The beginning of 2013 saw unseasonal mild temperatures in some part of Europe, up to 10 °C in places. That all changed on 12-13 January when temperatures plummeted and many places had snow.
According to our UK Member State NMS, the Met Office, this change of type was caused by a phenomenon called Sudden Stratospheric Warming (SSW), which they spotted with the help of satellite imagery.
A sudden stratospheric warming (SSW) is an event where the polar vortex of westerly winds in the winter hemisphere abruptly (i.e. over the course of a few days) slows down or even reverses direction, accompanied by a rise of stratospheric temperature by several tens of degrees (up to 50 °C). This is considered to be the most dramatic meteorological event in the stratosphere.
The Met Office, and many other NMSs around the globe, receive data from a variety of polar orbiting satellites, including EUMETSAT’s Metop, NOAA POES, Suomi NPP and other satellites from US, Europe, China, Japan and India.
Using these observations and the past forecast data, they create the first step of an accurate forecast — the current state of the atmosphere. To make a numerical weather forecast, Met Office supercomputers start with these initial conditions and then work out billions of calculations per second to see how the atmosphere will behave over the next few days.
Experts at the Met Office explained how satellites helped them spot the SSW: “Observations are crucial to the process of creating forecasts, whether these are for the UK, the globe or even high up in the atmosphere. Satellite observations are a source of rich data about the state of the atmosphere.
"The satellite data give us temperature information about vertical layers 10–15 km thick in the stratosphere. Merging this with a six hour model forecast gives us an analysis of the state of the stratosphere, with better vertical resolution (1–2 km).
“We are alerted to an SSW by the changes to the analyses, created using a range of data and a model forecast. These changes can be modelled at the surface and through the atmosphere, including the stratosphere, where we can see how temperature is changing. You can get stratosphere temperature information from GPS radio occultation (RO) (GPSRO) satellites, which give higher vertical resolution than the other satellite data.
“The use of satellite data, therefore, enables us to observe increases in stratospheric temperatures, including sudden stratospheric warming events, and to forecast the evolution of these warming events and their possible effects on the weather across Northern Europe. The key point is that if the satellite data were not available it would be very hard to see the SSW signature in the analyses.”