Dynamical Perspectives on the Formation and Intensification of Polar Lows
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- Geophysical Institute 
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 idealized baroclinic channel model is utilized to investigate the role of latent heating during polar low development in forward shear conditions. Within this idealized setup, a low-level, weak, cyclonic perturbation is able to amplify in absence of upper-level forcing, radiation, or surface fluxes. Crucial for rapid development is sufficient latent heat release in the north-eastern quadrant of the cyclone. The potential for latent heat release to occur depends on the environmental relative humidity, baroclinicity and static stability. Due to the Arctic configuration of the setup, the perturbation depth is relative shallow, which in turn improves the effectiveness of latent heat release on cyclone amplification.
Består avPaper I: Annick Terpstra, Clio Michel, Thomas Spengler, Atmospheric environments associated with polar low genesis in the North-East Atlantic. Full text not available in BORA.
Paper II: Annick Terpstra, Thomas Spengler, A novel method to initialize idealized channel simulations, Montly Weather Review. Full text not available in BORA.
Paper III: Annick Terpstra, Thomas Spengler, Richard Moore, Idealised simulations of polar low development in an Arctic moist-baroclinic environment, Quarterly Journal of the Royal Meteorological Society. Full text not available in BORA.