Dynamical Perspectives on the Formation and Intensification of Polar Lows

• Main Author: Terpstra, Annick
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: The University of Bergen 2014
• Subjects: Arctic
• Online Access: https://hdl.handle.net/1956/8987

This thesis consists of a collection of scientific publications addressing dynamical aspects of the formation and intensification of polar lows. Polar lows are small scale, intense, short-lived cyclones developing over ice-free oceans at high latitudes. In the first paper, environmental atmospheric conditions during polar low genesis are identified. These environmental conditions are classified based on the direction between the thermal wind and the mean flow in the lower troposphere. If the thermal wind and mean flow are in opposite direction the environment is classified as reverse shear, if they are in the same direction the environment is classified as forward shear. The two types of pre-polar low environments exhibit distinctly different features in terms of synoptic scale patterns, baroclinicity, configurations of sea surface temperature, depth and stratification of the troposphere, polar low propagation directions, and surface fluxes. The ambient polar low environment during forward shear conditions resembles typical mid-latitude baroclinic cyclogenesis, whereas the reverse shear conditions are characterized by the presence of a synoptic scale, occluded low over the genesis location, and a strong low-level jet. In the second paper, a method to initialize baroclinic channel models is devised. The initial conditions are hydrostatically and geostrophically balanced, which makes them particularly suitable for studying rapid cyclogenesis. Furthermore, the method allows the definition of an arbitrary windfield, hence the initialization method exhibits a large degree of freedom in defining the initial conditions. We performed an unperturbed simulation which features minimal gravity wave activity, illustrating the balanced initial conditions. We also performed a perturbed simulation to examplify the utility of the setup. In this simulation baroclinic instability is triggered resulting in perturbation growth with time- and length-scales in agreement with observed polar low development. In the third paper, an ...