Downscaling of Regional Climate Model Precipitation for Urban ...

11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
CONCLUSIONS AND FUTURE WORK
From this preliminary evaluation, it appears possible to combine RCM precipitation output,
separated into components representing convective and large-scale precipitation, with a
simple stochastic scheme reflecting spatial coverage, to create reasonably accurate realisations
of the associated point rainfall process. The downscaled point precipitation series compare
well with observations both in terms of general descriptive statistics and extreme values. The
results support the possibility to statistically relate RCM results to short-term point
observations. One possible application discussed above is to use dynamically downscaled
GCM output to assess future changes in point precipitation, which however will require a
very careful treatment of the calibration for present climate. However, also other applications
may be envisioned, e.g. simulating point precipitation properties in ungauged areas.
We believe the present methodology may constitute a step forward in the search for tools to
estimate future point precipitation. Most efforts to date have involved different versions of socalled
delta change, where an observed rainfall time series is modified in line with changes
found in RCM output. The proposed, process-based methodology is likely more flexible than
delta change, which is heavily constrained by the properties of the available observations. It
should however be emphasised that it is associated with uncertainty, e..g. because of the
simplified descriptions of rainfall generating emchanisms in climate models.
Future work will include the following activities: (1) optimisation of the coverage parameters
(cc and cl) for the different cities, sensitivity testing, split sample calibration, (2) estimation of
coverage not as fixed areas but as given by additional RCA3 output parameters, e.g.
cloudiness, (3) evaluation and optimisation for not only ERA-40-driven RCA3 results but for
GCM-driven RCA3 results representing present climate, (4) application of optimised
downscaling scheme to GCM-driven RCA3 results representing future climate.
ACKNOWLEDGEMENT
The study was funded by The Swedish Research Council for Environment, Agricultural
Sciences and Spatial Planning (FORMAS) and The Foundation for Strategic Environmental
Research (MISTRA).
REFERENCES
Hernebring C. (2006). 10-års regnets återkomst förr och nu (Design storms in Sweden then and now). VA-forsk
publ. 2006-04, Svenskt Vatten, Stockholm (in Swedish).
IPCC (2007). Climate Change 2007: The Physical Science Basis, Summary for Policymakers. Contribution of
Working Group I to the Fourth Assessment Report of the IPCC
Kjellström E., Bärring L., Gollvik S., Hansson U., Jones C., Samuelsson P., Rummukainen M., Ullerstig A.,
Willén U. and Wyser K. (2005). A 140-year simulation of European climate with the new version of the
Rossby Centre regional atmospheric climate model (RCA3). Reports Meteorology and Climatology
108, SMHI, Sweden, 54 pp.
Räisänen J., Hansson U., Ullerstig A., Döscher R., Graham L.P., Jones C., Meier H.E.M., Samuelsson P. and
Willén U. (2004). European climate in the late twenty-first century: regional simulations with two
driving global models and two forcing scenarios. Clim. Dyn., 22, 13-31.
Uppala S.M., Kållberg P.W., Simmons A.J., Andrae U., da Costa Bechtold V., Fiorino M., Gibson J.K., Haseler
J., Hernandez A., Kelly G.A., Li X., Onogi K., Saarinen S., Sokka N., Allan R.P., Andersson E., Arpe
K., Balmaseda M.A., Beljaars A.C.M., van de Berg L., Bidlot J., Bormann N., Caires S., Chevallier F.,
Dethof A., Dragosavac M., Fisher M., Fuentes M., Hagemann S., Hólm E., Hoskins B.J., Isaksen L.,
Janssen P.A.E.M., Jenne R., McNally A.P., Mahfouf J.-F., Morcrette J.-J., Rayner N.A., Saunders
R.W., Simon P., Sterl A., Trenberth K.E., Untch A., Vasiljevic D., Viterbo P. and Woollen J. (2005).
The ERA-40 re-analysis. Quart. J. R. Meteorol. Soc., 131, 2961-3012.
6 Downscaling of climate model precipitation