Mesoscale aspects of atmospheric flow in complex orography | Þættir á millikvarða í vindasviði yfir fjöllóttu landi

The weather and climate in Iceland is to a large degree governed by synoptic scale weather systems and orographic forcing. This thesis is composed of 15 peer-reviewed papers pertaining to atmospheric processes in complex terrain, with a special focus on Iceland. Severe weather is the subject of most of the papers, either in the context of primary weather parameters such as wind, or in relation to secondary parameters such as atmospheric turbulence and icing. In two papers, numerical simulations and observations of winds are used to map and analyze katabatic winds during a heatwave in South-Iceland as well as near the Hofsjökull ice cap in central Iceland. Observations of weak orographically forced winds are also the subject of another paper where asymmetric atmospheric vortices are shed in the lee of a large mountain in West-Iceland and advected 120 km towards and over Reykjavík. Five papers analyze simulations and observations of winds during severe windstorms in Southeast- and Northwest-Iceland. The performance of the atmospheric model with regard to model setup and the parameterizations of moisture physics and boundary layer processes is investigated and analyzed in two of the papers. The strongest winds are generally found below amplified and/or breaking gravity waves, as well as hydraulic jump-like features, on the lee side of large mountains. The papers reveal the importance of high horizontal resolution for resolving downslope windstorms in complex terrain and that interpolation from coarse-resolution simulations may lead to large errors, even if the mountains are to some extent correctly reproduced. The downstream extent of downslope windstorms depends strongly on the upstream structure of the atmosphere and its prediction is one aspect of numerical weather prediction that needs improvement. These papers also reveal that fine scale numerical simulations are not only needed to capture windstorms at the surface in complex terrain but also to correctly reproduce turbulence aloft, both at lower tropospheric levels in Iceland as well as at the tropopause, e.g. above Greenland. Wind gusts are analyzed and parameterized in two papers, where the observed gustiness is on average reproduced but how well depends strongly on the accuracy of the simulated turbulence aloft. Two papers show that observations from small unmanned aerial systems can be used to force and improve simulations of local weather in the lee of a mountain as well as regionally during a sea-breeze event. In the first case, the atmospheric model failed to capture the observed flow without the additional forcing, due to an error in the sharpness and strength of an inversion aloft. Sensitivity experiments pertaining to persistent downslope flows on the large Icelandic ice caps, and their dependance on surface friction and temperature, are discussed in one paper. The mass balance of an ice cap at the south coast of Iceland is analyzed and compared with simulated precipitation which is found to reproduce the observed winter accumulation on the ice cap. Finally, observed and simulated climatologies of wet-snow accretion in Southeast-Iceland are used to improve previous parameterization methods for wet-snow accretion, highlighting the dependance of the accretion process on wind speed and the liquid water content of the snow. Additionally, the thesis includes wet-snow and in-cloud icing maps which were prepared within the scope of the thesis work. The maps identify regions prone to icing and highlight the complex spatial structure of the icing field, resulting from the orographic forcing on the weather of Iceland. For the sake of completeness, five conference papers on atmospheric icing are reproduced here, but conference papers are standard fare within the icing society.