Characterisation of Low-Level Jets and their Influence on Low-Level Clouds over the Baltic Sea; Östergarnsholm Observatory, Sweden

Sammanfattning: In this study, about two years of data from Östergarnsholm in the Baltic Sea are used to analyse offshore low-level clouds (LLCs) influenced by low-level jets (LLJs) in the marine atmospheric boundary layer. The LLJs are passages of strong horizontal winds near the surface and are measured using a Doppler LiDAR, which provides information about the wind speed and wind direction up to 300 meters. In this study we define two groups of LLJs depending on the height at which they occur; 1) LLJs with maximum wind speed at heights below 300 meters that are clearly identified from the LiDAR data, and 2) LLJs that occur at heights around 300 meters or higher and are not clearly captured by the LiDAR but are assumed by visual inspection. The clouds are quantified using a ceilometer which provides information about the cloud base heights (CBH). The main objectives of this study were to identify periods of coincident LLJs and LLCs, to classify these identified LLJs and LLCs and to provide general statistics of their interaction. The combination of LLJ- and LLC presence, and the interaction between these, are new research. The analysis showed that the highest LLJ frequency regarding all LLJs was 47.7 %, with a maximum speed of 17.5 ms^-1 and occurred during winter. Only 4.38 % of these LLJs occurred below 300 m. The largest share of all LLJs below 300 m occurred during summer (75.6 %), where the mean maximum speed was 11.1 ms^-1. Largest LLC presence during active LLJs occurred during the winter (56.0 % regarding all LLJs, 64.5 % regarding LLJs below 300 m). The possible LLJ influence on the formation of LLCs was large during late autumn, winter, and early spring since the LLJ mean height and the $CBH$ were at approximately the same height. It was also found that the LLJs induced an increased turbulence intensity below and above the LLJs cores. The increased mixing above the jet core could lead to turbulent transport of cold and humid air, leading to LLC formation. The results of this study may contribute to improved weather forecasts for the processes related to LLJ- and LLC interaction.