title: A daily reconstruction of historical weather to study past climate variability and impacts
Creators: name: Imfeld, Noemi
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 500 Science
subject: 550 Earth sciences &amp; geology
subject: 940 History of Europe

description: Studying daily weather of the past can provide relevant insights into the decadal variability of weather events and climate impacts that are not resolved in current climate reconstructions. While monthly to seasonal reconstructions have been evaluated for the past several hundred years, the daily time scale has received little attention, mainly because the necessary data sets are scarce.
In this thesis, a daily reconstruction of high-resolution (1x1 km²) temperature and precipitation fields from 1763 to 1960 is presented, that forms together with present-day fields a 258-year-long gridded data set for Switzerland (Chapter 2). Further, reconstructions for sea level pressure and temperature for several short periods in the late 18ᵗʰ and early 19ᵗʰ century for Europe are presented (Sect. 1.3 and Appendix A). These data sets allow for new analyses of daily weather and daily-based climate indices that were hitherto not possible. The meteorological fields were reconstructed using the Analogue Resampling Method (ARM) to generate a first guess of the meteorological fields, and subsequently improved by assimilating temperature and pressure observations and bias-correcting precipitation fields. For Switzerland, cross-validation results of the temperature reconstruction show good skill even for the very early periods before 1864, when observations were sparse. The reconstruction skill for precipitation is lower than for temperature, but wet days frequencies compare well to independent observations.
Based on this gridded data set, we calculated climate indices and two phenological phases to evaluate spring weather over the 258-year-long period (Chapter 3). Although it receives less attention than winter and summer, the spring season is important because adverse weather conditions in spring delay plant growth, and late frosts can damage vegetation. Climate and phenological indices impressively depict the warming of the recent decades compared to the pre-industrial reference period 1871 to 1900. Cherry flowering, for example, advanced by up to 20 days in the Swiss Plateau since the pre-industrial reference period. In the 258-year-long series, the spring of 1785 stands out of the reconstruction with a mean  temperature of only 4.10 °C and up to 30 days of frost registered in the Swiss Plateau. Further data sets and historical sources confirm that this was a record-breaking cold spring, with prolonged inversion conditions in the Swiss Plateau.
Among the ten warmest summer half years in Switzerland, only one summer from the 20ᵗʰ century remains, the summer of 1947 (Chapter 4). It still ranks as the fifth warmest summer on record based on a time series for the Swiss Plateau since 1756. In some parts of Switzerland, precipitation deficits are still the lowest since 1864. The repeated occurrence of blocking anticyclones led to a total of five heat waves in some parts of Switzerland between May and September, contributing to the anomalously warm temperatures. The warm and dry conditions had severe consequences such as extensive glacier ice loss, drying out of lakes, and forest damage. If we compare the warm summers to their mean climatic state, the summer of 1947 was indeed as extreme as 2003. However, the summer of 2022, the second warmest summer on record, was a fairly normal summer compared to its mean climate. Compared to the summer temperatures expected by the end of the century (2070 - 2099), the record summers of 1947, 2003, and 2022 would only be fairly normal summers under an emissions reduction scenario (RCP2.6), and such summers would be exceptionally cold if no emissions reductions are achieved (RCP8.5).