A statistical–dynamical regionalization approach is developed to assess possible changes in wind storm impacts. The method is applied to North Rhine‐Westphalia (Western Germany) using the FOOT3DK mesoscale model for dynamical downscaling and ECHAM5/OM1 global circulation model climate projections. The method first classifies typical weather developments within the reanalysis period using K‐means cluster algorithm. Most historical wind storms are associated with four weather developments (primary storm‐clusters). Mesoscale simulations are performed for representative elements for all clusters to derive regional wind climatology. Additionally, 28 historical storms affecting Western Germany are simulated. Empirical functions are estimated to relate wind gust fields and insured losses. Transient ECHAM5/OM1 simulations show an enhanced frequency of primary storm‐clusters and storms for 2060–2100 compared to 1960–2000. Accordingly, wind gusts increase over Western Germany, reaching locally +5% for 98th wind gust percentiles (A2‐scenario). Consequently, storm losses are expected to increase substantially (+8% for A1B‐scenario, +19% for A2‐scenario). Regional patterns show larger changes over north‐eastern parts of North Rhine‐Westphalia than for western parts. For storms with return periods above 20 yr, loss expectations for Germany may increase by a factor of 2. These results document the method's functionality to assess future changes in loss potentials in regional terms.

The quantification of subgrid land surface heterogeneity effects on the scale of climate and numerical weather prediction models is of vital interest for the energy budget of the atmospheric boundary layer and for the atmospheric branch of the hydrological cycle. This paper focuses on heterogeneity effects for the exchange processes between land surfaces and the atmosphere. The results are based on high-resolution non-hydrostatic model simulations for the LITFASS area near Berlin. This area represents a highly heterogeneous landscape of 20 × 20 km 2 around the Meteorological Observatory Lindenberg of the German Weather Service (DWD). Model simulations were carried out using the non-hydrostatic model FOOT3DK of the University of Köln with resolutions of 1 km and 250 m.The performance of different area-averaging methods for the turbulent surface fluxes was tested for the LITFASS area, namely the aggregation, mosaic and tile methods. For one tile method (station-tile), the experimental setup of the surface energy balance stations of the LITFASS98 experiment was investigated. Two different simulation types are considered: (1) realistic topography and idealized synoptic forcing; (2) realistic topography and realistic synoptic forcing for LITFASS98 cases. A double one-way nesting procedure is used for nesting FOOT3DK in 'Lokalmodell' of the DWD.The mosaic method shows good results, if the wind speed is sufficiently high. During weak-wind convective conditions, errors are particularly large for the latent heat flux on the 20 × 20 km 2 scale. The aggregation method yields generally higher errors than the mosaic method, which even increase for higher wind speeds. The main reason is the strong surface heterogeneity associated with the lakes and forests in the LITFASS area. The main uncertainty of the station-tile method is the knowledge of the area coverage in combination with the representativity of the stations for the land-use type and surface conditions. The results of this study lead to the recommendation to use a mosaic approach or at least a tile approach for downscaling fluxes over heterogeneous surfaces in mesoscale and regional climate models.

A field experiment combined with numerical studies provides a better understanding of the area-averaged evaporation and leads to improved parameterization schemes.

A physically based gust parameterisation is added to the atmospheric mesoscale model FOOT3DK to estimate wind gusts associated with storms over West Germany. The gust parameterisation follows the Wind Gust Estimate (WGE) method and its functionality is verif ed in this study. The method assumes that gusts occurring at the surface are induced by turbulent eddies in the planetary boundary layer, def ecting air parcels from higher levels down to the surface under suitable conditions. Model simulations are performed with horizontal resolutions of 20 km and 5 km. Ten historical storm events of different characteristics and intensities are chosen in order to include a wide range of typical storms affecting Central Europe. All simulated storms occurred between 1990 and 1998. The accuracy of the method is assessed objectively by validating the simulated wind gusts against data from 16 synoptic stations by means of "quality parameters". Concerning these parameters, the temporal and spatial evolution of the simulated gusts is well reproduced. Simulated values for low altitude stations agree particularly well with the measured gusts. For orographically exposed locations, the gust speeds are partly underestimated. The absolute maximum gusts lie in most cases within the bounding interval given by the WGE method. Focussing on individual storms, the performance of the method is better for intense and large storms than for weaker ones. Particularly for weaker storms, the gusts are typically overestimated. The results for the sample of ten storms document that the method is generally applicable with the mesoscale model FOOT3DK for mid-latitude winter storms, even in areas with complex orography.

No abstract