The role of cyclone clustering during the stormy winter of 2013/2014

The winter season of 2013/2014 was the stormiest on record for the British Isles. In this article we show that there was an unprecedented amount of cyclone clustering during this season, corresponding to an average of one intense cyclone affecting the country every 2.5 days. An intensely clustered period from 6 to 13 February 2014 that was associated with one specific cyclone family is analysed in detail. This cyclone family is shown to be associated with a strong and straight upper level jet that is flanked by Rossby wave breaking on both its northern and southern sides for the duration of the clustering event. This mechanism is also identified for other periods in this season. The persistence of these conditions resulted in the clustered cyclone activity, and it was accompanied by record-breaking rainfall, widespread flooding and large socio-economic losses

Generalised additive point process models for natural hazard occurrence

ArticleThis is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.Point processes are a natural class of model for representing occurrences of various types of natural hazard event. Flexibly implementing such models is often hindered by intractable likelihood forms. Consequently, rates of point processes tend to be reduced to parametric forms, or the processes are discretised to give data of readily modelled `count-per-unit' type. This work proposes generalised additive model forms for point process rates. The resulting low-rank spatio-temporal representations of rates, coupled with the Laplace approximation, makes the restricted likelihood relatively tractable, and hence inference for such models possible. The models can also be interpreted from a regression perspective. The proposed models are used to estimate di erent types of Cox process and then spatio-temporal variation in European windstorms. Through a combination of thin plate and cubic regression splines, and their tensor product, established relationships between where windstorms occur and the state of the North Atlantic Oscillation are con rmed, and then expanded to bring detailed understanding of within-year variation, which has otherwise not been possible with count-based models.Willis Research Networ

A multimodel assessment of future projections of North Atlantic and European extratropical cyclones in the CMIP5 climate models

The response of North Atlantic and European extratropical cyclones to climate change is investigated in the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). In contrast to previous multimodel studies, a feature-tracking algorithm is here applied to separately quantify the re- sponses in the number, the wind intensity, and the precipitation intensity of extratropical cyclones. Moreover, a statistical framework is employed to formally assess the uncertainties in the multimodel projections. Under the midrange representative concentration pathway (RCP4.5) emission scenario, the December–February (DJF) response is characterized by a tripolar pattern over Europe, with an increase in the number of cyclones in central Europe and a decreased number in the Norwegian and Mediterranean Seas. The June–August (JJA) response is characterized by a reduction in the number of North Atlantic cyclones along the southern flank of the storm track. The total number of cyclones decreases in both DJF (24%) and JJA (22%). Classifying cyclones according to their intensity indicates a slight basinwide reduction in the number of cy- clones associated with strong winds, but an increase in those associated with strong precipitation. However, in DJF, a slight increase in the number and intensity of cyclones associated with strong wind speeds is found over the United Kingdom and central Europe. The results are confirmed under the high-emission RCP8.5 scenario, where the signals tend to be larger. The sources of uncertainty in these projections are discussed

Serial clustering of extratropical cyclones

The clustering in time (seriality) of extratropical cyclones is responsible for large cumulative insured losses in western Europe, though surprisingly little scientific attention has been given to this important property. This study has investigated and quantified the seriality of extratropical cyclones in the Northern Hemisphere using a point-process approach. A possible mechanism for serial clustering is the time-varying effect of the large-scale flow on individual cyclone tracks. Another mechanism is the generation by one 'parent' cyclone" of one or more 'offspring through secondary cyclogenesis. A long cyclone-track database was constructed for extended October to March winters from 1950 until 2003 using 6-hourly analyses of 850-mb relative vorticity derived from the NCEP/NCAR reanalysis. A dispersion statistic based on the variance-to-mean ratio of monthly cyclone counts was used as a measure of clustering. It reveals extensive regions of statistically significant clustering in the European exit region of the North Atlantic storm track and over the central North Pacific.EThOS - Electronic Theses Online ServiceGBUnited Kingdo