Local flow conditions in the Bergen valley based on observations and numerical simulations
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- Geophysical Institute 
The complex topography and variability in surface roughness in the Bergen valley aff-ects the flow conditions in a multitude of ways. The steep mountains give shelter during strong synoptic flow, but the orography of Bergen can also lead to problems, such as accumulation of pollutants. A network of seven automatic weather stations has been installed to represent the horizontal and vertical variety of meteorological conditions in the Bergen valley. More than 17 months of data from 2010-2012 have been evaluated to give a general overview of the flow conditions. Case studies covering the local effect of strong synoptic wind and thermally induced circulation have also been investigated. Numerical simulations using the Weather Research and Forecasting Model (WRF) have been compared with the observations for the two case studies covering the thermally induced flow, namely the land-sea breeze circulation and an inversion. The quality of the WRF model has also been tested, which showed that the model's accuracy was good when simulating the sea breeze, but less accurate during inversions. The dense network and long time series have resulted in some new and interesting results. The results show that a typical vertical extension of the sea breeze exceeds the height of Løvstakken, and at the Ulriken side of the valley, anabatic flow is enhanced by the sea breeze. There are also strong local differences in the directional distribution of the flow, as the station downstream of Isdalen reveals a completely different wind distribution than at Florida, located only 1.7 km apart. The results also show that the wind speed at the lower elevated stations exceeds the wind speed at Ulriken for particular wind directions at Ulriken. This is linked to the direction of the large-scale flow and the local topography. Typical wind directions and wind speeds for inversions to develop are found, revealing that southerly and easterly directions are the most common ones. Observations of fronts passing a location confirm well established theory on the subject. The numerical simulations have led to new results for the sea breeze in the Bergen valley, for example the existence of a stronger sea breeze on a larger scale outside the coast of Bergen. Numerical simulations also show the reason for the strong inversion during southeasterly flow in November 2011, as warm air was transported toward the valley during high atmospheric stability. This study has resulted in a broader understanding of the flow conditions in the Bergen valley, but it has also resulted in more questions to be answered in the future.