Climatology of Low-Level Jets Over Scandinavia and the Nordic Seas Using Model Datasets and Radiosondes

Abstract

Vertical wind speed profiles occasionally exhibit a maximum within the first 1–2 km above the surface, called a low-level jet (LLJ). This phenomenon can occur anywhere in the world. Here, we detect such jets in rawinsonde data as well as in two reanalyses (ERA5 and CERRA) and one hindcast (NORA3) over a large Scandinavian domain during the period 2000–2015 in order to find the datasets best reproducing the observed LLJs, although keeping in mind that some observed jets are assimilated in the two reanalyses. We find that both the reanalyses and hindcast exhibit the same climatology pattern in frequency, height, speed and annual cycle of LLJs, though their values differ. However, all three reanalysis/hindcast datasets present some differences compared to the rawinsondes, especially in terms of the jet height, which tends to be underestimated by the models. When considering all seven rawinsonde stations, CERRA seems to best simulate the LLJs frequencies and their features, probably because the observations are assimilated in the reanalysis. The analysis of three cases shows that their direction is driven by the large-scale atmospheric circulation, while their intensity is enhanced by the local topography. In most cases investigated, the atmosphere is in a stable state with a temperature inversion when LLJs occur. These conditions are known to be common during the formation of LLJs. In one location (Novaya Zemlya), the LLJ is associated with downslope winds and a Foehn effect. Finally, we show that, for a region over the North Sea where offshore wind farms may be built, there is large spatial variability in the LLJs height and speed. This could be taken into consideration in wind energy production estimates instead of prescribing a fixed profile of wind.