Abstract
Extreme values of wind speed were studied based on the highly detailed ERA5 dataset covering the central part of the Kara Sea. Cases in which the ice coverage of the cells exceeded 15% were filtered. Our study shows that the wind speed extrema obtained from station observations, as well as from modelling results in the framework of mesoscale models, can be divided into two groups according to their probability distribution laws. One group is specifically designated as black swans, with the other referred to as dragons (or dragon-kings). In this study we determined that the data of ERA5 accurately described the swans, but did not fully reproduce extrema related to the dragons; these extrema were identified only in half of ERA5 grid points. Weibull probability distribution function (PDF) parameters were identified in only a quarter of the pixels. The parameters were connected almost deterministically. This converted the Weibull function into a one-parameter dependence. It was not clear whether this uniqueness was a consequence of the features of the calculation algorithm used in ERA5, or whether it was a consequence of a relatively small area being considered, which had the same wind regime. Extremes of wind speed arise as mesoscale features and are associated with hydrodynamic features of the wind flow. If the flow was non-geostrophic and if its trajectory had a substantial curvature, then the extreme velocities were distributed according to a rule similar to the Weibull law.
Highlights
A large part of the Kara Sea (Figure 1) is covered with ice year-round
Extreme values of wind speed were studied based on the highly detailed ERA5 dataset covering the central part of the Kara Sea
Our study shows that the wind speed extrema obtained from station observations, as well as from modelling results in the framework of mesoscale models, can be divided into two groups according to their probability distribution laws
Summary
This study focuses on the Kara Sea, a small part of the Pan-Arctic domain, to more clearly delineate its regional characteristics For this purpose, a horizontally detailed re-analysis of ERA5 was used. Our analysis demonstrated that each set of wind speed extremes (observed in both coastal and open-ocean locations) is a mixture of two different subsets, with each neatly described by the Weibull distribution. Apart from the statistical approach, an explanation of the observed wind speed probability distribution should be based on theoretical ideas from hydrodynamic peculiarities of the atmospheric motion This justification can be obtained by studying the products of numerical simulations or by studying equations that are sufficiently simplified to obtain their analytical solutions.
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