Squall line crossing Catalonia

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).

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.

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 100km/h.

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 500hPa), there were high PV values and a positive vorticity advection at 300 and 500hPa. All these dynamic processes in the higher levels of the atmosphere were crucial for providing a synoptic-scale uplift.

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.

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).

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 Θ_e 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 25m/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 30m/s (Figure 7).

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.

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

Additional content

Absolutely incredible thunderstorm structure from Spain (YouTube/Forum Atmosfer)
Incidències pel fort vent i la ràpida línia de tempestes que ha travessat Catalunya (CCMA, in Catalan)
El temporal Hortense colpeja de ple Barcelona i fa caure diversos arbres a la ciutat (Beteve, in Catalan)
Una forta tempesta a tot el pla de Lleida (avencsdelpalau)
Airmass RGB Quick Guide

References

Severe Convective Storms - An Overview (Charles A. Doswell)
A 10-year spatial climatology of squall line storms across Oklahoma (James E. Hocker and Jeffrey B. Basara)
Formation of a Squall Line over Germany (EUMeTrain/Wilfried Jacobs)
General Features of Squall Lies in East China (Zhiyong Meng, Dachun Yan, and Yunji Zhang)
About derechos (NOAA-NWS-NCEP Storm Prediction Center)
Vertical wind shear and convective modes (Estofex)