Storm Xaver batters Europe

By Ian Mills, Remko Scharroo (EUMETSAT) and Luciana Fenoglio (Technische Universität, Darmstadt)

Xaver, a winter Atlantic storm, crossed the UK on 5 December, leaving devastation and severe flooding in its wake. Overnight the east coast of the UK was hit by a massive storm surge, caused by the convergence of high tides, high winds and waves.

On 6 December (St Nicolas Day) Xaver (also nicknamed Sinterklaasstorm) moved on to northern Europe, again causing major flooding and devastation. At least nine people died.

The Jason Significant Wave Height (SWH) product (see Figure 1) shows significant wave height of 4m near the coast of England, rising to 8m on the coast of Norway, with a maximum of 9m in the middle of the North Sea. This surge caused flooding on both sides of the North Sea. Along the Wadden Sea, the barrier islands along the north of the Netherlands and the northwest of Germany, record storm surges were measured.

The Mean Sea Level Pressure (MSLP) at 18:00 UTC on 5 December shows the deep depression with a central pressure of 959hPa over southern Sweden. The strong gradient on the western side of the depression produced very strong winds (up to mean speeds of 93km/h (57mph) reported in the North Sea) which drove the storm surge south to affect coasts of England, Northern Germany and the Netherlands. Gusts of 148km/h (92mph) were reported in northern Germany.

Meteosat-10 and Jason products

The storm surge along the North Sea was also picked up by the satellite altimeter AltiKa onboard the SARAL satellite.

SARAL is a French-Indian satellite mission launched on 25 February 2013. Among other instruments, it carriers a Ka-band satellite radar altimeter named AltiKa. EUMETSAT, in cooperation with the French space agency CNES, is processing and distributing in near-real time AltiKa’s measurements of significant wave height, wind speed and sea level height.

A comparison has been made of the storm surge measured by the AltiKa and two tide gauges along the German North Sea coast — one in the southern harbour of the island of Helgoland and one on an off-shore lighthouse Alte Weser.

The differences between these two ways of measuring sea-level had to be taken into consideration. While tide gauges measure sea level constantly at a single location, an altimeter captures the variation along its ground track. Thus, the altimeter can display the shape of the storm surge as a function of the distance to the coast, but the tide gauge can't. The tide gauge provides repeated measurements at that location, showing the full time history, while the altimeter revisits the same location at longer time intervals, in this case 35 days.

However, despite these differences the resulting data was found to be comparable.

Figure 3 (top graphics) shows a pass of the SARAL mission crossing the North Sea from south to north at 04:47 UTC on 6 December (Pass 629, Cycle 8). On the right of the map, shown in blue, are the wind speed, significant wave height, and sea level anomaly along the pass, as a function of latitude. The grey lines show the values of eight previous cycles, each separated by 35 days.

The AltiKa data showed:

• The significant wave height of up to 13m was much larger than had been measured before.
• The wind speed of about 65km/h (18m/s) exceeded all wind speeds measured by the altimeter over the previous nine months.
• The sea level anomaly (the level above a long-term mean), obtained by removing the main components of the ocean tide, rose by as much as 3m near the coast and was still noticeable as far as 200km into the North Sea.

Figure 3 (middle and bottom graphics) shows the data from the tide gauges at light tower Alte Weser (red) and Helgoland’s southern harbour (green), located about 10 and 50km from the coast. Both the astronomical tides and the long-term mean were removed from the tide gauge records to show only the effect of the storm on sea level.

Tide gauge data showed:

• Highest surge levels of 3.4m and 3.2m.
• A maximum wind speed of 83km/h (23m/s) preceded the height of the wind surge by half a day.

Additional content

Deadly storm and tidal surge batter northern Europe (BBC News)
Winter storm 'Xaver' batters Northern Europe (Spiegel Online)
Winter storm Xaver (CIMSS Satellite Blog)