FRIO: FRont Interaction with Orography

FRIO aims to provide a better understanding of how mountains modify the development of atmospheric fronts. Atmospheric fronts shape the general perception of weather, because they are associated with abrupt changes in temperature. In addition, fronts go along with often strong and sometimes extreme precipitation and wind gusts. Both the precipitation and the wind gusts can lead to substantial damage. The question how mountains modify the development of atmospheric fronts is one of the most fundamental open questions in meteorology and of special importance for Norway. Norway's mountainous coast line is very exposed to low pressure systems and their fronts approaching from the Atlantic Ocean. To be able to make reliable forecasts, it is therefore important to know how the Norwegian mountain range modifies the approaching fronts. In particular, will the movement of the front be slowed down or accelerated by the mountains? Will the front weaken or intensify when it hits the coast line? One of the main challenges in addressing this question is the limited front type classification, which currently generally only distinguishes between warm, cold and occluded fronts depending on the temperature contrast across the front. For all three front types, is has been demonstrated that fronts can both be slowed down or accelerated, weakened or intensified. The first step in FRIO has therefore been to develop a more detailed front type classification, to be able to distinguish between many additional front types. FRIO has shown that in addition to the temperature contrast across the front, also (1) the wind contrast across the front, (2) the strength of the airstreams preceding the front and (3) the warming or cooling effect of the ocean should be considered to classify a front. This classification has been applied to analyse the New Year Day's Storm, a particularly intense cyclone that made landfall on the Norwegian coast line 1 January 1992. The air masses approaching the Norwegian mountain range with this storm were partly diverted around the mountains, and partly lifted over them. The flow diversion around the mountains lead to an initial strengthening of the wind contrast across both for the warm and cold fronts. Later, when the front moved over the mountains, turbulent mixing induced by the rough topography weakened both the temperature and wind contrast across the fronts. Further, the airstream ahead of the cold front was partly interrupted and partly lifted off the ground such that the lowest part of the cyclone core was cut-off from the cyclone's warm sector.

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Fronts are a key feature of the lower atmosphere, as they are one of the main factors influencing perceived weather. They are associated with abrupt changes in temperature and go along with often strong and at times extreme precipitation and wind gusts. Both the precipitation and the wind gusts can lead to substantial damage. The question how fronts interact with orography is a long-standing one in atmospheric dynamics and of special importance for Norway. Its mountainous coast line is very exposed to low pressure systems and their fronts approaching from the Atlantic Ocean. Therefore, an improved conceptual understanding of the modifications that a front undergoes when impinging on the Norwegian coast will aid forecasters in the weather service to interpret model predictions. The proposed project will improve our understanding of front-orography interactions by approaching the problem from a new perspective. In a first step, an objective front detection algorithm will be adapted to improve its reliability close to orography. The improved algorithm will in a second step be applied in a composite analysis to study the typical behaviour of fronts when they interact with (a) relatively smooth and monolithic orography like Greenland and (b) complex, rugged orography like the Norwegian coastal range. The composite analysis will be complemented by idealised model experiments with both two and three-dimensional models that will help clarity the pertinent mechanisms apparent in the composites.