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Arnold Tafferner
and
Joseph Egger

Abstract

Theories of lee cyclogenesis are tested for two cases of lee cyclogenesis observed during ALPEX. A numerical forecast model is used to simulate these events and perform tests of these linear theories. It is found that none of the theories is capable of explaining the observed 24 hour cyclone developments. Problems with the evaluation of the theories are discussed.

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Christian Keil
,
Arnold Tafferner
,
Hermann Mannstein
, and
Ulrich Schättler

Abstract

The forecasting performance of the operational Lokal Modell (LM) of the German Weather Service (DWD) is evaluated for two severe winter storms that crossed central Europe in December 1999. Synthetic satellite images constructed from model output fields are compared with observed imagery from a Meteosat satellite. Also, synthetic radar images constructed from forecast precipitation fields are taken for validation against observed precipitation as represented by the European radar composite of the DWD. Comparisons are performed by visual inspection of satellite and radar imagery and by calculating statistical measures such as frequency histograms of observed and synthetic brightness temperature. Whereas the visual inspection allows detection of even finescale details in both synthetic satellite and radar imagery, for example, the position of fronts and rainbands, the statistical analysis reveals model deficiencies with respect to the representation of upper-level cloudiness. The operational LM did not incorporate a cloud-ice parameterization scheme at the time that these storms occurred. Additional experiments were performed to investigate the impact of the cloud-ice parameterization on brightness temperature. As expected, the longwave radiative impact is seen to be strongly influenced by the presence of cloud ice. Furthermore, changing the value of the ice-to-snow autoconversion threshold in the microphysics scheme by a factor of 2 leads to a significant improvement in synthetic brightness temperature as compared with observations. The results suggest that synthetic satellite and radar images could be used to perform quality control on numerical weather forecasts in real time. Basic ideas are proposed for an automated quality-control system using an image-matching tool developed at the German Aerospace Center (DLR).

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Michael Frech
,
Frank Holzäpfel
,
Arnold Tafferner
, and
Thomas Gerz

Abstract

A 1-yr meteorological dataset for the terminal area of Frankfurt Airport in Germany has been generated with a numerical weather prediction system to provide a synthetic though realistic database for the evaluation of new operational aircraft arrival procedures and their associated risks. The comparison of the 1-yr dataset with a local surface wind climatology indicates that the main climatological features are recovered. A subset of 40 days is validated against measurements from a sound detection and range/radio acoustic sounding system (SODAR/RASS) taken at Frankfurt Airport. The RMS errors of wind speed and direction are between 1.5 m s−1 at the surface and 2 m s−1 at 300 m and 40°, respectively. The frequency distribution of meteorological parameters, such as the wind component perpendicular to the glide path, shear, and thermal stratification, show good agreement with observations. The magnitude of the turbulent energy dissipation rate near the surface is systematically overestimated, whereas above 100 m the authors find on average a slight underestimation. The analysis of the database with respect to crosswind conditions along the glide path indicates only a time fraction of 12% for which the crosswind is above a threshold of 2 m s−1. A similar result is obtained using a grid point near the airport that mimics a wind profiler, which suggests that in a majority of cases a wind profiler appears sufficient to cover the expected crosswind conditions along the glide path. A simple parameterization to account for the crosswind variability along the glide path is proposed.

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