Séminaire
Polar lows are severe, maritime mesoscale cyclones, developing over the ice-free ocean at high-latitudes. Despite recent improvements in numerical weather prediction, polar lows still represent a major challenge for forecasting in polar areas. Hence, understanding the underlying dynamical pathway and identifying favourable atmospheric conditions are essential to make improvements in our forecasting skills.
We discuss the synoptic conditions in which polar lows form, where we utilise a dynamical feature of the sub-synoptic environment associated with the genesis of polar lows to distinguish between different dynamical pathways of polar low development and determine the characteristics of those environment. Based on the angle between the thermal wind and the mean wind, two subsets of polar lows are identified: forward-shear polar lows, where the thermal and mean wind are in the same direction, and reverse shear polar lows, for which the thermal and mean wind are in opposing directions. Both environments are equally likely associated with polar low development. However, the environments exhibit distinctly different synoptic and sub-synoptic configurations.
Furthermore, we present idealised simulations to examine the influence of moisture, baroclinicity, and static stability on development utilising an idealised baroclinic channel model. The experiments show that disturbances are able to amplify in the absence of an upper-level perturbation, surface fluxes, friction, or radiation. Separation between developing and non-developing disturbances is feasible by considering the baroclinic and diabatic contributions to eddy available potential energy. Developing disturbances show a clear diabatic dominance during the early stage of development. A comparison with the conceptual framework of the Diabatic Rossby Vortex (DRV) growth mechanism provides insight into the dynamical pathway potentially underlying the enhanced amplification. We also identified threshold values for moisture content and baroclinicity.
Gwendal Rivière