1. EGS-AGU-EUG Joint Assembly Nice, France, 10th April 2003
New Developments in Flood Risk Estimation
Session 11: Revised FSR Rainfall-Runoff Method
EGS-AGU-EUG Joint Assembly Nice, France, 10th April 2003
Changes in UK extreme rainfall projected by regional climate models and implications for flood risk
Hayley J. Fowler and Chris G. Kilsby
WRSRL, University of Newcastle, UK

2. Introduction
SWURVE project (Sustainable Water : Uncertainty, Risk and Vulnerability in Europe) funded under the EU Environment and Sustainable Development programme
Past decade, perceived increase in rainfall intensities – widespread flooding and landslides
Future predicted increases in frequency and intensity in northern latitudes – GCMs/RCMs
Increases have huge economic and social implications in terms of increased flooding
Aim of study: examine representation of UK extreme rainfall by RCMs by comparison with observations and produce estimates of future change in extreme rainfall

3. Data available for UK extreme rainfall study
Observed daily rainfall data
Daily rainfall data from RCMs and

4. Growth curve production
Standard regional frequency analysis
Based on :
Extraction of annual maximum series
standardisation by RMED (median annual maximum event) at each station
Pooling of data by region
Single-site L-moment ratios then combined - weighted regional averaging by record length
GEV distribution fitted using L-moments
Growth curves produced for 1-, 2-, 5-, and 10-day events

5. Control run comparison
New Developments in Flood Risk Estimation
Session 11: Revised FSR Rainfall-Runoff Method
Control run comparison
Observed 5km climatology
(UK Met Office/UKCIP)
Control HadRM3 overestimates rainfall in winter and spring months, particularly at high elevations, but underestimates rainfall in summer and autumn
underestimation of annual mean rainfall - eastern coastal regions, and in the Cheshire plain, where simulated annual rainfall is as low as 250 mm (observed values are between 600 and 700 mm)
Future simulations suggest increasing seasonal contrasts
Summer reductions greatest in SE, winter increases largest in NW Scotland
little change in mean annual rainfall other than a slight drying of the southeast England and East Anglia.

6. Mean rainfall
Proportion dry days

7. Mean rainfall
Control HadRM3 overestimates rainfall in winter and spring months, particularly at high elevations, but underestimates rainfall in summer and autumn
underestimation of annual mean rainfall - eastern coastal regions, and in the Cheshire plain, where simulated annual rainfall is as low as 250 mm (observed values are between 600 and 700 mm)
Future simulations suggest increasing seasonal contrasts
Summer reductions greatest in SE, winter increases largest in NW Scotland
little change in mean annual rainfall other than a slight drying of the southeast England and East Anglia.

8. Comparison of control and observed growth curves
New Developments in Flood Risk Estimation
Session 11: Revised FSR Rainfall-Runoff Method
Comparison of control and observed growth curves
South-east England
. It can be seen that in the regional climate models the growth curves are too flat, particularly at the 1- and 2-day level. The underestimation of extreme 1-day rainfall in southeast England by HadRM2 was also noted by Jones and Reid (2001). This may be caused by poor representation of convective rainfall processes within the regional climate models since 1- and 2-day AM in these regions tend to be a result of convective summer and autumn storms. Further north, 1- and 2-day AM are generally a result of frontal rainfall in autumn or winter months (see Figure 7 for example) causing less curvature at the upper end of the growth curve. Therefore, the regional climate models are able to simulate extreme 1-day annual maxima in these regions better than in more southerly regions of the UK. This problem also translates through to the future simulations.

9. Representation of extremes
1-day, 10 year RP
10-day, 50 year RP
Observed
HadRM2
Observed
HadRM2
HadRM3
HadRM3

10. (c) and (d) show same for HadRM3 (UKCIP02) scenarios
New Developments in Flood Risk Estimation
Session 11: Revised FSR Rainfall-Runoff Method
Future projections
(a) and (b) show HadRM2 projected % change in magnitude of 10-yr and 50-yr 1 day event during
(c) and (d) show same for HadRM3 (UKCIP02) scenarios
In HadRM2, for the 1-day event, southern England, and western regions show a large increase in threshold values for both the 10% and 2% chance event, although increases are much larger for the 2% chance event
HadRM3 shows a very different pattern of future change in extreme rainfall, that is much more akin to trends noted in observations during the 1990s The most noticeable difference between the prospective changes in threshold values for HadRM3 and those of HadRM2 is that the future changes projected by HadRM3 are of a much lower magnitude. For the 10% chance event, threshold values increase by a maximum of 5 and 15 mm for the 1- and 10-day event respectively (Figures 12 and 13) compared to increases of 20 and 55 mm for HadRM2. These are increased at the 2% chance event. The pattern of change in HadRM3 suggests a small increase in threshold values across the UK for the 10% chance event. However, for shorter recurrence intervals such as the 2% chance event there are increasing threshold values in northern and western regions, with decreasing values in southern and eastern regions.

11. Future projections – 1 day
HadRM2 (UKCIP98)
S England, and west show large increase in threshold values for both the 10-yr and 50-yr RP event
HadRM3 (UKCIP02)
Very different pattern of future change
More akin to trends noted in observations during the 1990s
Future changes are of a much lower magnitude.
Pattern of change suggests small increase in threshold values across the UK for the 10-yr event. But for 50-yr event increasing in N and W regions, with decreases in S and E regions.

12. Future projections
(a) and (b) show HadRM2 projected % change in magnitude of 10-yr and 50-yr 10 day event during
(c) and (d) show same for HadRM3 (UKCIP02) scenarios

13. Summary of HadRM3 projections
‘New estimates’ of future changes from HadRM3 suggest increases of up to 20% in the threshold values of 1-day events across the UK.
Largest increases over Scotland, NI, SEE and lowest over NEE
For longer duration events, small increase across UK of up to 10% in threshold values for the 10-yr event. However, for more infrequent events, this increases to 20% in Scotland, reduces by 10% in England.
This differs substantially from HadRM2 which shows large increases across the UK. Care in interpretation due to uncertainties in GCM, poor reproduction of means/extremes

14. Implications for flooding
Future increasing thresholds of short duration rainfall events across the UK – combined sewer overflows / storm drainage (10% in England, 20% Scotland)
Future increase in longer duration event thresholds - fluvial flood defence schemes (20% Scotland, some increase in England)
Increases in frequency and severity of flooding are therefore very likely