Daily Weather Reconstructions to Study Decadal Climate Swings
On top of anthropogenic changes, the climate system exhibits mulitannual-to-multidecadal variability that is relevant for society, but not well understood. Decades of warm, dry European summers, cold winters or storm or flood rich periods have been observed in the past and will recur in the future. Long records are required to study this variability. To better understand the underlying processes, we will analyse such periods on a daily time scale by generating daily weather reconstructions and ana-lysing them together with monthly reconstructions and model simulations. Atmospheric processes operate on short time scales and it is often weather rather than climate that matters for society. Recon-structing weather rather than climate is thus closer to the mechanisms and closer to impacts. Several long European measurement series are available, but more information is required for daily weather reconstruction. The first part of the project will provide additional daily measurement series as well as weather diaries for central Europe. Our work will focus on the period 1683-1737; we will rely on available sources for later periods. The second part of the project will provide daily weather reconstructions back to 1683 in two ways. (1) We will provide daily weather types for Central Europe back to the 17th century based on measurements and observations from weather diaries using a machine-learning classification algorithm. (2) We will provide daily fields of temperature, precipi-tation, and sea-level pressure for central-western Europe back to the 18th or 17th century using an analog approach. Analogs will be sampled from daily 0.1° resolved fields (E-Obs) and augmented using a data assimilation (Ensemble Kalman Filter) algorithm.
The fields will be used for the analyses. We will also generate impact-relevant indices such as growing degree days, frost days, and dry spells.The third part of the project provides subdaily high-resolution fields using data assimilation and dy-namical donwscaling. We will use Version 3 of the „Twentieth Century Reanalysis“ (20CR), which provides global weather data back to 1804. We will dynamically downscale 20CRv3 for periods in the early 19th century (including the warm summers of the 1800s and the “Year Without a Summer” of 1816) and will test 4D-Var assimilation within WRF to assimilate additional observations. The daily data from the three project parts, combined with monthly reconstructions and climate model simulations, will be used to study European climate variability of the past 400 years. We will focus on decadal periods of warm and dry summers, which will be analysed with respect to links to the (sub-)tropical circulation (monsoons, tropical belt) using other reconstructions and models. We will focus on the sequence of warm summers around 1730, 1780, and 1800 and will compare them with the 1940s summers. A second focus are cold winters, such as 1683/4, 1708/9 and 1829/30, which will be analysed with respect to a meridionalisation of atmospheric circulation and relations to sea-surface temperatures or snow cover in Eurasia. Detailed studies will also be performed on the impcats of the volcanic eruptions of Serua (1693), Laki (1783/4), Tambora (1815) and others. Our data will yield new insights into the workings of the climate systems and bridge the gap from weather to climate.