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Günther Heinemann

Abstract

Preliminary results of investigations of Antarctic mesocyclones in the Weddell Sea region are presented for the Antarctic summer periods 1983–88. Based on NOAA and METEOR satellite images, a total of 195 mesoscale vortices (scale less than 1000 km) were observed and classified using a simple classification scheme, depending on lifetime and cortex diameter. The highest frequency of vortices was found over the ice free part of the eastern Weddell Sea. Mean synoptic conditions for vortices with scales less than 400 km near Halley Station indicate an offshore flow being most favorable for their development. A case study with satellite images, surface measurements, synoptic analyses derived from TOVS data shows the evolution of several mesoscale vortices in a pool of cold air in the eastern Weddell Sea.

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Günther Heinemann

Abstract

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.

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Günther Heinemann and Thomas Klein

Abstract

Mesocyclones (MCs) are a frequently observed phenomenon in the coastal regions of Antarctica. Numerical simulations of topographically forced MCs in the Weddell Sea and the Ross Sea region are presented using a three-dimensional mesoscale weather forecast model. Simulations are performed for an idealized case without synoptic forcing, three realistic cases of smaller MCs with diameters of 200–300 km, and two larger systems with diameters of up to 1000 km. The simulation results show that the orography of the coastal regions can play an important role in mesocyclogenesis. One key factor is the katabatic wind system, which is able to initiate low-level MCs in areas of suitable orography structure. The second key factor is the support of the synoptic environment, leading to vorticity production by vertical stretching of the synoptically supported katabatic winds. Besides this stretching mechanism, katabatic winds can have a second impact on the generation of MCs by transporting cold air into the coastal areas and thereby enhancing the low-level baroclinicity. A large fraction of short-lived coastal MCs seem to be generated by these mechanisms. For larger-scale and long-lived MCs, the amplification of a near-surface perturbation is found to occur in association with the approach of an upper-level potential vorticity anomaly. The initial low-level perturbations, however, seem to be strongly connected to specific topographical features. Since satellite-based climatologies only include MCs associated with cloud formation, a considerable fraction of topographically forced MCs will remain undetected in areas without a mesoscale observational network.

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Michael Lieder and Günther Heinemann

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The development of three summertime mesoscale cyclones (MCs) over the northern Amundsen and Bellingshausen Seas from 10 to 12 January 1995 (during FROST SOP-3) is studied by means of AVHRR data, ERS and SSM/I retrievals, and mesoscale numerical model data. The most pronounced MC is investigated in detail. It had a diameter of about 800 km, a lifetime of more than 24 h, and reached the intensity of a polar low. The developments take place far away from the sea-ice front or topography. The MCs are detected as cyclonic cloud signatures in the AVHRR imagery, and SSM/I retrievals show a distinct mesoscale signal in the fields of cloud liquid water, wind speed, and integrated water vapor (IWV). The frontal structure of the most intense MC is depicted by high IWV gradients and a large near-surface wind shear. The collocation of ERS- and SSM/I-derived wind speeds shows good agreement (bias, 1.1 m s−1; std dev, 1.2 m s−1). ERS-derived wind vectors give no insight into the structure of the MCs, because of missing direct overpasses over the MCs by the narrow ERS scatterometer swaths, but they are used to validate numerical simulations. The numerical simulations using the mesoscale Norwegian Limited Area Model (NORLAM) show two of the MCs as short-wave baroclinic developments triggered by an upper-level trough, while a less significant third MC is not simulated by the model. In contrast to the satellite retrievals, the simulations give insight into the three-dimensional structure of the MCs. Model results are validated using satellite retrievals and some few available in situ observations. This validation study shows the good quality of the numerical simulations for the IWV and the near-surface wind speed from SSM/I as well as for the near-surface wind vector from ERS over the simulation time of 36 h. The differences between ERS and NORLAM wind vectors are 1.1 ± 2.5 m s−1 (mean bias ± std dev) and −3 ± 25° for wind speed and direction, respectively. The validation using SSM/I retrievals yields a mean bias of 0.3 m s−1 (std dev, 2.9 m s−1) for the wind speed, and of −2.5 kg m−2 (std dev, 2.9 kg m−2) for the IWV.

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Gunther Heinemann and Chantal Claud

A workshop on theoretical and observational studies of polar lows was held in St. Petersburg, Russia, 23–26 September 1996. An international group with scientists and students from Canada, Denmark, Germany, Norway, Russia, the United Kingdom, and the United States participated in the workshop. The papers presented covered the fields of climatological studies, studies using numerical models, and satellite studies for the Northern and Southern Hemispheres. Two polar low cases were investigated in more detail during working groups sessions: The polar low “Le Cygne” occurring 13–16 October 1993 over the Norwegian Sea, and a Southern Hemisphere polar low occurring 30 July–1 August 1986 over the Weddell Sea. Programs related to polar low research were presented in order to coordinate the international efforts in investigating polar mesocyclones. The workshop showed the progress achieved by using 1) the synergetic combination of the satellite data presently available, and 2) mesoscale numerical models for the understanding of the development and the physical processes of polar mesocyclones.

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Clemens Drüe and Günther Heinemann

Abstract

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.

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Günther Heinemann and Øyvind Saetra
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Günther Heinemann, Chantal Claud, and Thomas Spengler
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Rolf Zentek, Günther Heinemann, and Ekkehard Sachs

Abstract

Spectra of wind, kinetic energy, and temperature are investigated for a dataset of 10 years of regional climate simulations for mid-Europe. The nonhydrostatic Consortium for Small-Scale Modeling (COSMO) model in Climate Mode [COSMO-CLM (CCLM)] climate model is used in a hindcast mode for 1991–2000. The three-step nesting chain starts with a CCLM run with 18-km resolution covering all of Europe nested in ERA-40 reanalyses and then a run with a resolution of 4.4 km is performed for mid-Europe. Finally, the 1.3-km run focuses on the region of mid-Germany and Luxembourg. In the present study, only results for the 4.4- and 1.3-km runs are shown. Different methods based on the Fourier and cosine transformations for the computation of the spectra are evaluated.

The kinetic energy spectra show the expected slope in the mesoscale (up to the effective resolution), while the spectrum of the vertical wind shows a zero-slope behavior. The spectra of the horizontal wind components and temperature compare well to the observations. The effective model resolution was found to be about 7–10 (5–7) times the horizontal grid spacing for the one-dimensional (two-dimensional) spectral methods. A comparison between the different model resolutions shows a benefit of the 1.3-km simulations for the boundary layer for horizontal scales up to 25 km. The multiyear time-scale simulations allow for a climatological study of the seasonal cycle. The kinetic energy spectrum is found to have the largest values in summer.

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Thomas Spengler, Chantal Claud, and Günther Heinemann
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