ALBERT

Description: The 13th European Polar Low Workshop was organized by the European Polar Low Working Group (www.uni-trier.de/index.php?id=20308)and gathered scientists from nine countries focusing on polar mesocyclones in both hemispheres and other mesoscale weather phenomena such as katabatic winds, tip jets, boundary layer fronts, cold air outbreaks, and weather extremes in polar regions. Topics included experimental, climatological, theoretical, modeling, and remote sensing studies. The aim was to bring together scientists and forecasters to present their latest work and recent findings on these topics and to encourage discussions on improving forecasting and understanding of these phenomena.

Description: The regional climate model COSMOin Climate Limited-AreaMode (COSMO-CLM or CCLM) is used with a high resolution of 15km for the entire Arctic for all winters 2002/03–2014/15. The simulations show a high spatial and temporal variability of the recent 2-m air temperature increase in the Arctic. The maximum warming occurs north of Novaya Zemlya in the Kara Sea and Barents Sea between March 2003 and 2012 and is responsible for up to a 208C increase. Land-based observations confirm the increase but do not cover the maximum regions that are located over the ocean and sea ice.Also, the 30-km version of theArctic SystemReanalysis (ASR) is used to verify the CCLM for the overlapping time period 2002/03–2011/12. The differences between CCLM and ASR 2-m air temperatures vary slightly within 18C for the ocean and sea ice area. Thus,ASR captures the extreme warming as well. The monthly 2-m air temperatures of observations and ERA-Interim data show a large variability for the winters 1979–2016. Nevertheless, the air temperature rise since the beginning of the twenty-first century is up to 8 times higher than in the decades before. The sea ice decrease is identified as the likely reason for the warming. The vertical temperature profiles show that the warming has a maximum near the surface, but a 0.58Cyr21 increase is found up to 2 km. CCLM, ASR, and also the coarser resolved ERA-Interim data show that February and March are the months with the highest 2-m air temperature increases, averaged over the ocean and sea ice area north of 708N; for CCLM the warming amounts to an average of almost 58C for 2002/03–2011/12.

Description: A large number of quantities have to be measured and processed to determine the atmospheric-state variables, which are the actual measurands, from aircraft-based measurements. A great part of the dependencies between these quantities depends on the aerodynamic state of the aircraft. Aircraft-based meteorological measurements, hence, require in-flight calibration. Most operators of research aircraft perform some kind of calibration, but the schemes used and the degree they are documented greatly vary. The flight maneuvers and calculation methods required, however, are published in a number of partly overlapping and partly contradictory publications. Some methods are only presented as a minor issue in publications mainly focused on atmospheric processes and are therefore hard to find. For an aircraft user, it is hence challenging to either perform or verify a calibration because of missing comprehensive guidance. This lack was stated on occasion of the in-flight calibration of the German research aircraft Polar5 carried out for the field experiment Investigation of Katabatic Winds and Polynyas during Summer (IKAPOS). In the present paper, a comprehensive review of the existing literature on this field and a practical guide to the wind calibration of a research aircraft to be used for turbulent flux measurements are given.

Description: 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.

Description: European Polar Low Working Group This workshop summarized the current state of PL research in the Arctic and Antarctic. A couple of related projects are in the planning phase or already funded. The creation of a PL database for the Norwegian Sea in the frame of the Sea Surface Temperature and Altimeter Synergy (STARS) project (http://projects.met.no stars) will provide a valuable resource for future research and, potentially, predictability improvements. The maintenance of this database and the creation of similar databases for other polar areas including satellite and NWP data are highly recommended. There is also a need for free and timely access to satellite data, in particular to SAR data to fill the gap caused by the mission end of Envisat. With the increasing resolution of climate models, mesoscale processes such as polar MCs will have to be considered in international research programs such as the World Climate Research Programme (WCRP) Polar Climate Predictability Initiative and the World Weather Research Programme (WWRP) Polar Predictability Project.

Description: A polar-adjusted version of the regional climate model COSMO-CLM was used at a horizontal resolution of about 15km (0.125°; C15) to simulate and analyse extreme 10m wind speeds from 1979-2016 in winter (Nov to Apr) in the Arctic and around Greenland. Based on daily maximum 10m wind speeds, extreme indices (95% percentiles, average strong gale and hurricane days per winter) and return levels up to the 20year return period are calculated and compared with state-of-the-art reanalysis data sets (ERA-Interim, ASR version 1 and 2) and with the satellite product CCMP version 2. The return levels were calculated by the 'peaks-over-threshold' (POT) method which fits a General Pareto distribution (GPD) to extreme values exceeding a specified high threshold (for this study the local 95% and 99% percentiles). The model comparison was based on the overlapping winter period 2000-2012 (Nov to Apr). Before the analysis all data sets were interpolated onto the rotated model grid of C15 (450x350 grid boxes, with the rotated pole located at 100°W and 0°N).

Description: Sea ice leads play an essential role in ocean-ice-atmosphere exchange, in ocean circulation, geochemistry, and in ice dynamics. Their precise detection is crucial for altimetric estimations of sea ice thickness and volume. This study evaluates the performance of the SARAL/AltiKa (Satellite with ARgos and ALtiKa) altimeter to detect leads and to monitor their spatio-temporal dynamics. We show that a pulse peakiness parameter (PP) used to detect leads by Envisat RA-2 and ERS-1,-2 altimeters is not suitable because of saturation of AltiKa return echoes over the leads. The signal saturation results in loss of 6–10% of PP data over sea ice. We propose a different parameter—maximal power of waveform—and define the threshold to discriminate the leads. Our algorithm can be applied from December until May. It detects well the leads of small and medium size from 200 m to 3–4 km. So the combination of the high-resolution altimetric estimates with low-resolution thermal infra-red or radiometric lead fraction products could enhance the capability of remote sensing to monitor sea ice fracturing.

Description: During the winter season 2014/2015 a field campaign at the Tiksi observatory in the Laptev Sea area was carried out using SODAR/RASS measurements to investigate the atmospheric boundary layer (ABL) over a time period of six months with the focus on low-level jets (LLJ). The campaign took place at the Tiksi observatory (71.60N, 128.89E, 7 m asl) about 5 km south of Tiksi. A SODAR (Sound Detection And Ranging) and a RASS (Radio Acoustic Sounding System) were installed in addition to the existing instrumentation at the observatory.

Description: During the winter season 2014/2015 a field campaign at the Tiksi observatory in the Laptev Sea area was carried out using SODAR/RASS measurements to investigate the atmospheric boundary layer (ABL) over a time period of six months with the focus on low-level jets (LLJ). The campaign took place at the Tiksi observatory (71.60N, 128.89E, 7 m asl) about 5 km south of Tiksi. A SODAR (Sound Detection And Ranging) and a RASS (Radio Acoustic Sounding System) were installed in addition to the existing instrumentation at the observatory.

Description: During the winter season 2014/2015 a field campaign at the Tiksi observatory in the Laptev Sea area was carried out using SODAR/RASS measurements to investigate the atmospheric boundary layer (ABL) over a time period of six months with the focus on low-level jets (LLJ). The campaign took place at the Tiksi observatory (71.60N, 128.89E, 7 m asl) about 5 km south of Tiksi. A SODAR (Sound Detection And Ranging) and a RASS (Radio Acoustic Sounding System) were installed in addition to the existing instrumentation at the observatory.