In this Chapter extra-tropical cyclones are explored in high resolution simulations with a Global Climate Model (GCM) in the Northern Hemisphere (NH) using the tracking method described in x3.2.3, with a particular focus on precipitation and precipitation extremes associated with the cyclones. The GCM data are taken from two relatively high resolution time slice simulations of the current and future climate with the ECHAM5 model, as described in Chapter 3. In Chapter 1 the relationship between extra-tropical cyclones and extreme precipitation events that can and have led to widespread disruption and loss of life, due to the associated flooding, in many parts of the world was discussed. In this Chapter this is explored further using an event identification approach that directly identifies cyclones in the GCM data and explores the precipitation associated with them, as described in x3.2.3. This is preferred to the approach of investigating the gridded timeseries of the precipitation field directly, although this is also explored, as the main aim of this project is to downscale particular events in this data, rather than downscale the whole timeseries, as discussed in x3.6. In this Chapter the impact of the GCM resolution on extra-tropical cyclone properties and extremes, in particular for precipitation, will be explored for both the current and future climate.
The GCM used in this study is the ECHAM5 GCM integrated at a spectral resolution of triangular truncation T319 (. 40km) and described in x3.2.1, this is a higher resolution simulation than that used in previous climate studies of extra-tropical cyclones (e.g. Geng and Sugi, 2003; Bengtsson et al., 2009). The representation of small scale fields, such as precipitation and the precipitation associated with extra-tropical cyclones, will likely be different in this simulation of the model than in lower-resolution simulations and other models. The impact of an increase in the horizontal resolution is explored in x4.4 for extra-tropical cyclones in the NH by contrasting cyclone properties in this simulation of the ECHAM5 model and the lower resolution T213 simulation used in the previous study of Bengtsson et al. (2009) using the same model. The study of Bengtsson et al. (2009) also compared this model at two different resolutions of T63 and T213 and found large differences in the cyclone properties between the two resolutions. The difference in resolutions between the T319 and T213 resolutions is not so large and therefore the difference in cyclone properties may not appear so large. Several fields associated with extra-tropical cyclones are considered although primarily focussing on the wind and precipitation fields. The hemispheric analysis of resolution on cyclones is extended to focussing separately on Western Europe and the UK regions in x4.6.1, which also discusses the need to downscale this information for a region such as the UK.
One of the aims of the thesis is to investigate the effect of a warmer climate on the precipitation associated with extra-tropical cyclones, and in particular those that may have an impact on the UK. In x4.5 the effect of a warmer climate on extra-tropical cyclones in the NH is investigated, again looking at several fields associated with extra-tropical cyclones although with a particular emphasis on the wind and precipitation fields and their extremes. This work is extended into looking specifically at Western Europe and the UK region in x4.6.2 which again explores the need to downscale this information to gain further insight for a region such as the UK. The Chapter proceeds with a brief validation of cyclones in this model by comparing with a re-analysis followed by a discussion of the impact of resolution on cyclone properties, in particular precipitation, followed by the impact of a warmer climate.
Champion, A.J., Hodges, K.I., Bengtsson, L.O., Keenlyside, N., Esch, M., 2011: Impact of increasing resolution and a warmer climate on extreme weather from northern hemisphere extra-tropical cyclones, Tellus 63A, 893-905, doi:10.1111/j.1600-0870.2011.00538.x. http://dx.doi.org/10.1111/j.1600-0870.2011.00538.x