Variability within cold air outbreaks and implications for parametrization

• Main Author: Chylik, Jan
• Format: Thesis
• Language: English
• Published: 2017
• Subjects: Arctic; Sea ice
• Online Access: https://ueaeprints.uea.ac.uk/id/eprint/67058/; https://ueaeprints.uea.ac.uk/id/eprint/67058/1/thesis_chylik_2017_digital.pdf

The changing structure of the marginal sea-ice zone (MIZ), together with high temperature gradients between the cold Arctic air and relatively warm sea water, contribute to uncertainty in numerical weather prediction (NWP). Since cold-air outbreaks (CAO) over the MIZ contribute strongly to heat transfer in the polar areas, assessment of variability in them is of a great importance. This thesis deals with extending our understanding of the variability within CAO by means of large eddy simulations, performed in The Met Office Large Eddy Model (LEM). The novel approach of this study lies within: Firstly, introducing three different patterns of heterogeneity in surface temperatures that represent conditions in MIZs; secondly, investigating both the spatial and temporal variability in the developing boundary-layer convection. A set of idealised scenarios and a case study are analysed. The case study is performed for a weak CAO event observed during ACCACIA field campaign on 21 March 2013. The study shows a profound impact of the inhomogeneous surface on both the spatial organisation of the boundary-layer convection and the latent heat flux at the surface and the boundary layer. The effect of heterogeneities depends strongly on the wind shear, the size and the orientation of the heterogeneity, and the initial stratification. When active cumuli clouds form, the effect of heterogeneities quickly diminish due to a top-driven mixing. In a stronger stratification that inhibits cumulus formation, the effect of heterogeneity is maintained. Stripes of temperature anomalies parallel to mean wind direction drive the formation of forced convective rolls and facilitate significantly higher latent heat flux. The impact of this heterogeneity usually increases with increased wind-shear, while the impact of other heterogeneities generally decreases. The impact of temperature heterogeneity is generally stronger than the impact of varying ice roughness. The implications for the parametrization of convective boundary layer are discussed.