Summary and conclusions
Between the 15th and 16th of November 2001 Finland experienced very strong wind gusts and, locally, heavy sleet and/or snowfall. The storm that caused the severe weather in Finland (known as Janika) originated near Greenland. Rapid Cyclogenesis that took place was influenced by Greenland and a very distinctive tropopause fold developed, resulting in the intrusion of stratospheric air down into the troposphere. A sign of this intrusion was the very extensive Water Vapour Eye that developed near the tropopause folding. The jet stream maximum increased from 60 m/s to above 80 m/s. Two low level jets appeared in the troposphere; one in the warm sector and another in the cold airmass to the rear of the low pressure centre.
The Water Vapour Eye began to deform and several diverging Dark Stripes developed. At that time the low centre was already passing northern Scandinavia, towards the middle parts of Finland. The Dark Stripes on the clockwise rotating side broke up and formed a group of small dark hooks with slight to moderate turbulence near the ground. However, the hooks that developed on the anticlockwise rotating side of the low grew large and dark. A hook that moved just in front of a white water vapour arc caused very strong gusts. The gusty winds were strongest at the hook head, varying from 22-28 m/s over land and approximately 25-35 m/s over the sea.
The white arc became more distinct in the water vapour image as it propagated southwards at 43 m/s from Lapland, Northern Finland, causing severe turbulence in southern Finland. The destruction of forests resembled that typically caused by downbursts of a scale F2 on the Fujita scale.
When compared with Dark Stripes, the white arc differs as follows:
- Temperature advection (TA) increases in the vicinity of the tropopause and a PVA maximum dominates in the same area
- No clear sign of tropopause folding exists
- The increase of TA takes place over a short period of time, as the gravity wave deepens
- A trough develops and moves parallel to the white arc line
The gravity wave marked by the white water vapour arc appeared in ideal surroundings:
- Scandinavian mountain barrier to the west
- Favourable stability differences in the horizontal and vertical; weaker mid-tropospheric static stability overlaid a strong stable lower-tropospheric layer.
- Nearly parallel air stream throughout the troposphere.
- Air stream from Barents sea over Lapland and once again from open Gulf of Bothnia to western Finland
The evolution of this episode may have been a coincidence. However, there were many important events, supporting Rapid Cyclogenesis, which took place nearly simultaneously:
- Ideal position and angle between the surface and upper level lows
- Existence of cold air outbreaks at the most favourable time of year
- Wave propagation to the Occlusion point just as the Dark Stripe moved over southern tip of Greenland to the leeside.
In order to understand the developments that took place in Finland we can conclude the following:
It seems that ideal surroundings and dynamical events caused a chain reaction:
- The Rapid Cyclogenesis to the lee of Greenland resulted in a deep tropopause fold and a pool of cold air
- Thermal advection and fronts intensified and strong low-level jets formed
- The formation of several diverging Dark Stripes and the enhancement of convection in the Water Vapour Eye region caused heavy showers of snow and sleet and strong wind gusts
- Arctic air flow from the open Barents Sea south towards the Scandinavian mountains resulted in the right circumstances for large scale gravity wave amplification
- Vertical wind fields became nearly parallel and were also parallel to the propagation of a developing gravity wave.