h.

Meteosat-11 SEVIRI 10.8 µm image overlaid with isotachs and wind barbs at 300 hPa, 22 Jan 06:00 UTC
Figure 3: Meteosat-11 SEVIRI 10.8 µm image overlaid with isotachs and wind barbs at 300 hPa, 22 Jan 06:00 UTC. The black line shows the location of the squall line (Credit: EUMeTrain)

The synoptic situation reveals that the convective line was formed in an unstable environment. As seen in Figure 3, the squall line developed in the left exit region of the jet stream. In this area, ahead of short and cold wave trough (geopotential height at 500 hPa), there were high PV values and a positive vorticity advection at 300 and 500 hPa. All these dynamic processes in the higher levels of the atmosphere were crucial for providing a synoptic-scale uplift.

Figure 4: Meteosat-10 RSS SEVIRI Airmass RGB loop, 22 January 00:00–09:00 UTC

The loop of Airmass RGB images (Figure 4) shows the movement of two air masses. In the hours before squall-line development (00:00 UTC) the warm and moist air mass (green) covers a large area of the Iberian Peninsula. In contrast, in the north-west, behind the cold front, dry intrusion (red) appears, associated commonly with the potential vorticity (PV) anomaly and the position of the jet stream. At 03:00 UTC the short and cold wave trough moves further east, and the polar air mass replaces the tropical one. Three hours later (06:00 UTC) close to Catalonia an increase of the cloud development at the boundary of these air masses can be observed, probably supported by the convergence line at surface level. The end of the loop shows the squall line approaching the Mediterranean Sea.

At surface level, the convection associated to the cloud band was triggered by upward motions of the cold front. The squall line displacement eastwards followed the areas where the pressure drop was significant (Figure 5). This frontal system could develop the well-defined line of convective cells.

Figure 5: 3-hourly surface pressure change, 22 January, 00:00 UTC. Credit: WRF–ARW.

Conversely, due to the complex orography, a convergence line was formed at Ebro’s Valley near ground level (until 950 hPa) that contributed to the intensification of the convective line. Moreover, the westerly flow behind the squall line, and the persistence of the easterly wind ahead of it helped the maintenance of the squall line structure (Figure 6).

Streamlines (950hPa) and radar product, 22 January 06:00 UTC
Figure 6: Streamlines (950hPa) and radar product, 22 January 06:00 UTC. Credit: EUMeTrain.

Because of the time of year (January) the stability indices were not as unstable as they would have been in a summer case. Nevertheless, looking for the thermodynamic characteristics in the area where the event was initiated, the pseudo-sounding of Palencia (located on the northern plateau of Spain) showed favourable conditions for multi-cell development. The air stratification near the surface was unstable (warm and wet air with Θe decreased with height). The hodograph shows a clockwise rotation with height and strong vertical wind shear at low-levels which are characteristics for the development of multi-cell storms.

The squall line produced strong winds with maximum wind gusts in Catalonia related to a bow-echo shape seen in radar data. Values over 25 m/s were estimated with the radial wind radar product (not shown) for more than two hours. This coincides with wind gusts recorded by the automatic weather station network from Meteorological Service of Catalonia. In several locations along the squall line wind speeds exceeded 30 m/s (Figure 7).

Wind gusts measured over Catalonia, 22 January 2021
Figure 7: Barbs: Wind gusts measured over Catalonia, 22 January. Blue triangles: hail observed. Credit: Meteorological Service of Catalonia.

Furthermore, 300 incidents were reported by emergency services related to the three people being injured, falling trees and branches, removal of roofs and fallen walls.

Lightning detection, 22 January 2021, 00:00 to 12:00 UTC
Figure 8: Lightning detection, 22 January 00:00 to 12:00 UTC. The different colours indicate temporal evolution of lightning events (green near 00:00 UTC and red 12:00 UTC). Credit: LINET.

A squall line is often accompanied by lighting. Figure 8 reveals a length of about 400 kilometres of the squall line. Severe features were inferred with the detection of a Lightning Jump warning in Catalonia.


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