Reconstruction and preliminary analysis of the temperature record of Trento (Italy) 1816-2007

A long record of monthly mean temperature data from the city of Trento (Central-Eastern Italian Alps), covering with continuity the time period 1816-2007, is presented. Many other cities started having regular observations at the beginning of the eighteenth century in the surroundig region, such as Mantova (1827), Modena (1830), Verona (1741: see related abstract ID 140774 in the 17th Conference on Applied Climatology), but most of them are located in plain regions and relatively far from mountainous areas. Therefore they are representative of weather effects related to the Mediterranean climate, and on their basis little can be inferred on processes typical of the Alpine areas. However the Alps provide a very peculiar climatological system and are a region separating two areas where contrasting climatological trends have been detected in the last two centuries, namely the tendency to increase precipitation in central-northern Europe and decreasing precipitation in the larger Mediterranean area. The position of the city of Trento, in the middle of a wide glacial valley where the River Adige flows from North to South, is strategic to verify the different climatological evolution between these two regions. Furthermore, as the urban area remained relatively small during the last couple of centuries, any urban heat island effect may reasonably be neglected, at least at a first step. The temperature record of Trento presented here has been composed on the basis of shorter series available from different sources. The latter include temperature measurements earlier performed by private researchers and later by official istitutions, such as, among others, the Austrian Central Institute of Meteorology and Geodynamics and the Italian Hydrographic and Mareographic Office. A thorough historical analysis allowed to collect data from many unpublished local sources. The database thus obtained includes subseries of data collected independently by different observers and in some cases displaying partly overlapping edges, in the city of Trento or nearby surroundigs. In summary, the reconstructed time series displays remarkable length and continuity and the overall record of Trento spans the period 1816-2007 with only a few gaps (1871-72, 1883-84, June 1915-June 1919), the major one being due to the first World War. By application of the Standard Normal Homogeneity Test (SNHT), existing inhomogeneities have been detected and related to changes occurring in either the observers or the procedures or the institutions recording the observations. Accordingly standard homogeneisation procedures have been applied. The resulting homogeneous series have been analyzed for regression, randomness and statistical significance, on both annual and seasonal basis using Durbin-Watson, Mann-Kendall and Spearman's tests. Before performing linear regression and trend estimation, the series has been averaged using a first-order recursive filter with the same spectral characteristics as a decadal moving average with gaussian weights and width equal to three times the variance (see figure). Regression analysis shows a significant positive trend in the annual mean temperatures (0,59 C°/100y over the timespan of the series). This is particularly remarkable in the last 20 years, in agreement with findings from other long series. On a seasonal basis, regression analysis shows that estimated trends for winter, summer and autumn are smaller than the spring trend.