A continuous-wave LiDAR adjusts the focus of the laser beam to a specific height, thereby increasing the probability that backscatter on particles comes from that particular height. The emitted laser beam can be either continuous or pulsed. By aiming the laser at an angle from the vertical (typically around 30 ∘) in at least three different directions and recording the frequency of the backscattered signal, it is possible to calculate the wind speed using the Doppler shift. The Doppler-wind LiDAR sends out a laser beam, which is reflected from moving particles in the air. The observational data used in this study came from wind-profiler LiDAR measurements, which have an advantage over those from traditional meteorological masts, in that they can reach higher, typically up to 300 m for wind energy purposes. We partly discuss these differences in the physical models where applicable however, the aim of the paper is not to describe each reanalysis in detail, but to highlight the essential challenges when comparing observations with the selected reanalyses. The reanalyses used within this study handle the physics differently, which in turn affects the resulting quantities (e.g., wind speed, low-level jets, and shear) to varying extents. We analyze four commonly used and freely available state-of-the-art reanalyses and compare with LiDAR (light detection and ranging) measurements up to a 300 m height from four locations spread over the Baltic Sea. A reanalysis is an optimized gridded description of the atmosphere at a given time, and by comparing with observations, it is possible to study the performance of the reanalysis under different meteorological conditions. Since measurements of wind speed at hub height are rare (especially offshore), gridded computer models are needed to describe the wind resource and assist in the decision about where to build a wind park. In this paper, the wind conditions over the Baltic Sea are examined at heights relevant for wind power (hub height, as well as the heights swept by the blades), and low-level jets are studied in detail. In terms of general wind characteristics, ERA5, UERRA, and NEWA are similar, and the best choice depends on the application. The observed frequency of low-level jets is best captured by UERRA. It is also shown that low-level jets, a very frequent mesoscale phenomenon in the Baltic Sea during late spring, can appear in a wide range of wind speeds. MERRA2 suffers from insufficient vertical resolution, which limits its usefulness in evaluating the wind profile. The average shear is too low in NEWA, while ERA5 and UERRA predominantly overestimate the shear. In general, the reanalyses underestimate the average wind speed. The findings provide insight into the accuracy of reanalyses for wind resource assessment. The comparison is made for four sites in the Baltic Sea with wind profiles up to 300 m. This paper presents a comparison between data from four state-of-the-art reanalyses (MERRA2, ERA5, UERRA, NEWA) and observations from LiDAR. The meteorology of this inland sea in particular is strongly affected by the surrounding land, creating mesoscale conditions that are important to take into consideration when planning for new wind farms. With an increasing interest in offshore wind energy, focus has been directed towards large semi-enclosed basins such as the Baltic Sea as potential sites to set up wind turbines.

Zx flux ocean waves