Cyclone Friedhelm: a potential sting jet case

Written by Laura

Wednesday, 07 December 2011 15:07

synop 12z_081211

Planning is well underway for a double flight through the explosively deepening cyclone that is forecast to pass over Scotland tomorrow afternoon and into the evening. The cyclone has been named Friedhelm, by the Free University of Berlin's "Adopt a vortex" scheme. The system is forecast to deepen by more than 24hPa in 24 hours, and thus satisfies the conditions for a "bomb".

The Met Office surface chart for 1200UTC tomorrow shows the low pressure centre located over Lewis, off the west coast of Scotland, and it will then continue to track across the UK. Very strong winds are forecast, associated with the passage of the bent-back front (the hooked-round part of the occluded front on the surface chart).

The cyclone is forecast to develop a T-bone frontal structure and a strong cold conveyor-belt associated with the bent-back front and strong low-level winds. This development resembles the development of sting jet cyclones, so we are hoping that our observations of this case could be the first in situ measurements of a sting jet cyclone so far.

gudrun pic

The sting jet is a mesoscale jet that descends from the tip of the cloud head into the dry intrusion air ahead. Sting jets can result in very strong, potentially damaging surface winds. The picture below is a satellite image of windstorm Gudrun, a sting jet storm that occurred in January 2005. The annotations show the position of the sting jet (labelled "SJ") relative to the cloud head and dry intrusion. The sting jet in this case resulted in surface wind gusts of more than 40m/s (around 90mph). However, even stronger winds in this case were associated with the cold conveyor-belt (CCB), which hooked round as the cyclone continued to deepen, and these strong winds caused a large amount of damage over Denmark and Scandanavia as the cyclone continued to move eastwards. Another notable case of a sting jet cyclone was the Great Storm of October 1987 which caused huge amounts of damage in the south of England, felling millions of trees and damaging many buildings.

Current forecasts for tomorrow's system show wind gusts exceeding 40m/s which appear to be associated with the CCB. However, there are also strong winds ahead of the cloud head, in the region where a sting jet could occur.

The plan for the flight is to fly across the bent-back front to drop sondes through the cloud head, to investigate the banded structure and possible slantwise circulations, which are thought to be important in the development of sting jets. We will then fly through the dry intrusion, above the region where the sting jet might occur. If we're lucky, we may be able to drop sondes through the sting jet itself!