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This work was performed as part of the SWURVE project in collaboration with Marie Ekström and Phil Jones at CRU and published in:

Fowler, H.J., Ekström, M., Kilsby, C.G. and Jones, P.D. 2005. New estimates of future changes in extreme rainfall across the UK using regional climate model integrations. 1: Assessment of control climate. Journal of Hydrology, 300(1-4), 212-233. [Abstract]

Ekström, M., Fowler, H.J., Kilsby, C.G. and Jones, P.D. 2005. New estimates of future changes in extreme rainfall across the UK using regional climate model integrations. 2. Future estimates and use in impact studies. Journal of Hydrology, 300(1-4), 234-251. [Abstract]

The Intergovernmental Panel on Climate Change (Giorgi et al., 2001) suggest that in the future there may be more intense rainfall events over many areas in Europe. Changes to the magnitude, character and spatial distribution of extreme rainfall may have serious impacts upon many sectors such as agriculture, industry, transport, power generation, the built environment and ecosystems. Similarly, changes in many of these sectors will affect hydrology and water resources by altering the flow paths of both surface and groundwater.

Recent extreme rainfall events have pushed urban structures beyond their design limits (Pagliara et al., 1998) and caused failure of many systems, including fluvial flood defences (Lawrimore et al., 2001).  A possible increase in the occurrence of such events under climate change may exacerbate these impacts. It is important therefore, to understand not only the current spatial and temporal patterns of extreme rainfall (e.g. Osborn et al., 2000; Osborn and Hulme, 2002) but also how they are changing (e.g. Fowler and Kilsby, 2003a, b) and how the distributions may further change during the planning horizon for system design (~20–100 years). Moreover, uncertainties in these future estimates should be assessed.

Here, two methods have been used to examine results from HadRM3H for a future scenario ensemble of enhanced greenhouse conditions: regional frequency analysis (RFA) and individual grid box analysis (GBA). Both methods used L-moments (Hosking and Wallis, 1997) to produce rainfall growth curves with an extreme value distribution for 1-, 2-, 5- and 10-day events.  Previous work assessed the performance of HadRM3H in the simulation of UK extreme rainfall. It was found that HadRM3H provided a good estimate of event magnitude at a given return period for most parts of the UK.

This work provides an assessment of projected changes in extreme rainfall and an estimation of the related uncertainty: an approach that may be used for impact assessments related to future changes in extreme rainfall in the UK.

The projected change in the event magnitude of a given return period in the future integrations of the HadRM2 and HadRM3H using the regional analysis are detailed in Table 1. Figures 1 and 2 present the change in magnitude for future 1- and 10-day extreme rainfall events as an anomaly (in mm) from the control value, whereas Figures 3 and 4 present the future change in magnitude as a percentage change from the control value. These can be considered complementary, the first looking at the future as an anomaly from the present magnitudes, with the second estimating the change in magnitude as climate changes.

Table 1 Estimated changes in extreme rainfall event magnitude for the period 2070-2100 relative to 1960-1990 from HadRM3H using RFA in the 9 UK rainfall regions: North Scotland (NS), East Scotland (ES), South Scotland (SS), Northern Ireland (NI), Northwest England (NWE), Northeast England (NEE), Central and Eastern England (CEE), Southeast England (SEE) and Southwest England (SWE).

Figure 1 Comparison of absolute difference (mm) in  1-day rainfall event magnitudes between control and future simulations for (a) HadRM2, 10-year, (b) HadRM2, 50-year, (c) HadRM3H, 10-year and, (d) HadRM3H, 50-year. Note that as no data is available for Ireland it has been given a value of zero change.	Figure 2 Comparison of absolute difference (mm) in  10-day rainfall event magnitudes between control and future simulations for (a) HadRM2, 10-year, (b) HadRM2, 50-year, (c) HadRM3H, 10-year and, (d) HadRM3H, 50-year. Note that as no data is available for Ireland it has been given a value of zero change.

The HadRM3H future integration shows a very different pattern of change in extreme rainfall than the HadRM2 model, which is much more akin to trends noted in observations during the 1990s (see section on extreme rainfall). Although both show increases in extreme rainfall event magnitude for the same return period event, it is clear that the future changes projected by HadRM3H are of a much lower magnitude compared to those of HadRM2.  For the 10-year return period event, magnitudes increase by a small amount across most of the UK; a maximum of 5 and 15 mm for the 1- and 10-day event respectively (Figures 1 and 2). This compares to projected increases of 20 and 55 mm from the HadRM2 model. Figure 3 shows that for 1-day events there is little difference in terms of either the spatial pattern of change or the relative change between higher and lower return periods. This is again different to HadRM2, which shows greater relative change at higher return periods.

Figure 3 Percentage change in 1-day rainfall event magnitudes between control and future simulations for (a) HadRM2, 10-year, (b) HadRM2, 50-year, (c) HadRM3H, 10-year and, (d) HadRM3H, 50-year. Note that as no data is available for Ireland it has been given a value of zero change.	Figure 4 Percentage change in 10-day rainfall event magnitudes between control and future simulations for (a) HadRM2, 10-year, (b) HadRM2, 50-year, (c) HadRM3H, 10-year and, (d) HadRM3H, 50-year. Note that as no data is available for Ireland it has been given a value of zero change.

For longer duration events (e.g. 10-days) (Figure 4) there is a variable spatial pattern of change across the UK. At lower return periods there is a small percentage increase in magnitude in all regions (up to ~10 %) excepting central and east England. However, at higher return periods, the relative increase in northern and western regions is greater than that at lower return periods. The largest changes are found in Scotland, with an increase of 20 % in east Scotland for the 10-day, 50-year event. This increase concurs with observed trends in extreme longer duration rainfall (Fowler and Kilsby, 2003a,b), which found that the east Scotland region has shown the greatest increase in event magnitude for a given return period over the 1990s. In southern and eastern regions however, the relative change is much lower and is actually negative for higher return period events.

The future integrations of HadRM3H produce a very different pattern and magnitude of change in extreme rainfall than HadRM2. These results should be viewed with caution due to the significant differences in future changes generated by the two models. It is also possible that future changes in scenario development and improvements of model parameterisation may produce different estimates to those presented here. However as, from a simple physical viewpoint, global warming will allow the atmosphere to hold more moisture then it is extremely unlikely that they will alter the sign of the change.

These new estimates have implications for the design of flood defence and drainage infrastructure. The increase in event magnitude for a given return period for shorter duration events across the UK has severe implications for systems affected by short duration intense rainfall, such as combined sewer systems and storm drainage. At the other end of the scale, an increase in longer duration event magnitude of a given return period will have implications for fluvial flood defence schemes.

Further details of how to apply these estimates for design calculations and how to apply uncertainty estimates can be found in the paper.

References

Fowler, H.J. & Kilsby, C.G. 2003a. A regional frequency analysis of United Kingdom extreme rainfall from 1961 to 2000. International Journal of Climatology, 23, 1313-1334.

Fowler, H.J. & Kilsby, C.G. 2003b. Implications of changes in seasonal and annual extreme rainfall. Geophysical Research Letters 30, 1720, doi:10.1029/2003GL017327.

Giorgi, F., Hewitson, B., Christensen, J., Hulme, M., Von Storch, H., Whetton, P., Jones, R., Mearns, L. and Fu, C. 2001. Chapter 10. Regional Climate Information – Evaluation and Projections. In: Climate change 2001: The scientific basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P. van der Linden, X. Dai, K. Maskell, and C.I. Johnson (eds.), Cambridge University Press, p. 583-638.

Hosking, J.R.M. & Wallis, J.R. 1997. Regional frequency analysis: an approach based on L-moments, Cambridge University Press, Cambridge, 224 pp.

Lawrimore, J.H., Halpert, M.S., Bell, G.D., Menne, M.J., Lyon, B., Schnell, R.C., Gleason, K.L., Easterling, D.R., Thiaw, W., Wright, W.J., Heim, R.R., Robinson, D.A. and Alexander, L. 2001. Climate assessment for 2000. Bulletin of the American Meteorological Society 82, S1–S62.

Osborn, T.J., Hulme, M., Jones, P.D. and Basnett, T.A. 2000. Observed trends in the daily intensity of United Kingdom precipitation. International Journal of Climatology 20, 347–364.

Osborn, T.J. and Hulme, M. 2002. Evidence for trends in heavy rainfall events over the UK. Philosophical Transactions of the Royal Society, Series A 360, 1313-1325.

Pagliara, S., Viti, C., Gozzini, B., Meneguzzo, F. and Crisci, A. 1998. Climatic change – uncertainties and trends in extreme rainfall series in Tuscany, Italy: effects on urban drainage networks design. Water Science and Technology 37, 195-202.