An inter-comparison exercise of mesoscale flow models applied to an ideal case simulation

An exercise is described aiming at the comparison of the results of seven mesoscale models used for the simulation of an ideal circulation case. The exercise foresees the simulation of the flow over an ideal sea–land interface including ideal topography in order to verify model deviations on a controlled case. All models involved use the same initial and boundary conditions, circulation and temperature forcings as well as grid resolution in the horizontal and simulate the circulation over a 24-h period of time. The model differences at start are reduced to the minimum by the case specification and consist mainly of the parameterisation and numerical formulation of the fundamental equations of the atmospheric flow. The exercise reveals that despite the reduction of the differences in the case configuration, the differences in model results are still remarkable. An ad hoc investigation using one model of the original seven identifies the treatment of the boundary conditions, the parameterisation of the horizontal diffusion and of the surface heat flux as the main cause for the model deviations. The analysis of ideal cases represents a revealing and interesting exercise to be performed after the validation of models against analytical solution but prior to the application to real cases

Die Untersuchung von gross- und mesoskaligen Einfluessen auf die Entwicklung polarer Mesozyklonen mit Hilfe des Modells METRAS

Polare Mesozyklonen sind haeufig mit hohen Windgeschwindigkeiten und starken Niederschlaegen verbunden. Sie stellen nicht nur ein extremes Wetterereignis dar, sondern sind auch fuer das Klimasystem von Bedeutung, indem sie Waerme und Feuchte polwaerts transportieren und einen wichtigen Beitrag zum Niederschlag in den Polargebieten leisten. Die Bedeutung gross- und mesoskaliger Einfluesse auf die Entwicklung von polaren Mesozyklonen wird mit Hilfe des mesoskaligen Atmosphaerenmodells METRAS gekoppelt mit einem Eismodell bestimmt. Im Gegensatz zu bisherigen Untersuchungen ist es mit Hilfe des gekoppelten Modells moeglich, die Wechselwirkung zwischen Zyklone und Eisbedeckung zu untersuchen. Die Guete des Atmosphaere-Eismodells wird anhand eines waehrend des FRAMZY'99-Experiments beobachteten Zyklonendurchgangs beurteilt. Der Vergleich der Modellergebnisse mit Flugzeug- und Eisbojenmessungen zeigt eine gute Uebereinstimmung sowohl fuer die atmosphaerischen Groessen als auch fuer die Eisgroessen. Die Untersuchung der Einflussfaktoren wird fuer eine idealisierte mesoskalige Zyklone durchgefuehrt, die sich aufgrund barokliner Instabilitaet vertieft. Die Ergebnisse zeigen, dass die Zugbahn stark durch den Eisrand bestimmt wird und eine Auflockerung der Eisbedeckung durch den Zyklonendurchzug stattfindet. (orig.)Polar mesocyclones are often connected with high wind speeds and strong precipitation. They represent an extreme weather event but furthermore play an important part in the climate system by transporting heat and humidity polewards and they are important for precipitation in the polar regions. The synoptic and mesoscale influences on the development of polar mesocyclones are investigated by using the mesoscale atmosphere model METRAS coupled with a sea ice model. Compared to former investigations, the application of the coupled model allows studying the interaction between cyclones and ice coverage. The quality of the model results is judged by simulating a situation observed during the FRAMZY'99-experiment. The comparison of model results and plane and ice buoy measurements reveals a good agreement for the atmospheric quantities as well as the ice quantities. The investigation of the different scale influences is performed for an idealized mesoscale cyclone, which deepens due to baroclinic instability. The cyclone track is strongly affected by the ice edge and the ice coverage is broken up during the cyclone passage. (orig.)SIGLEAvailable from TIB Hannover: RR 1857(35) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Wind turbines in icing conditions: performance and prediction

Icing on structures is an important issue for wind energy developments in many regions of the world. Unfortunately, information about icing conditions is mostly rare due to a lack of measurements. Additionally, there is not much known about the operation of wind turbines in icing conditions. It is the aim of the current study to investigate the effect of icing on power production and to evaluate the potential of icing forecasts to help optimizing wind turbine operation. <br><br> A test site with two Enercon E-82 turbines was set up in the Jura region in Switzerland in order to study the turbines' behaviour in icing conditions. Icing forecasts were performed by using an accretion model driven by results of the mesoscale weather forecast model WRF. <br><br> The icing frequency at the test site is determined from pictures of a camera looking at the measurement sensors on the nacelle. The results show that the site is affected by frequent icing: 11.5 days/year of meteorological icing and 41.5 days/year of instrumental icing were observed corresponding to a factor of about four. The comparison of power production with and without blade heating shows that blade heating results in a 3.5% loss and operation without blade heating results in a 10% loss of the annual power production due to icing. Icing forecasts are performed for winter 2009/2010. Simulated and observed icing events agree well and also coincide with periods of power drop. Thus, the results suggest that icing forecasts can help to optimize the operation of wind parks in icing conditions

Sensitivity of the WRF model to PBL parametrisations and nesting techniques: evaluation of wind storms over complex terrain

Simulating surface wind over complex terrain is a challenge in regional climate modelling. Therefore, this study aims at identifying a set-up of the Weather Research and Forecasting Model (WRF) model that minimises system- atic errors of surface winds in hindcast simulations. Major factors of the model configuration are tested to find a suitable set-up: the horizontal resolution, the planetary boundary layer (PBL) parameterisation scheme and the way the WRF is nested to the driving data set. Hence, a number of sensitivity simulations at a spatial resolution of 2 km are carried out and compared to observations. Given the importance of wind storms, the analysis is based on case studies of 24 historical wind storms that caused great economic damage in Switzerland. Each of these events is downscaled using eight different model set-ups, but sharing the same driving data set. The results show that the lack of representation of the unresolved topography leads to a general overestimation of wind speed in WRF. However, this bias can be substantially reduced by using a PBL scheme that explicitly considers the effects of non-resolved topography, which also improves the spatial structure of wind speed over Switzerland. The wind direction, although generally well reproduced, is not very sensitive to the PBL scheme. Further sensitivity tests include four types of nesting methods: nesting only at the boundaries of the outermost domain, analysis nudging, spectral nudging, and the so-called re-forecast method, where the simulation is frequently restarted. These simulations show that restricting the freedom of the model to develop large-scale disturbances slightly increases the temporal agreement with the observations, at the same time that it further reduces the overestimation of wind speed, especially for maximum wind peaks. The model performance is also evaluated in the outermost domains, where the resolution is coarser. The results demonstrate the important role of horizontal resolution, where the step from 6 to 2 km significantly improves model performance. In summary, the combination of a grid size of 2 km, the non-local PBL scheme modified to explicitly account for non-resolved orography, as well as analysis or spectral nudging, is a superior combination when dynamical downscaling is aimed at reproducing real wind fields

Sensitivity of the WRF model to PBL parametrisations and nesting techniques: evaluation of wind storms over complex terrain

Simulating surface wind over complex terrain is a challenge in regional climate modelling. Therefore, this study aims at identifying a set-up of the Weather Research and Forecasting Model (WRF) model that minimises systematic errors of surface winds in hindcast simulations. Major factors of the model configuration are tested to find a suitable set-up: the horizontal resolution, the planetary boundary layer (PBL) parameterisation scheme and the way the WRF is nested to the driving data set. Hence, a number of sensitivity simulations at a spatial resolution of 2 km are carried out and compared to observations. Given the importance of wind storms, the analysis is based on case studies of 24 historical wind storms that caused great economic damage in Switzerland. Each of these events is downscaled using eight different model set-ups, but sharing the same driving data set. The results show that the lack of representation of the unresolved topography leads to a general overestimation of wind speed in WRF. However, this bias can be substantially reduced by using a PBL scheme that explicitly considers the effects of non-resolved topography, which also improves the spatial structure of wind speed over Switzerland. The wind direction, although generally well reproduced, is not very sensitive to the PBL scheme. Further sensitivity tests include four types of nesting methods: nesting only at the boundaries of the outermost domain, analysis nudging, spectral nudging, and the so-called re-forecast method, where the simulation is frequently restarted. These simulations show that restricting the freedom of the model to develop large-scale disturbances slightly increases the temporal agreement with the observations, at the same time that it further reduces the overestimation of wind speed, especially for maximum wind peaks. The model performance is also evaluated in the outermost domains, where the resolution is coarser. The results demonstrate the important role of horizontal resolution, where the step from 6 to 2 km significantly improves model performance. In summary, the combination of a grid size of 2 km, the non-local PBL scheme modified to explicitly account for non-resolved orography, as well as analysis or spectral nudging, is a superior combination when dynamical downscaling is aimed at reproducing real wind fields