Estimating Regional Precipitation Trends Comparison of two Methods

Two different methods were applied to estimate long-term precipitation trends representative for regions in Norway. A new method, comparative trend analysis (CTA), was applied on 142 homogeneous precipitation series of 70-100 years. In this way 12 precipitation trend regions were identified. Principal component analysis (PCA) was applied on a subset of 30 series during the period 1896-1994 The results from both analyses were used to estimate precipitation trend series at several locations. The estimates based upon the PCA were of same quality as the estimates based upon the CTA. However, by CTA it is possible to visualize the trend in a distinct region by using just one trend curve, while trend curves based on PCA are composed of contributions from 5 principal components. The resulting trend curves document that the annual precipitation level has increased by 8-14% throughout this century in most Norwegian regions. The increase occurred not simultaneously all over the country. The regional differences in precipitation trends and variability are probably connected to variations in the atmospheric circulation patterns.