The role of frontogenetic and frontolytic wind field effects during cyclonic development

Abstract

With the aid of a generalised frontogenesis vector and an expanded Q-vector diagnosis, the frontogenetic and frontolytic effects in numerical model analysed winds are determined for the case of a typical cyclogenesis over the North Atlantic. These effects, when occurring in the geostrophic part of the flow, give rise to circulations transverse to the frontal zone and the resulting vertical motions are of importance for the dynamics of the cyclone. The forcing of the transverse circulatory motions, which is given by Q-vector divergences across the isotherms, is computed and compared with the forcing resulting from divergences along the frontal zone. The analyses for 850 and 500 hPa show that frontogenesis dominates at the beginning of the cyclogenesis as well as at its final stage, whereas in the meantime different patterns result for both levels. A significant feature in this case was the development of a new frontal zone north of the deepening low through interaction with the parent low further north. Comparison of the different parts of the omega forcing reveals that the forcing through Q-vector divergences along the isotherms is the stronger one during the mature stage, but that the transverse circulations contribute significantly to the total forcing. This is especially true for the cold front in the lower troposphere and for the newly developing frontal zone in both levels. At the final stage the influence of the transverse circulatory forcing is even more important and determines to a great extent the shape of the total forcing. The outcome of this study corresponds very well with the findings from other investigations and with the fields derived from a vortex model.