Improving ECMWF forecasts of sudden stratospheric warmings
Diamantakis, Michail
During winter a strong circumpolar cyclonic flow of cold air develops in the stratosphere which is known as the 'stratospheric polar vortex'. Observation records dating back to the 1950s show that on a number
of occasions each winter this apparently stable circulation pattern can be suddenly disrupted. A warming of up to 50 K occurs in the space of just a few days (at altitudes around 30 km and above), weakening the westerly zonal-mean flow or even reversing it to be easterly in the most extreme cases. This phenomenon is called a sudden stratospheric warming (SSW) and primarily occurs in the northern hemisphere. [...] It is important that our forecast model is capable of<br>accurately predicting these dramatic events for two reasons. Firstly, despite being a stratospheric phenomenon, there is strong evidence to suggest that SSWs influence the largescale tropospheric circulation below and therefore the winter weather that we experience. Secondly, a failure to represent the large thermal changes associated with SSWs<br>can lead to a significant discrepancy between the model and observations (primarily from satellites). Under these conditions the data assimilation system may incorrectly interpret the mismatch as a problem with the measurements and wrongly reject perfectly good observations.
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