Untersuchungen zum Einfluss erzwungener atmosphärischer Rollenkonvektion in Kaltluftausbrüchen auf den vertikalen turbulenten Transport in der atmosphärischen und der ozeanischen Grenzschicht mittels Large-Eddy Simulationen

authored by: Jens Fricke
supervised by: Siegfried Raasch
Abstract: Weather and climate are significantly influenced by the turbulent exchange of heat, momentum, water and trace substances between the atmosphere and the ocean. These exchange processes take place within the atmospheric and upper oceanic boundary layer and depend essentially on the temperature difference between the two systems and the respective flows. A typical meteorological situation with large temperature differences are polar cold air outbreaks (CAO), where cold and dry air is advected by synoptic-scale wind from an ice-covered landmass over the warmer ocean. With structured ice edges, the convection over the ocean is typically organized into rolls, which extend vertically across the entire boundary layer and horizontally up to several hundreds of kilometers. The turbulent exchange processes within the boundary layers cannot be resolved in large-scale models, such as global numerical weather, climate and ocean models, and thus must be parametrized. Therefore, a deeper understanding of these processes is necessary. Although previous studies have shown that convective rolls play a significant role for the vertical turbulent transport, it is not clear if the boundary layer rolls affect the total turbulent transport. This question will be investigated in the current study by means of large-eddy simulations (LES). In the first part of this study, 12 different polar CAO situations were simulated, by varying the synoptic-scale wind and the ocean temperature. By means of different sea ice distributions, a case with and without rolls could be simulated under the same meteorological conditions for each CAO situation. The properties of the simulated rolls agreed well with observed convective rolls in CAO. The boundary layer development was compared between the roll and the nonroll case, and the roll contribution to the vertical turbulent transport was analyzed for each investigated CAO situation. Results show that although the rolls make a substantial contribution to the vertical transport, they neither affect the total transport nor the development of the boundary layer. These results could also be confirmed for different roll wavelengths and additionally for a dry atmosphere. As known from observations, atmospheric rolls modify the structure of the sea surface, including the surface fluxes. Therefore, the influence of atmospheric convective rolls on the ocean was investigated in the second part of this study, and, if feedback effects with the atmosphere could occur. For the simulations of the upper oceanic boundary layer, the flow was driven by the near-surface fluxes of one of the CAO simulations, again with and without roll signals at the sea surface. The ocean simulations with a roll signal showed that a very weak roll-like circulation is induced, which extends over the whole oceanic boundary layer. A comparison between the simulations with and without roll signals at the sea surface showed no significant differences in the boundary layer development of the ocean. This also applies to the sea surface, so that there is no feedback on the atmosphere in the form of a changing heat or moisture input. This result was also confirmed by considering the Langmuir circulation. The results of this study showed that atmospheric rolls triggered by upstream heterogeneities contribute substantially to the vertical turbulent transport, without increasing the total transport. Therefore, such rolls do not have to be considered in parametrization schemes of large-scale models, as long as these models do not resolve such large-scale turbulence.
Organisation(s): Institute of Meteorology and Climatology
Type: Doctoral thesis
No. of pages: 121
Publication date: 2018
Publication status: Published
Sustainable Development Goals: SDG 13 - Climate Action
Electronic version(s): https://doi.org/10.15488/3637 (Access: Open)