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Sie sind hier: Startseite Bibliothek Bonner Meteorologische Abhandlungen (BMA) Katabatic winds over Greenland and Antarctica and their interaction with mesoscale and synoptic-scale weather systems: investigations using three-dimensional numerical models

Katabatic winds over Greenland and Antarctica and their interaction with mesoscale and synoptic-scale weather systems: investigations using three-dimensional numerical models


Katabatic winds over Greenland and Antarctica and their interaction with mesoscale and synoptic-scale weather systems:
investigations using three-dimensional numerical models


In this Ph.D. thesis, the katabatic wind systems over the Greenland and Antarctic ice sheets are investigated.
Besides their four-dimensional structure, the interaction of katabatic winds with synoptic-scale and mesoscale weather systems is examined by means of numerical modeling and intercomparison of model results and observational data.
Simulations of the katabatic wind system over the Greenland ice sheet for the two months April and May 1997 were
performed using the Norwegian limited area model (NORLAM).

In Chapter 2 the model results are intercompared and validated against observational data from automatic weather stations (AWS), global atmospheric analyses and instrumented aircraft observations of individual cases during that period.
The NORLAM is able to simulate the synoptic developments and daily cycle of the katabatic wind system realistically.
For most of the cases covered by aircraft observations, the model results agree very well with the measured developments and structures of the katabatic wind system in the lowest 400~m. Despite NORLAM's general ability of reproducing the
four-dimensional structure of the katabatic wind, problems occur in cases, where the synoptic background is not well captured by the analyses used as initial and boundary conditions for the model runs or where NORLAM fails to correctly predict the synoptic development. The katabatic wind intensity in the stable boundary layer is underestimated by the model in cases, where the simulated synoptic forcing is too weak. An additional problem becomes obvious in cases, where the model simulates clouds in contrast to the observations or where the simulated clouds are too thick compared to the observed cloud cover. The excessive cloud prediction prevents the development of the katabatic wind in the model. Sensitivity studies were performed for these cases by artificially suppressing the cloud development in the model, leading to an improvement of the simulated katabatic winds.

In Chapter 3 the structure of the Greenland katabatic wind system is investigated for one winter (positive NAO index) and two spring months and compared to an idealized case without synoptic forcing and a real case with strong synoptic forcing.
The impact of transient synoptic cyclones is mainly confined to the southeast of Greenland, i.e. the vicinity of the Icelandic low. In the other parts of Greenland, the katabatic wind system turned out to be the dominant boundary layer phenomenon even on monthly timescales. In the two spring months, a mean diurnal cycle of the katabatic wind system is present with the strongest katabatic flow during the early morning.
Investigations of the dynamics of the Greenland boundary layer are presented using the concept of a two-layer atmosphere after Ball (1960). Simulation results of the idealized simulation, the case study with real synoptic forcing and the winter month are analysed with a special focus on the horizontal structure of the different contributions of the integrated boundary layer accelerations. For the case of the winter month, the missing of the katabatic wind signal in Southeast Greenland in the monthly mean is shown to be a result of an only weakly stable boundary layer due to clouds associated with synoptic low pressure systems.
NORLAM simulations of mesocyclones (MCs) in the Weddell Sea and the Ross Sea region of Antarctica are discussed in Chapter 4. MCs with diameters of 200-300~km represent a frequent phenomenon in the Eastern Weddell Sea Region. The simulation results of such a MC case show the effect of vertical stretching of the synoptically supported katabatic winds in that area to be important for the MC development. Larger systems with diameters of up to 1000~km are generally slightly
less frequent but more intense. In a case study of a larger MC, an amplification of a near surface perturbation occurs in association with the approach of an upper level potential vorticity anomaly. Upper level support is also found to be important for the case of a MC development over the Filchner/Ronne Ice Shelf. Yet, low-level vortices and baroclinicity associated with katabatic flows were present at the initial stages of the latter two cases, suggesting katabatic winds to be important for the initiation of the developments.
For the Ross Sea area, two case studies of MC developments in the vicinity of the western coast are presented.
The combination of synoptic forcing and the impact of the katabatic wind system over the ice slopes is a key factor for these
cases. In all the cases examined, the synoptic forcing is of importance during the development cycle of the MCs, which underlines the need of a high quality of the operational analyses used for the MC forecasts. In addition, the forecast models have to accurately represent physical processes in the strongly stable boundary layer over the Antarctic ice as well as cloud processes. Satellite images occasionally show the existence of MCs close to the eastern coast of Greenland, especially in the region of Angmagssalik/Tasiilaq.

In Chapter 5 the forcing mechanisms of such MCs are investigated by means of numerical simulations with the NORLAM. The special characteristics of the East Greenland topography are shown to be a key factor for the generation of such MCs. The channeling of the synoptically supported flow in large valleys along the east coast can induce cold air outbreaks over the adjacent maritime region and can also lead to convergences associated with vertical stretching and the generation
of cyclonic vorticity. The convergence can be especially strong during intense katabatic storms, so-called Piteraqs, which are a much-feared phenomenon in thearea of Angmagssalik. The results of numerical simulations suggest a close
relationship between the occurrence of Piteraqs and the generation of mesoscale vortices close to the East Greenland coast.

In order to investigate the detail structure of the katabatic wind in the coastal areas of Greenland, the non-hydrostatic Lokal-Modell (LM) of the Deutscher Wetterdienst (DWD) is nested into NORLAM forecasts.
In Chapter 6 results of LM simulations are presented for an idealized case using an atmosphere at rest as initial conditions and two real cases of the katabatic flow in the coastal areas of Greenland near Kangerlussuaq (Southwest Greenland) and Angmagssalik (SoutheastGreenland), respectively. As well the idealized case as the real case for the Kangerlussuaq
region suggest a complex pattern of fjord winds to be typical for this area with the wind intensity varying according to the
specific synoptic environment. For the real case, the simulation results are in good agreement with instrumented aircraft and AWS observations. In the case study for the Angmagssalik region, the interaction of synoptically supported katabatic winds at gale force and a MC development is simulated. The detail structure of the atmospheric flow in that area obtained from the simulation results can explain the existence of coastal polynias in the sea ice coverage as visible on satellite imagery.