Barbara Chimani

A Mesoscale Data Analysis and Downscaling Method over Complex Terrain

Abstract A mesoscale data analysis method for meteorological station reports is presented. Irregularly distributed measured values are combined with measurement-independent a priori information about the modification of analysis fields due to topographic forcing. As a physical constraint to a thin-plate spline interpolation, the so-called “fingerprint method” recognizes patterns of topographic impact in the data and allows for the transfer of information to data-sparse areas. The results of the method are small-scale interpolation fields on a regular grid including topographically induced patterns that are not resolved by the station network. Presently, the fingerprint method is designed for the analysis of scalar meteorological variables like reduced pressure or air temperature. The principles for the fingerprint technique are based on idealized influence fields. They are calculated for thermal and dynamic surface forcing. For the former, the effects of reduced air volumes in valleys, the elevated heat s...

Numerical Simulations of the Foehn in the Rhine Valley on 24 October 1999 (MAP IOP 10)

Abstract This paper presents numerical simulations of the MAP IOP 10 foehn case (24 October 1999) for the lower Alpine Rhine Valley, which was one of the two target areas selected for foehn observations. The simulations have been performed with a modified version of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model MM5. Most importantly, the horizontal diffusion of temperature and the mixing ratios of water vapor and cloud water is computed truly horizontally rather than along the model surfaces, as is the case in the original MM5. In the Rhine Valley, the foehn case under consideration was characterized by a complex and rapidly changing flow field. In the early morning, the foehn was restricted to the upper parts of the valley while a cold air pool was present in the lower parts and around the exit toward Lake Constance. During the following hours, the cold air pool was gradually eroded, and the foehn reached Lake Constance around noon. Only 3 h later, a shallow cold front passing f...

Thermally and Dynamically Induced Pressure Features over Complex Terrain from High-Resolution Analyses

Abstract Within the Vienna Enhanced Resolution Analysis (VERA) Climatology (VERACLIM) project, the complex influence of topographic structures on the spatial distribution of meteorological parameters has been investigated and evaluated climatologically. VERACLIM is aimed to generate a set of high-resolution analyses (lower meso-β-scale) of various meteorological parameters on a climatological basis. It tried to combine both the high spatial resolution provided by the VERA scheme that was used and the high temporal resolution of a comprehensive synoptic dataset of the last two decades, which was retrieved from ECMWF’s Meteorological Archival and Retrieval System (MARS). In the present study, the interpolated fields of reduced pressure of 3-hourly synoptic data over the Alpine region are evaluated climatologically. Using high temporal and spatial resolution, the authors were able to investigate both thermally and dynamically induced mesoscale pressure phenomena such as “Stau,” associated with trans-Alpine f...

Van Bebber's cyclone tracks at 700 hPa in the Eastern Alps for 1961-2002 and their comparison to Circulation Type Classifications

In this study, a systematic tracking of atmospheric cyclones at the pressure level of 700 hPa has been performed to determine three different track types and their climatologic characteristics over Central Europe from 1961 to 2002 by using ERA-40 (ECMWF – European Centre for Medium Range Weather Forecast, 40 year Re-Analysis) and ERA-Interim data. The specif c focus is on cyclone tracks of type V as suggested by Van Bebber in 1891 and congeneric types, because of their association with extreme large-scale precipitation amounts and related f ooding in Central and Eastern Europe. The tracking procedure consists of detecting isolated minima of the band-pass f ltered 700 hPa geopotential height f eld and combining them over time to continuous tracks by nearest neighbour approach. Results show that the common Vb cyclone track is a rare event (3.5/year) and the probability of occurrence is largest in April with a secondary maximum in autumn. Furthermore, there is no temporal trend of any of these track types noticeable throughout the 42years investigation period. The new 700 hPa cyclone track catalogue is used as a reference to verify the hypothesis of a coherence of Vb-tracks with certain circulation types (CTs). Selected objective and subjective circulation type classif cations (CTCs) from COST (European Cooperation in Science and Technology) action 733 as well as a manual Vb-classif cation from the ZAMG (Zentralanstalt für Meteorologie und Geodynamik) are utilized. This study shows both the shortcomings and the potential of CTCs to discriminate Vb cyclone tracks by certain classes of stationary circulation patterns. Subjective CTCs rank higher than objective ones in terms of Brier Skill Score, which demonstrates the power to enhance relevant synoptic phenomena instead of favouring mathematical criteria. For the f rst time, an objective catalogue with the famous cyclone track Vb and its climatology is now on hand for the period 1961–2002, offering a valuable basis to further improve our knowledge on the issues of Vb and related hydro-meteorological aspects.