A numerical, high-resolution study of the life cycle of the severe storm over Denmark on 3 December 1999

Abstract

A severe cyclone with destructive effects moved across Denmark during the evening of 3 December1999. A study, based on a numerical high-resolution limited area model simulation of this event, is presented. The development is of the frontal wave type. It can be interpreted qualitatively as aninteraction between a PV wave at the tropopause, a surface temperature wave on the polar frontand a low-level positive and upper-level negative PV anomaly generated by release of latent heat ofcondensation. In a run without this heat source the minimum sea level pressure in the cyclone increasedfrom 954 to 978 hPa. As a result of the PV anomaly interactions the surface cyclone moves from theanticyclonic to the cyclonic shear side of the upper-level jet during its evolution from an incipientlow to a mature cyclone. Its frontal structure evolves from an anticyclonic barotropic shear type toa barotropic type without shear. In the former state a low-level jet is confined to the warm sector ofthe surface cyclone. In the latter state a low-level bent-back front jet develops and becomes the mostintense low-level jet. The time-scale from initiation to maximum intensity of the jet is about 9 h. Therun without latent heat release develops similar, but weaker frontal structures, most notably a muchweaker bent-back front.