Improving reconstructions of historical extreme weather events in 20CRv3 by rescuing undigitized observations with citizen scientists

Our understanding of past changes in weather and climate relies on the availability of observations made over many decades. However, millions of historical weather observations are effectively hidden from science as they are still only available in their original paper form in various archives around the world. The large-scale digitisation of these observations would substantially improve gridded reconstructions and atmospheric reanalyses back to the 1850s and earlier.

Recently, the citizen science project WeatherRescue.org has recovered 1.8 million observations of sea level pressure, temperature and rainfall at 73 locations across Great Britain, Ireland and western Europe during the 1900-1910 period. The value of these data will be assessed by considering case studies of extreme weather events.

We compare the recovered observations to the new NOAA-CIRES-DOE Twentieth Century Reanalysis version 3 (20CRv3) which assimilates surface pressure observations from the International Surface Pressure Databank (ISPD). In the early 20^th century, the spatial coverage of pressure observations in ISPD over western Europe is poor which results in uncertain representation of extreme weather events and a large ensemble spread. However, we demonstrate that the quality of the reanalysis for extreme events can be substantially improved by adding these newly recovered pressure observations into 20CRv3.

As one example, we consider a particularly destructive windstorm which crossed Scotland on February 27^th 1903. The reanalyzed storm in the 20CRv3 reanalysis is not strong enough to cause the damage known to have occurred, but an additional 20CR simulation with some of the additional pressure observations assimilated produces a stronger storm, matching the known regions of damage with greater fidelity. Similar analyses for major flooding events and a heatwave will be also presented.