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  • Predictability and Dynamics of Weather Systems in the Atlantic-European Sector (PANDOWAE) x
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Lisa-Ann Quandt, Julia H. Keller, Olivia Martius, and Sarah C. Jones

Abstract

The Euro–Russian atmospheric blocking pattern in the summer of 2010 was related to high-impact weather, including a mega–heat wave in Russia. A set of scenarios for the synoptic evolution during the onset, mature stage, and decay of the block are extracted from the THORPEX Interactive Grand Global Ensemble multimodel ensemble forecast. These scenarios represent the key features of the forecast variability of the block and of the resulting surface impacts. Two heat indices and a fire index are computed to highlight the forecast variability in societal impacts. The study is a proof of concept, showing how information about surface impacts can be derived from available operational ensemble forecasts in an effective manner, and pointing to possible difficulties in this approach. Comparing the forecast for the heat wave’s impact on large spatial domains, and on a near-gridpoint scale, identifies challenges forecasters may face when predicting the development of a heat wave.

Although the block’s onset was highly predictable, the increase in temperature and the extension of the heat-affected area differed between the scenarios. During the mature stage of the block, the variability of its western flank had a considerable influence on the precipitation and heat distribution. Since the blocking was maintained after the analyzed decay in two of three scenarios, the predictability of the decay was low in this forecast. The heat wave ended independently from the block’s decay, as the surface temperature and the impact indices decreased in all scenarios.

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Paraskevi Giannakaki and Olivia Martius

Abstract

An accurate representation of synoptic-scale Rossby waves in numerical weather forecast models is very important as these waves are closely linked to weather formation at the surface. Enhanced potential vorticity (PV) gradients at the tropopause levels act as waveguides for synoptic-scale Rossby waves, so spatial errors in the waveguides imply errors in the amplification and propagation of Rossby waves. This paper focuses on evaluating the forecast representation of these waveguides and presents an object-based forecast verification tool. In both forecast and the verification data, Rossby waveguide objects are defined based on enhanced PV gradient fields on isentropic surfaces. The tool automatically pairs the complex objects, compares their properties, and assesses the number of objects without a matching partner in either the forecast or the reanalysis. In the last step, error measures are calculated for the area and the location of the objects. As proof-of-concept application of the method for the year 2008, five lead times of the Integrated Forecast System (IFS) from the ECMWF are compared with the ECMWF reanalysis dataset. The majority of the waveguide objects are found to be in the correct position, and there are no systematic positional errors; however, the forecast objects and hence the areas of enhanced PV gradients are smaller.

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Lars Wiegand, Arwen Twitchett, Cornelia Schwierz, and Peter Knippertz

Abstract

Around 26 May 2008 a pronounced potential vorticity (PV) streamer penetrated from the North Atlantic into the western Mediterranean Sea followed by widespread dust mobilization over the Maghreb region of northwest Africa and a subsequent northward transport into central Europe. At the same time, strong southerly flow over the Mediterranean Sea caused heavy precipitation and flooding at the windward side of the European Alps. Using continuous and feature-based error measures, as well as ensemble correlation techniques, this study investigates the forecast quality and predictability of synoptic and mesoscale aspects of this high-impact event in operational ensemble predictions from nine meteorological centers participating in The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) project. TIGGE is a recently established program providing ensemble forecasts in a standardized format, which allows for an exciting new multimodel approach to investigating the predictability of, for example, high-impact weather and its dynamics. The main conclusions from this study are that 1) the quality of the PV streamer forecasts degrades with lead time showing a general tendency toward too weak Rossby wave; 2) when focusing on the region around the streamer, most models show root-mean-square errors of the same magnitude or larger than the ensemble spread (underdispersive behavior); 3) errors are reduced by about 50% if the comparison is made to each center’s own analysis instead of the ECMWF analysis; 4) peak wind speeds over the Sahara tend to be underpredicted, with differences in model formulation dominating over differences in the representation of the PV streamer; and 5) ensemble-mean multimodel forecasts of 4-day accumulated precipitation appear accurate enough for a successful severe-weather warning.

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