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Historical sources report manifold on hazardous past climate and weather events that had considerable impacts on society. Studying changes in the occurrence or mechanisms behind such events is, however, hampered by a lack of spatially and temporally complete weather data. In particular, the spring season has received less attention in comparison to summer and winter but is nevertheless relevant, since weather conditions in spring can delay vegetation and create substantial damage due to late-frost events. For Switzerland, we created a daily high-resolution (1 × 1 km 2) reconstruction of temperature and precipitation fields from 1763 to 1960 that forms, together with present-day meteorological fields, a 258-year-long gridded data set. With this data set, we study changes in long-term climate and historical weather events based on climate and phenological indices focusing on the spring season. Climate and phenological indices show few changes in the mean during the first 200 years compared to the most recent period from 1991 to 2020, where climate change signals clearly emerged in many indices. We evaluate the climate and phenological indices for three cases of extreme spring weather conditions: an unusually warm spring, two late-frost events, and three cold springs. Warm springs are much more frequent in the 21st century, but a very warm and early spring also occurred in 1862. Spring temperatures, however, do not agree on how anomalously warm the spring was when comparing the Swiss temperature reconstruction with reanalyses that extend back to 1868. The three springs of 1785, 1837, and 1853 were particularly cold, with historical sources reporting, for example, prolonged lake freezing and abundant snowfall. Whereas the springs of 1837 and 1853 were characterized by cold and wet conditions, in the spring of 1785 wet days were below average, and frost days reached an all-time maximum, in particular in the Swiss Plateau, indicating inversion conditions. Such conditions are in line with a high occurrence of northeasterly and high-pressure weather types and historical sources describing Bise conditions, a regional wind in the Alpine area related to inversions. Studying such historical events is valuable, since similar atmospheric conditions can lead to cold springs affecting vegetation growth and agricultural production. [ABSTRACT FROM AUTHOR]

Historical sources report manifold on hazardous past climate and weather events that had considerable impacts on society. Studying changes in the occurrence or mechanisms behind such events is, however, hampered by a lack of spatially and temporally complete weather data. Especially, the spring season has received less attention in comparison to summer and winter, but is nevertheless relevant since weather conditions in spring can delay vegetation and create substantial damage due to for example late frost events. For Switzerland, we created a daily high-resolution (1x1 km2) reconstruction of temperature and precipitation fields from 1763 to 1960, that forms together with present-day meteorological fields a 258-year-long gridded data set. With this data set, we study changes in longer-term climate and historical weather events based on climate and phenological indices focusing on the spring season. Climate and phenological indices show few changes in the mean during the first 200 years, but climate change signals clearly emerge in all indices in the most recent period. We evaluate the climate and phenological indices for three cases of extreme spring weather conditions, an unusually warm spring, two late frost events, and three cold springs. Warm springs are much more frequent in the 21st century, but also in 1862 a very warm and early spring occurred. Spring temperatures, however, do not agree on how anomalously warm the spring was when comparing the Swiss reconstruction with reanalyses that extend back to 1868. The three springs of 1785, 1837, and 1853, were particularly cold with historical sources reporting for example prolonged lake freezing and abundant snowfall. Whereas the springs of 1837 and 1853 were characterized by cold and wet conditions, in the spring of 1785 wet-days were below average and, in particular, in the Swiss Plateau, frost days reached an all-time maximum. Such inversion conditions are confirmed by mostly north-easterly and high pressure weather types and historical sources describing prolonged Bise conditions. Studying such historical events is valuable since similar atmospheric conditions can also nowadays lead to cold springs affecting vegetation growth and agricultural production. [ABSTRACT FROM AUTHOR]

Climate reconstructions give insights in monthly and seasonal climate variability of the past few hundred years. However, for understanding past extreme weather events and for relating them to impacts, for example through crop yield simulations or hydrological modelling, reconstructions on a weather time scale are needed. Here, we present a data set of 250 years of daily temperature and precipitation fields for Switzerland for 1763 to 2020. The data set was reconstructed with the analogue resampling method which resamples meteorological fields for a historical period based on the most similar day in a reference period. These fields are subsequently improved with data assimilation for temperature and bias correction for precipitation. Even for an early period prior to 1800 with scarce data availability, we found good validation results for the temperature reconstruction especially in the Swiss Plateau. For the precipitation reconstruction, skills are considerably lower, which can be related to the few precipitation measurements available and the heterogeneous nature of precipitation. By means of a case study on the wet and cold years from 1769 to 1772, which triggered wide-spread famine across Europe, we show that this dataset allows more detailed analyses than hitherto possible. [ABSTRACT FROM AUTHOR]

In the southern Peruvian Andes, communities are highly dependent on climatic conditions due to the mainly rain‐fed agriculture and the importance of glaciers and snow melt as a freshwater resource. Longer‐term trends and year‐to‐year variability of precipitation or temperature severely affect living conditions. This study evaluates seasonal precipitation and temperature climatologies and trends in the period 1965/66–2017/18 for the southern Peruvian Andes using quality‐controlled and homogenized station data and new observational gridded data. In this region, precipitation exhibits a strong annual cycle with very dry winter months and most of the precipitation falling from spring to autumn. Spatially, a northeast–southwest gradient in austral spring is observed, related to an earlier start of the rainy season in the northeastern part of the study area. Seasonal variations of maximum temperature are weak with an annual maximum in austral spring, which is related to reduced cloud cover in austral spring compared to summer. On the contrary, minimum temperatures show larger seasonal variations, possibly enhanced through changes in longwave incoming radiation following the precipitation cycle. Precipitation trends since 1965 exhibit low spatial consistency except for austral summer, when in most of the study area increasing precipitation is observed, and in austral spring, when stations in the central‐western region of the study area register decreasing precipitation. All seasonal and annual trends in maximum temperature are larger than trends in minimum temperature. Maximum temperature exhibits strong trends in austral winter and spring, whereas minimum temperature trends are strongest in austral winter. We hypothesize, that these trends are related to precipitation changes, as decreasing (increasing) precipitation in spring (summer) may enhance maximum (minimum) temperature trends through changes in cloud cover. El Niño Southern Oscillation (ENSO), however, has modifying effects onto precipitation and temperature, and thereby leads to larger trends in maximum temperatures. [ABSTRACT FROM AUTHOR]