The entire 7‐year archive of ECMWF operational analysis and forecast data is used to assess the skill of the Centre's model in short‐ and medium‐range forecasting of atmospheric blocking. The assessment covers 7100‐day periods, from 1 December to 10 March of all winters from 1980‐81 to 1986‐87, inclusive. A slightly modified version of the Legenäs and Økland objective zonal index is used to quantify both observed and forecast occurrence of blocking. The study is performed on 500 hPa geopotential height and on Euro‐Atlantic and Pacific blocking separately. It is found that blocking frequency is severely underestimated in medium‐range forecasts; the model is, on average, reasonably skilful if the initial conditions are blocked, but blocking onset is poorly represented if it occurs more than a few days into the forecast. This inability in entering the blocking regime has a substantial impact on the systematic error of the model.

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A case of deep and rapid cyclogenesis in the lee of the Alps is analysed by means of cross‐sections, isentropic maps and trajectories based on synoptic data, to investigate the three‐dimensional structure of the phenomenon and the nature of the processes that are responsible for it. The results of the analysis are also compared with available theoretical models. Considerations of the various scales involved in the development lead to a description of the phenomenon in terms of two distinct phases: a very rapid ‘trigger’ phase due to interaction between the frontal layer and the Alps, and a more usual ‘baroclinic development’ phase. Through a selection process typical of baroclinic instability, the lee cyclone acquires the observed horizontal and vertical scales and undergoes the normal life of a mid‐latitude depression. The insufficiency of the present synoptic network for a satisfactory analysis of such a meteorological phenomenon is also stressed.

A high-resolution ensemble system, based on five runs of a limited-area model (LAM), is described. The initial and boundary conditions for the LAM integrations are provided by the representative members (RMs) selected from the European Centre for Medium-Range Weather Forecasts Ensemble Prediction System (EPS). @S meinbeax are grouped in five clusters; then, from each cluster, an RM is selected, according to the methodology described in the companion paper. The ability of the high-resolution ensemble system to predict the occurrence of heavy rainfall events (either five or six days ahead) is tested for four cases of floods over the Alpine region. Results show that, in two case-studies, the LAM integration corresponding to the RM of the highly populated cluster predicts the observed rainfall with a very good degree of time and spatial accuracy. In the other two cases, the extreme events are captured by at least one of the runs nested on the members of the less populated clusters. Probability maps constructed from LAM integrations provide great detail on the location of the regions affected by heavy precipitation and the information gained with respect to EPS probability maps and LAM deterministic forecasts is highlighted. The probabilistic estimates based on the LAM ensembles are also shown to be of valuable assistance to forecasters in issuing early flood alerts, contributing to the definition of a flood-risk alarm system. t In November 2000, the horizontal resolution of the operational EPS was increased to T~2 5 5 , Corresponding to a grid scale of approximately 80 km. @ Royal Meteorological Society, 2001. 209s 2096 C. MARSIGLI et al.and boundary conditions provided by the representative members (hereafter, RMs) of the ECMWF EPS. The RMs are selected first by applying a cluster analysis to the 51member EPS to define five clusters and, then, by identifying the RM of each cluster. Clusters are defined by considering the atmospheric flow at 700 hPa and by using the wind vector as clustering variable. Once the five clusters have been constructed, for each cluster the RM is defined as the member closest to all members of its own cluster and most distant from the members of the other clusters, with distances computed using an Ll norm applied to the precipitation field. The reader is referred to the companion paper, Molteni et al. (2001), and to Marsigli (1998) for a detailed description of the selection methodology. LEPS is based on integrations of the limitedarea model LAMBO (Limited Area Model Bologna), operational at ARPA-SMR since 1993. LAMBO runs are performed at high horizontal resolution (about 20 km) in order to resolve those orographic and mesoscale processes responsible for heavy -precipitation events. A probability of occurrence is assigned to each scenario, based on the population of the corresponding EPS cluster. In this way, it is possible to combine the ability of the EPS to highlight a set of possible evolution scenarios (keeping account of the intrinsic predictability of a particular synoptic situation...

Can multi-annual variations in the frequency of North Atlantic atmospheric blocking and mid-latitude circulation regimes be skilfully predicted? Recent advances in seasonal forecasting have shown that mid-latitude climate variability does exhibit significant predictability. However, atmospheric predictability has generally been found to be quite limited on multi-annual timescales. New decadal prediction experiments from NCAR are found to exhibit remarkable skill in reproducing the observed multi-annual variations of wintertime blocking frequency over the North Atlantic and of the North Atlantic Oscillation (NAO) itself. This is partly due to the large ensemble size that allows the predictable component of the atmospheric variability to emerge from the background chaotic component. The predictable atmospheric anomalies represent a forced response to oceanic low-frequency variability that strongly resembles the Atlantic Multi-decadal Variability (AMV), correctly reproduced in the decadal hindcasts thanks to realistic ocean initialization and ocean dynamics. The occurrence of blocking in certain areas of the Euro-Atlantic domain determines the concurrent circulation regime and the phase of known teleconnections, such as the NAO, consequently affecting the stormtrack and the frequency and intensity of extreme weather events. Therefore, skilfully predicting the decadal fluctuations of blocking frequency and the NAO may be used in statistical predictions of near-term climate anomalies, and it provides a strong indication that impactful climate anomalies may also be predictable with improved dynamical models.

SUMMAraYA case of deep and rapid Cyclogenesis in the lee of the Alps is analysed by means of cross-sections, isentropic maps and tdectories based on synoptic data, to investigate the three-dimensional structure of the phenomenon and the nature of the processes that are responsible for it. The results of the analysis are also compared with available theoretical models. Considerations of the various scales involved in the development lead to a description of the phenomenon in terms of two distinct phases: a very rapid 'trigger' phase due to interaction between the frontal layer and the Alps, and a more usual 'barclinic development' phase. Through a selection process typical of baroclinic instability, the lee cyclone acquires the observed horizontal and vertical scales and undergoes the normal life of a mid-latitude depression. The insufficiency of the present synoptic network for a satisfactory analysis of such a meteorological phenomenon is also stressed.