Cold wave along the Spanish Mediterranean coast

By Nicolás Bermejo (AEMET)

This event began on 18 January with the arrival of a very cold Continental airmass near Spain.

In the water vapour imagery from 19 January, 06:00 UTC (Figure 1), an elongated and curved deformation zone can be seen in the East Atlantic (zone marked by sharper moisture gradient) with a mushroom-shaped pattern.

The Meteosat-9 Airmass RGB, with 300hPa geopotential height overlaid (Figure 2), shows a cut-off low over Spain. The PV anomaly is marked by a red colour. Potential Vorticity (PV) is a good indicator for the dynamic processes. The tropopause dynamic anomalies are areas where dry stratospheric air descends from the upper troposphere, into the lower levels.

When comparing the Airmass RGB to the model PV=1.5 PVU surface height, a clear correspondence can be observed between the red areas and PV anomalies.

Convective clouds are the bright white cloud formations seen over the east of Spain. The areas where the deep convection is seen, is where the heaviest rainfall can be expected.

The presence of a cut-off low with a cold pool at 500hPa (-32°C) promotes the advection of moist and relatively warm airmass from the east at surface level over the sea.

The upper air was very cold, coming from inside the European continent. This very cold airmass became unstable when heated and moistened in low layers in contact with the surface of the sea, and was responsible for heavy snowfall. The highest snowfall intensity was recorded on the 19th and some of the snowfall was accompanied by thunderstorms. Figure 3 is the precipitation estimate product MPE. The accumulated snow recorded in this first part of the event was important, generally in excess of 30cm. Some parts of the eastern Spanish coast saw their first snow in 90 years. The main problems occurred in the mountains near the coast where at least 3,000 people became stranded on roads and trains due to heavy snow.

Later, on 20 January, the cold intrusion which was aloft was the precursor of a cyclogenesis to the north of Algeria. In Figure 4 we can see the s-shaped appearance of the baroclinic leaf which is a signal that a cyclogenetic process is occurring. The formation of a baroclinic leaf typically occurs together with significant PV anomalies on the 1.5 PVU surface (red colours in RGB Airmass). On 21 January, the surface low moved northward with centre of 1,004hPa (Figure 5).

A high was above Central Europe with centre of 1,037hPa over Hungary. The result was a strong surface pressure gradient over the western Mediterranean Sea. Figure 6 shows the ASCAT 10m winds on 20 January between 19:45 and 21:30 UTC, overlaid on an MSG IR 10.6µm image and ECMWF MSLP chart from 21 January at 00:00 UTC. The ASCAT data indicates the wind speeds associated with the system. There are strong winds of about 30 to 35kt (red arrows) in several areas of Mediterranean Sea.

On 21 January at 21:00 UTC, ASCAT confirm gale force winds in the western Mediterranean area (Figure 7, pink arrows). One use of the ASCAT winds is to compare them with the wind speed surface forecast to check if the model is consistent with reality. Comparing ASCAT winds with the ECMWF nine-hour wind speed surface (10m) forecast (Figure 8), show the model underestimates the wind surface speeds, especially between the Gulf of Valencia and the Balearic Islands, and also in the Gulf of Genoa.

On the 21st several buoys broke their wave measurement records (Figure 9):

• The 'Cabo de Palos' buoy recorded a 6.33m wave, beating the previous record of 5.7m in 2013.
• The Tarragona buoy recorded a 6.33m wave, beating the previous record of 5.27m in 2010.
• The Valencia buoy recorded a 6.45m wave, beating its previous record of 5.63m in December 2009.

The highest wave height of the event was recorded by the Canal Ibiza buoy — 10.1m. Although close to the bouy's historical record of 10.3m recorded in December 2013, it wasn't near the highest wave recorded in the western Mediterranean — 13.6m in Mahon in 2001.

Figure 10 shows the wave heights with Jason altimeter data, on 21st between 14:00 and 21:00 UTC. The altimeter gives maximum values of wave height of 18ft (5.5m) at west of Corcega island.

The ECMWF analysis for 18:00 UTC and 00:00 UTC on 21st (Figure 11) coincides with the altimeter data but underestimates the values observed by the buoys, since it gives lower values than those observed by the buoys (6.3m at Catalonia, 6.2m at Cartagena, etc.)