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Lukas Strauss, Stefano Serafin, and Manfred Dorninger

Abstract

In this paper, a verification study of the skill and potential economic value of forecasts of ice accretion on wind turbines is presented. The phase of active ice formation on turbine blades has been associated with the strongest wind power production losses in cold climates; however, skillful icing forecasts could permit taking protective measures using anti-icing systems. Coarse- and high-resolution forecasts for the range up to day 3 from global (IFS and GFS) and limited-area (WRF) models are coupled to the Makkonen icing model. Surface and upper-air observations and icing measurements at turbine hub height at two wind farms in central Europe are used for model verification over two winters. Two case studies contrasting a correct and an incorrect forecast highlight the difficulty of correctly predicting individual icing events. A meaningful assessment of model skill is possible only after bias correction of icing-related parameters and selection of model-dependent optimal thresholds for ice growth rate. The skill of bias-corrected forecasts of freezing and humid conditions is virtually identical for all models. Hourly forecasts of active ice accretion generally show limited skill; however, results strongly suggest the superiority of high-resolution WRF forecasts relative to other model variants. Predictions of the occurrence of icing within a period of 6 h are found to have substantially better accuracy. Probabilistic forecasts of icing that are based on gridpoint neighborhood ensembles show slightly higher potential economic value than forecasts that are based on individual gridpoint values, in particular at low cost-loss ratios, that is, when anti-icing measures are comparatively inexpensive.

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Manfred Dorninger, C. David Whiteman, Benedikt Bica, Stefan Eisenbach, Bernhard Pospichal, and Reinhold Steinacker

Abstract

Meteorological events affecting the evolution of temperature inversions or cold-air pools in the 1-km-diameter, high-altitude (~1300 m MSL) Grünloch basin in the eastern Alps are investigated using data from lines of temperature dataloggers running up the basin sidewalls, nearby weather stations, and weather charts. Nighttime cold-air-pool events observed from October 2001 to June 2002 are categorized into undisturbed inversion evolution, late buildups, early breakups, mixing events, layered erosion at the inversion top, temperature disturbances occurring in the lower or upper elevations of the pool, and inversion buildup caused by the temporary clearing of clouds. In addition, persistent multiday cold-air pools are sometimes seen. Analyses show that strong winds and cloud cover are the governing meteorological parameters that cause the inversion behavior to deviate from its undisturbed state, but wind direction can also play an important role in the life cycle of the cold-air pools, because it governs the interaction with steep or gentle slopes of the underlying topography. Undisturbed cold-air pools are unusual in the Grünloch basin. A schematic diagram illustrates the different types of cold-air-pool events.

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Manfred Dorninger, Eric Gilleland, Barbara Casati, Marion P. Mittermaier, Elizabeth E. Ebert, Barbara G. Brown, and Laurence J. Wilson

Abstract

Recent advancements in numerical weather prediction (NWP) and the enhancement of model resolution have created the need for more robust and informative verification methods. In response to these needs, a plethora of spatial verification approaches have been developed in the past two decades. A spatial verification method intercomparison was established in 2007 with the aim of gaining a better understanding of the abilities of the new spatial verification methods to diagnose different types of forecast errors. The project focused on prescribed errors for quantitative precipitation forecasts over the central United States. The intercomparison led to a classification of spatial verification methods and a cataloging of their diagnostic capabilities, providing useful guidance to end users, model developers, and verification scientists. A decade later, NWP systems have continued to increase in resolution, including advances in high-resolution ensembles. This article describes the setup of a second phase of the verification intercomparison, called the Mesoscale Verification Intercomparison over Complex Terrain (MesoVICT). MesoVICT focuses on the application, capability, and enhancement of spatial verification methods to deterministic and ensemble forecasts of precipitation, wind, and temperature over complex terrain. Importantly, this phase also explores the issue of analysis uncertainty through the use of an ensemble of meteorological analyses.

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Reinhold Steinacker, Matthias Ratheiser, Benedikt Bica, Barbara Chimani, Manfred Dorninger, Wolfgang Gepp, Christoph Lotteraner, Stefan Schneider, and Simon Tschannett

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 sources, and the stability of the valley atmosphere are taken into account. The increase of temperature under ideal conditions in comparison to flat terrain is determined on a 1-km grid using height and surface geometry information. For the latter, a perturbation of an originally constant cross-Alpine temperature gradient is calculated by a topographical weighting. As a result, the gradient is steep where the mountain range is high and steep. If, during the interpolation process, some signal of the idealized patterns is found in the station data, it is used to downscale the analysis. It is shown by a cross validation of a case study that the interpolation of a mean sea level pressure field over the Alpine region is improved objectively by the method. Thermally induced mesoscale patterns are visible in the interpolated pressure field.

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C. David Whiteman, Bernhard Pospichal, Stefan Eisenbach, Philipp Weihs, Craig B. Clements, Reinhold Steinacker, Erich Mursch-Radlgruber, and Manfred Dorninger

Abstract

Comparisons are made between the postsunrise breakup of temperature inversions in two similar closed basins in very different climate settings, one in the eastern Alps and one in the Rocky Mountains. The small, high-altitude, limestone sinkholes have both experienced extreme temperature minima below −50°C and both develop strong nighttime inversions. On undisturbed clear nights, temperature inversions reach to 120-m heights in both sinkholes but are much stronger in the drier Rocky Mountain basin (24 vs 13 K). Inversion destruction takes place 2.6–3 h after sunrise in these basins and is accomplished primarily by subsidence warming associated with the removal of air from the base of the inversion by the upslope flows that develop over heated sidewalls. A conceptual model of this destruction is presented, emphasizing the asymmetry of the boundary layer development around the basin and the effects of solar shading by the surrounding ridgeline. Differences in inversion strengths and postsunrise heating rates between the two basins are caused by differences in the surface energy budget, with drier soil and a higher sensible heat flux in the Rocky Mountain sinkhole. Inversions in the small basins break up more quickly following sunrise than for previously studied valleys. The pattern of inversion breakup in the non-snow-covered basins is the same as that reported in snow-covered Colorado valleys. The similar breakup patterns in valleys and basins suggest that along-valley wind systems play no role in the breakups, since the small basins have no along-valley wind system.

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Benedikt Bica, Thomas Knabl, Reinhold Steinacker, Matthias Ratheiser, Manfred Dorninger, Christoph Lotteraner, Stefan Schneider, Barbara Chimani, Wolfgang Gepp, and Simon Tschannett

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 flows, blocking by the Alps, and local pressure extrema, as well as thermal lows and thermal high pressure zones. Comparisons are made between the mean course of reduced pressure at given grid points and the averaged divergence of the 10-m wind field in the Alpine region. It is shown that, climatologically, Alpine pumping and thermally induced pressure patterns have a similar frequency and intensity. For the latter ones, the buildup and cutback processes are described. Moreover, the frequency and intensity of pressure-related mesoscale features in the Alpine region over the last decades are investigated.

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Eric Gilleland, Gregor Skok, Barbara G. Brown, Barbara Casati, Manfred Dorninger, Marion P. Mittermaier, Nigel Roberts, and Laurence J. Wilson

Abstract

As part of the second phase of the spatial forecast verification intercomparison project (ICP), dubbed the Mesoscale Verification Intercomparison in Complex Terrain (MesoVICT) project, a new set of idealized test fields is prepared. This paper describes these new fields and their rationale and uses them to analyze a number of summary measures associated with distance and geometric-based approaches. The results provide guidance about how they inform about performance under various scenarios. The new case comparisons are grouped into four categories: (i) pathological situations such as when a variable is zero valued at all grid points; (ii) circular events aimed at evaluating how different methods handle contrived situations, such as equal but opposite translations, the presence of multiple events of same/different size, boundary effects, and the influence of the positioning of events in the domain; (iii) elliptical events representing simplified scenarios that mimic commonly encountered weather phenomena in complex terrain; and (iv) cases aimed at analyzing how the verification methods handle small-scale scattered events, very large events with holes (e.g., a small portion of clear sky on a cloudy overcast day), and the presence of noise in one or both fields. Results show that all analyzed measures perform poorly in the pathological setting. They are either not able to provide a result at all or they instigate a special rule to prescribe a value resulting in erratic results. The analysis also showed that methods provide similar information in many situations, but that each has its positive properties along with certain unique limitations.

Open access
Mathias W. Rotach, Paolo Ambrosetti, Felix Ament, Christof Appenzeller, Marco Arpagaus, Hans-Stefan Bauer, Andreas Behrendt, François Bouttier, Andrea Buzzi, Matteo Corazza, Silvio Davolio, Michael Denhard, Manfred Dorninger, Lionel Fontannaz, Jacqueline Frick, Felix Fundel, Urs Germann, Theresa Gorgas, Christoph Hegg, Alessandro Hering, Christian Keil, Mark A. Liniger, Chiara Marsigli, Ron McTaggart-Cowan, Andrea Montaini, Ken Mylne, Roberto Ranzi, Evelyne Richard, Andrea Rossa, Daniel Santos-Muñoz, Christoph Schär, Yann Seity, Michael Staudinger, Marco Stoll, Hans Volkert, Andre Walser, Yong Wang, Johannes Werhahn, Volker Wulfmeyer, and Massimiliano Zappa

Demonstration of probabilistic hydrological and atmospheric simulation of flood events in the Alpine region (D-PHASE) is made by the Forecast Demonstration Project in connection with the Mesoscale Alpine Programme (MAP). Its focus lies in the end-to-end flood forecasting in a mountainous region such as the Alps and surrounding lower ranges. Its scope ranges from radar observations and atmospheric and hydrological modeling to the decision making by the civil protection agents. More than 30 atmospheric high-resolution deterministic and probabilistic models coupled to some seven hydrological models in various combinations provided real-time online information. This information was available for many different catchments across the Alps over a demonstration period of 6 months in summer/fall 2007. The Web-based exchange platform additionally contained nowcasting information from various operational services and feedback channels for the forecasters and end users. D-PHASE applications include objective model verification and intercomparison, the assessment of (subjective) end user feedback, and evaluation of the overall gain from the coupling of the various components in the end-to-end forecasting system.

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Mathias W. Rotach, Paolo Ambrosetti, Christof Appenzeller, Marco Arpagaus, Lionel Fontannaz, Felix Fundel, Urs Germann, Alessandro Hering, Mark A. Liniger, Marco Stoll, Andre Walser, Felix Ament, Hans-Stefan Bauer, Andreas Behrendt, Volker Wulfmeyer, François Bouttier, Yann Seity, Andrea Buzzi, Silvio Davolio, Matteo Corazza, Michael Denhard, Manfred Dorninger, Theresa Gorgas, Jacqueline Frick, Christoph Hegg, Massimiliano Zappa, Christian Keil, Hans Volkert, Chiara Marsigli, Andrea Montaini, Ron McTaggart-Cowan, Ken Mylne, Roberto Ranzi, Evelyne Richard, Andrea Rossa, Daniel Santos-Muñoz, Christoph Schär, Michael Staudinger, Yong Wang, and Johannes Werhahn

Abstract

No Abstract available.

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