Idealized Simulations of Katabatic Flows in Iceland: Katabatic Winds or Land Breeze?

Local thermal flows may be organized where there is a local horizontal pressure gradient due to differential surface heating or cooling. According to a simple conceptual model of katabatic flows (e.g. Egger 1990), they develop in sloping topography where the air at the surface cools relative to the air aloft, e.g. due to radiative cooling on clear nights. The cold and heavy air flows downslope in a relatively shallow layer due to its negative buoyancy while the flow is dampened due to the turbulent drag or other more complex dynamics (e.g. Mahrt 1982). Thermal flows may also be generated on level ground and a typical example is the land breeze i.e. the cold surface flow from cooler land surfaces over warmer water bodies. During the night the ground may cool faster than the sea and consequently the surface pressure increases on land and the surface air flows towards the sea where it rises. Aloft, the pressure increases over the water and a return flow is setup towards the weaker pressure above land. Similar flows may also occur across other surfaces boundaries provided that the temperature or its development is different across it, e.g. at the edge of sea ice. In Iceland the studies of thermal flows have mostly been limited to studies of katabatic winds on the Breiðamerkurjökull outlet glacier of Vatnajökull (e.g. Van der Avoird and Duynkerke 1999) which are mostyly based on the observations described in Oerlemans et al. (1999). Another study used a numerical model to explore the local and regional katabatic flows organized by the complex topography in Iceland (Ágústsson et al. 2007) during an event with weak synoptic forcing and strong radiative surface cooling. There are several other studies that indicate that katabatic flows are of importance in Iceland (e.g. Bromwich et al. 2005). However, the authors are not aware of other extensive studies of thermal flows in Iceland or their interaction with the complex topography Here we use a mesocale model to numerically simulate the atmosphere from a state of rest with a realistic wintertime cooling as during a day with clear skies.