Dynamical Downscaling and Data Assimilation for the Early Instrumental Period - Challenges and Opportunities

Lucas Pfister

Institute of Geography and the Oeschger Centre for Climate Change Research University of Bern, Switzerland

Tuesday, Nov 21, 2023, 2:00 pm
DSRC Room GC402

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Abstract

In Europe, the 19th century was characterized by several events of extreme weather, ranging from cold spells, wet summers and floods to droughts. With the extension of atmospheric reanalyses back in time, dynamical downscaling of such datasets using a weather forecasting model has become an interesting approach in the field of weather reconstruction, shedding light on the detailed processes behind such extremes and their effects on the surface. Dynamical downscaling is particularly powerful, as it provides physically consistent, three-dimensional data on past weather at local and sub-daily scales, which is required for many scientific applications. Applying a 3D-Var data assimilation system with early instrumental surface station records, several sources of information can be combined to obtain refined reconstructions of past weather. While this approach had long been restricted to the 20th century due to a lack of prior atmospheric boundary conditions, the newest version of the 20th century reanalysis (20CRv3) opens a new chapter, allowing us to go back in time to the early 19th century.

In this talk, I will depict the challenges related to the various steps of such a weather reconstruction approach. Data rescue in the last decades brought to light a vast amount of meteorological station records covering over 300 years in total. However, tracking down early instrumental station records, digitizing and checking the available data, preparing the observations, which are often far from modern measuring and reporting standards, and finally using this data for assimilation in the WRF model can be a long and tedious journey. Also on the downscaling side, changes on the land surface over the last centuries, as well as larger uncertainties in initial conditions need to be addressed. Nevertheless, this approach holds many opportunities. High-resolution meteorological reconstructions at the dawn of the 19th century hold detailed information on atmospheric and surface processes in a pre-industrial climate, and can serve as a basis for studying and modelling the risks and impacts of extreme events on environment and economy in the past and today.