A Model-Based Climatology of Low-Level Jets in the Weddell Sea Region of the Antarctic

Abstract

Low-level jets (LLJs) are climatological features in polar regions. It is well known that katabatic winds over the slopes of the Antarctic ice sheet are associated with strong LLJs. Barrier winds occurring, e.g., along the Antarctic Peninsula may also show LLJ structures. A few observational studies show that LLJs occur over sea ice regions. We present a model-based climatology of the wind field, of low-level inversions and of LLJs in the Weddell Sea region of the Antarctic for the period 2002–2016. The sensitivity of the LLJ detection on the selection of the wind speed maximum is investigated. The common criterion of an anomaly of at least 2 m/s is extended to a relative criterion of wind speed decrease above and below the LLJ. The frequencies of LLJs are sensitive to the choice of the relative criterion, i.e., if the value for the relative decrease exceeds 15%. The LLJs are evaluated with respect to the frequency distributions of height, speed, directional shear and stability for different regions. LLJs are most frequent in the katabatic wind regime over the ice sheet and in barrier wind regions. During winter, katabatic LLJs occur with frequencies of more than 70% in many areas. Katabatic LLJs show a narrow range of heights (mostly below 200 m) and speeds (typically 10–20 m/s), while LLJs over the sea ice cover a broad range of speeds and heights. LLJs are associated with surface inversions or low-level lifted inversions. LLJs in the katabatic wind and barrier wind regions can last several days during winter. The duration of LLJs is sensitive to the LLJ definition criteria. We propose to use only the absolute criterion for model studies.

Global Climatology of Low‐Level‐Jets: Occurrence, Characteristics, and Meteorological Drivers

“…The average frequency of occurrence of PLLJ over Antarctica and Greenland is very high, about 59%; with some locations having values above 90%. This is broadly consistent with previous studies (e.g., Heinemann & Zentek, 2021; Tuononen et al., 2015). The development of PLLJs is similar to inertial oscillations that one expected due to near‐surface inversion formed by little irradiance in polar nights causing low surface temperatures throughout the day for several months (Figure 6c).…”

“…LLJs are known to occur in different regions around the world that favor the development of different types. Common locations for LLJ occurrence are coastal upwelling regions (Lima et al., 2022), hot deserts (Fiedler et al., 2013; Heinold et al., 2013) and continental polar regions (Heinemann & Zentek, 2021; Tuononen et al., 2015). LLJs in hot desert areas are usually NLLJs, although a fraction of the LLJs are connected to convective cold pools (Heinold et al., 2013).…”

“…LLJs in hot deserts have strong implications for forming dust storms (Fiedler et al., 2013; Z. Han et al., 2022). Likewise, LLJs happening in continental polar regions (PLLJs), or cold deserts, are generated by the stabilization of the atmospheric boundary layer via near‐surface cooling or warm air advection (Heinemann & Zentek, 2021). Other mechanisms for LLJ formation over cold deserts were also identified.…”

A Three-Year Climatology of the Wind Field Structure at Cape Baranova (Severnaya Zemlya, Siberia) from SODAR Observations and High-Resolution Regional Climate Model Simulations during YOPP

“…The comparison of the radiosonde data with the simulations for the lowest 600 m for the complete month of July 2018 showed that only a few LLJs were detected in the radiosonde data (Supplementary Material Figure S8), while the simulations showed frequent LLJs, which were associated with channeled flow in most cases (Supplementary Material Figure S9). The time resolution, as well as the problems of radiosonde wind measurements in the lowest 100 m, made it difficult to detect LLJs in operational radiosoundings [10,29,30]. It should be noted that measurements at Cape Baranova (synoptic data and radiosondes) were not used for the ERA5 reanalyses, which were used as initial and boundary conditions for the CCLM simulations.…”

“…The case studies already showed good examples of LLJs associated with channeling. The analysis of LLJs follows the method used in [30]. The lowest wind maximum was searched for below 1000 m, then the next minimum above the maximum was searched for below 1500 m, but limited by a search radius of 1.5 times the height of the maximum.…”