WRINCLE was an EU funded project to estimate the changes in precipitation, snowmelt and evapotranspiration in Europe resulting from the increased greenhouse gases in the atmosphere and to assess the impacts of these changes on river discharges, water resource availability, water quality and hydro-electric power. The project ran from 1 January 1998 till 1 January 2001. It aimed to present the results in the form of a digital atlas of European precipitation, river discharge and water resource impacts.

General Circulation Models (GCMs) have been used to predict future climate conditions. Because of their coarse resolution and simplified physics, these models are unable to produce realistic precipitation scenarios needed for assessment studies of the hydrological impacts. In the WRINCLE project use was made of statistical linkages between precipitation and other meteorological variables to translate the large scale information from GCMs into local time series data. This statistical downscaling approach is still in a developing stage. WRINCLE therefore contained a special work package to improve the meteorological basis of statistical downscaling. KNMI was involved in that work package.

Daily rainfall data for the period 1968-1997 were analysed for eleven stations across Europe: De Bilt and Maastricht (the Netherlands), Hamburg, Hanover and Berlin (Germany), Vienna (Austria), Berne, Neuchâtel and Payerne (Switzerland), and Salto de Bolarque and Munera (Spain). Two separate statistical models were used to describe the daily precipitation at a particular site: an additive logistic model for rainfall occurrence (1 for wet days and 0 for dry days) and a generalised additive model for wet-day rainfall. Both models are extensions of the standard linear regression model for data from a normal distribution. Typical predictor variables were the u-velocity (westerly flow), the v-velocity (southerly flow), the relative vorticity, the 1000-500 hPa thickness, the baroclinicity and atmospheric moisture. With respect to the latter two options for rainfall amount modelling were compared: (i) the use of the specific humidity at 700 hPa, and (ii) the use of both the relative humidity at 700 hPa and precipitable water. For rainfall occurrence modelling the relative humidity at 700 hPa was considered as moisture variable. The 1000-500 hPa thickness was only included in the model for rainfall occurrence.

For all stations the moisture variable appears to be one of the most significant predictors both for rainfall occurrence and rainfall amount. The u-velocity is an important predictor for the northern stations De Bilt, Maastricht, Hamburg and Hanover. Although precipitation usually has a strong link to relative vorticity, this is not the case for Vienna and the Swiss stations. The baroclinicity is an important predictor for these middle European stations. For the Spanish sites it was beneficial to derive the velocity components and the relative vorticity from the 850 hPa heights instead of the 1000 hPa heights or sea level pressure as was done for the other sites. Although the downscaling relationships for rainfall occurrence and wet-day rainfall are assumed constant over the year, they can reasonably reproduce the seasonal cycles in the probability of rain and mean wet-day rainfall.

For De Bilt, Hanover, Berlin, Berne and Salto de Bolarque the changes in the seasonal mean rainfall amounts for the end of the 21^st century were calculated using the fitted statistical models with perturbed predictor variables. The perturbations were derived from the output of a time-dependent greenhouse gas forcing experiment with the coupled ECHAM4/OPYC3 atmosphere-ocean GCM for the periods 1968-1997 and 2070-2099. For all these stations a decrease in the mean number of wet days was found which is mainly due to the increased 1000-500 hPa thickness in the latter period. The mean wet-day precipitation amounts are sensitive to the larger atmospheric water content in the future climate (increase in specific humidity and precipitable water). Therefore an increase in the seasonal mean rainfall amounts is often found despite the decrease in the mean number of wet days. The use of the rainfall amount model with specific humidity results in a larger increase in the mean wet-day precipitation amounts than that with precipitable water and relative humidity. In most cases the changes in the circulation variables have little effect on the mean number of wet days and the mean rainfall amounts. Moreover, the changes in the circulation variables are generally small compared with their bias. Except for an anomalous change in mean winter rainfall at Hanover, the estimated changes from the statistical models are comparable with those from the ECHAM4/OPYC3 model.

The use of the downscaling models for scenario production was examined for Berne. Scenarios of daily precipitation for the period 2070-2099 were obtained by perturbation of the observed rainfall record and by stochastic time series simulation. In the case of time series perturbation the standard method of scaling the observed rainfall amounts was extended to allow for a decrease in the number of wet days. The 90^th percentile of the distributions of the N-day annual maximum precipitation amounts for N = 1, 3, 10 and 30 was considered to compare the resulting scenarios. The representation of the coefficient of variation of the wet-day precipitation amounts in the stochastic model for time series simulation strongly influences the reproduction of this extreme-value characteristic and its changes. Besides difficulties with the description of the coefficient of variation, the model for wet-day rainfall often overpredicts the mean rainfall amounts in situations where extreme rainfall could be expected. Interaction between predictor variables has to be incorporated to reduce this bias.

More information can be found on the WRINCLE Homepage

Papers resulting from WRINCLE

Buishand, T.A. and B.-R. Beckmann, 2000. Development of daily precipitation scenarios at KNMI, In: (Beersma, J., Agnew, M., Viner, D. and Hulme, M., eds) Climate scenarios for water-related and coastal impacts, ECLAT-2 Workshop Report No. 3 KNMI, the Netherlands, 10-12 May 2000, CRU, Norwich, UK, pp. 79-91.

Beckmann, B.R. and T.A. Buishand, 2001. KNMI contribution to the European project WRINCLE: downscaling relationships for precipitation for several European sites. Technical Report TR-230, Royal Netherlands Meteorological Institute, De Bilt, pp. 47.

Beckmann, B.R. and T.A. Buishand, 2002. Downscaling relationships for precipitation for the Netherlands and North Germany. International Journal of Climatology, 22, 15-32..

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