EUMeTrain: Case Study on an unusual rapid cyclogenesis

Authors

ZAMG
Jarno Schipper
Veronika Zwatz-Meise

Introduction

This case study is about a rapid and intensive development of a synoptic scale frontal system originally from a small scale feature in the cold air. The two images below show an initial and an end stage.

Meteosat-8 IR 10.8μm: 1st March 2004: 12 UTC. The red arrow marks the initial cloudspiral. Meteosat-8 IR 10.8μm: 3rd March 2004: 00 UTC. The eventual developed cloudspiral covers southern Finland and the Baltic States.

The fully developed stage in the right image shows a synoptic scale frontal cloud system with a distinct broad cloud band extending from S-Finland to S-Poland and a well developed cloud spiral over the Baltic Sea. Usually the band would be classified as a cold and the spiral as an occlusion front. Only in the expected area of a warm front there is no extended cloudiness visible, but also this is not unusual in such a far developed stage of an occlusion development.

The discrepancy lies in the initial stage which can be seen in the left image. From the far developed stage one would expect originally a synoptic scale frontal cloud band along which a wave development taking place which then further develops into an occlusion spiral. But this is not the case here. In reality there is only a very weak W – E oriented cloudy band and some comma feature in the cold air behind.

However: the time of the image representing the initial stage is from 12 UTC and a very pronounced cloud spiral does already exist. If this cloud spiral is followed back to 00 UTC of this day it becomes clear that the spiral develops out of or within a much larger cloud system which cannot easily be diagnosed neither from the images nor from the parameters because of the very nordic location. But if NOAA images are inspected it becomes much clearer that the large cloud area does not look like a meridionally oriented frontal zone but rather like large scale cloudiness very probably in a thickness ridge and under WA; relevant numerical parameters will support this in a later chapter.

The aim of the study is to:

To be able to follow the case study from the beginning it would be preferable to study the chapters “occlusion: warm conveyor belt type” and “rapid cyclogenesis” from the “Manual of Synoptic Satellite Meteorology”.

A cyclogenesis is a rather complex process leading to an occlusion stage of a frontal system. There are at least two main theories: the “classical occlusion process” according to the”polar front theory” and the “rapid cyclogenesis” being a result of several processes where stratospheric and tropospheric features are acting together. Other processes of cyclogenesis are the “Instant Occlusion Process” and the “Cold Air Development”. From a quick look into the satellite images the development could also represent a “Cold Air Development” which starts from a cold air feature like a comma and grows into a synoptic scale cloud system.

To describe these processes with relevant numerical parameters, classical parameters like surface pressure (height of 1000 hPa) and upper level height (in this case study 500 hPa) but especially derived parameters like temperature advection (TA), vorticity advection (PVA – Positive Vorticity Advection), Jet streaks and Potential Vorticity (PV) give insight into the development mechanisms.