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Gap flows and the stable boundary layer were studied in northwest Greenland during the aircraft-based Investigation of Katabatic Winds and Polynyas during Summer (IKAPOS) experiment in June 2010. The measurements were performed using the research aircraft POLAR 5 of Alfred Wegener Institute (AWI; Bremerhaven). Besides navigational and basic meteorological instrumentation, the aircraft was equipped with radiation and surface temperature sensors and a turbulence measurement system. In the area of Smith Sound at the southern end of the Nares Strait, a stable, but fully turbulent, boundary layer with strong winds of up to 22 m s−1 was found during conditions of synoptically induced northerly winds through the Nares Strait. Strong surface inversions were present in the lowest 100–200 m. As a consequence of channeling effects, a well-pronounced low-level jet system was documented for each of four flights. The wind maximum is located at 20–50-km distance from the exit of Smith Sound. The 3D boundary layer structure past this gap is studied in detail. The channeling process is consistent with gap flow theory. The flow through the gap and over the surrounding mountains leads to the lowering of isotropic surfaces and the acceleration of the flow. The orographically channeled flow through Smith Sound plays a key role for the formation of the North Water polynya being the largest ice-producing polynya in the Arctic. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The concept of local scaling has proven to be a useful tool for the description of turbulence structures in the stable boundary layer (SBL) in mid-latitudes. In the present paper, local similarity properties of the continuously turbulent SBL over the Greenland ice sheet are investigated. The observations were obtained during the aircraft-based experiment KABEG ( Katabatic wind and Boundary-layer front Experiment around Greenland) under conditions of strong katabatic winds. For the first time, this dataset allows the study of the turbulence structure in the katabatic wind system over the whole boundary layer and over a horizontal scale of 80 km. Data from six flights during different synoptic conditions are summarized. In all situations, the katabatic flow was associated with a low-level jet, and maximum wind speeds exceed 20 m s-1 in several cases. The strong katabatic winds over the Greenland ice sheet represent conditions of the fully turbulent SBL under strong baroclinicity. Almost all data were collected above the surface layer and can be attributed to the regions of local scaling or z-less scaling. The local scaling concept was found to be valid also for these strong katabatic wind conditions. Profiles of scaled variances and fluxes agree with mid-latitude results, except for the total momentum flux. For the similarity functions and eddy diffusivities of the sensible heat flux, the use of the buoyancy length scale is found to be superior compared to the local Obukhov length. [ABSTRACT FROM AUTHOR]

A nonhydrostatic 3D meso-scale numerical model and a two-class sea ice model have been used for idealized numerical simulations with the goal to get an insight into the development of coastal polynyas driven by the katabatic wind and associated forcing and feedback processes. The simulations are performed for idealized orography structures and the realistic orography of East Greenland. Sensitivity studies with respect to synoptic forcing, sea ice coverage and polynya width are carried out using the uncoupled version of the model. The thermal forcing associated with reduced sea ice coverage and the synoptic forcing by the superimposed geostrophic wind are more important for the intensity of the katabatic low-level jet (LLJ) than the orography structure.For a moderate cold summertime situation, the air -- sea interaction is also moderate, but for a typical wintertime situation a strong feedback between the katabatic wind system and turbulent fluxes over the coastal sea ice is present. A coastal polynya acts also in the sense of intensifying the katabatic wind, but develops a secondary circulation opposed to the katabatic wind, which prohibits the katabatic wind to extend over the sea ice. Studies with the coupled atmospheric/sea ice model for the Angmassalik region of Greenland show that synoptically forced katabatic winds can result in a fast formation of a coastal polynya within 24 h. Sea ice advection is the main process during the first 12 h, but the production of frazil ice and the subsequent conversion to consolidated ice becomes important at later stages. Simulations for a scenario of an eastward moving cyclone being typical for piteraq events in the Angmassalik area reproduce the main features known from observational and realistic modeling studies, such as the development of the lee trough, its interaction with the katabatic wind and a development of a low-level mesoscale cyclone in the bay-like area southwest of Angmassalik. The simulations of the prese... [ABSTRACT FROM AUTHOR]

The aircraft-based experiment KABEG‘97 (Katabatic wind and boundary-layer front experiment around Greenland) was performed in April/May 1997. During the experiment, surface stations were installed at five positions on the ice sheet and in the tundra near Kangerlussuaq, West Greenland. A total of nine katabatic wind flights were performed during quite different synoptic situations and surface conditions, and low-level jets with wind speeds up to 25m s$^-1$ were measured under strong synoptic forcing of the katabatic wind system. The KABEG data represent a unique data set for the investigation of katabatic winds. For the first time, high-resolution and accurate aircraft measurements can be used to investigate the three-dimensional structure of the katabatic wind system for a variety of synoptic situations. Surface station data show that a pronounced daily cycle of the near-surface wind is present for almost all days due to the nighttime development of the katabatic wind. In a detailed case study the stably-stratified boundary layer over the ice and the complex boundary-layer structure in the transition zone ice/tundra are investigated. The katabatic wind system is found to extend about 10 km over the tundra area and is associated with strong wind convergence and gravity waves. The investigation of the boundary-layer dynamics using the concept of a two-layer katabatic wind model yields the results that the katabatic flow is always a ’shooting‘ flow and that the ’pure katabatic‘ force is the main driving mechanism for the flow regime, although a considerable influence of the large-scale synoptic forcing is found as well. [ABSTRACT FROM AUTHOR]

The parameterization of the boundary layer is a challenge for regional climate models of the Arctic. In particular, the stable boundary layer (SBL) over Greenland, being the main driver for substantial katabatic winds over the slopes, is simulated differently by different regional climate models or using different parameterizations of the same model. However, verification data sets with high-resolution profiles of the katabatic wind are rare. In the present paper, detailed aircraft measurements of profiles in the katabatic wind and automatic weather station data during the experiment KABEG (Katabatic wind and boundary-layer front experiment around Greenland) in April and May 1997 are used for the verification of the regional climate model COSMO-CLM (CCLM) nested in ERA-Interim reanalyses. CCLM is used in a forecast mode for the whole Arctic with 15 km resolution and is run in the standard configuration of SBL parameterization and with modified SBL parameterization. In the modified version, turbulent kinetic energy (TKE) production and the transfer coefficients for turbulent fluxes in the SBL are reduced, leading to higher stability of the SBL. This leads to a more realistic representation of the daily temperature cycle and of the SBL structure in terms of temperature and wind profiles for the lowest 200 m. [ABSTRACT FROM AUTHOR]

It is well-known that katabatic winds can be detected as warm signatures in the surface temperature over the slopes of the Antarctic ice sheets. For appropriate synoptic forcing and/or topographic channeling, katabatic surges occur, which result in warm signatures also over adjacent ice shelves. Moderate Resolution Imaging Spectroradiometer (MODIS) ice surface temperature (IST) data are used to detect warm signatures over the Antarctic for the winter periods 2002–2017. In addition, high-resolution (5 km) regional climate model data is used for the years of 2002 to 2016. We present a case study and a climatology of wind-induced IST anomalies for the Ross Ice Shelf and the eastern Weddell Sea. The IST anomaly distributions show maxima around 10–15K for the slopes, but values of more than 25K are also found. Katabatic surges represent a strong climatological signal with a mean warm anomaly of more than 5K on more than 120 days per winter for the Byrd Glacier and the Nimrod Glacier on the Ross Ice Shelf. The mean anomaly for the Brunt Ice Shelf is weaker, and exceeds 5K on about 70 days per winter. Model simulations of the IST are compared to the MODIS IST, and show a very good agreement. The model data show that the near-surface stability is a better measure for the response to the wind than the IST itself. [ABSTRACT FROM AUTHOR]