12 December 20116
One of the worst meteorological disasters in history took place in the southeastern Alps during the infamous winter of 1916 /17. Avalanches following a massive snowfall event killed thousands of soldiers as well as civilians. Novel insight into the event arises from an interdisciplinary study conducted at the Oeschger Centre. It provides detailed reconstruction based on weather forecast models and shows the potential of combining numerical techniques with historical documents.
A century ago, Europe was in the midst of World War I. On the Italian front, the Austro-Hungarian and Italian armies faced each other on some of the harshest battlefields in history – on the summits of the southeastern Alps. Here, during a large part of the year, the fighting would cease almost completely, as a different war took place: a war against cold, ice, and snow. With an average precipitation exceeding 2 m per year in some locations, this part of the Alps is one of the wettest places on the continent. Between November 1916 and January 1917, a rain gauge located on today’s Italy-Slovenia border measured 1432 mm of precipitation, about 80 % of the local mean annual total. Soldiers were literally buried by snow and their bodies, exposed by shrinking glaciers, still provide a touching reminder of that absurd carnage.
However, there was one particular day that tragically entered history books: 13 December 1916. On this day, following a week of abundant snowfall, advection of a warm and humid air mass from the Mediterranean brought intense precipitation and a rise of snow level, causing countless avalanches across the region. The number of human casualties was unprecedented for this kind of natural event. Having occurred in the midst of a greater tragedy – the Great War – this event passed almost unnoticed at the time. Nevertheless, it represents one of the worst weather-related disasters in European history in terms of loss of human life. It is the kind of event that can inform us about worst-case present and future extremes. With its well-documented impact, what is required is a detailed, quantitative understanding of the responsible atmospheric processes. Thus, the tragic event can become instructive.
The currently available information on the weather in December 1916 is primarily based on qualitative descriptions from diaries, memoirs, and anecdotes passed down through generations. Some quantitative information can be recovered from weather stations of national weather services and be used to reconstruct such extreme events. In recent years, however, numerical techniques have been developed that allow detailed, quantitative weather reconstructions.
These so-called reanalyses, which combine weather observations with a numerical weather prediction model, are standard data sets in atmospheric science. Until recently, they were restricted to reconstructions of past decades for which abundant observations from weather balloons are available. However, recent approaches require fewer observations. Today, several global data products can reproduce the mete-orological situation in December 1916. However, they have a coarse spatial resolution that is insufficient for analyzing a regional event in a complex topography such as that of the Alps. A further step is dynamical downscaling of the reanalysis, which is similar to operations by meteorologists to provide more accurate weather forecasts on a regional scale.
These numerical techniques do not replace, but rather complement the work of historians. Whereas reanalyses provide a dynamical interpretation for documented weather phenomena, historical documents provide impacts of the reanalyzed weather systems. This encourages interdisciplinary collaboration as is demonstrated by the December 1916 case. Taken alone, contemporary meteorological observations would draw a rather incomplete picture of the event, because very few daily observations are available for the most affected areas. Taken together, the downscaled reanalysis and the observations provide a detailed, comprehensive view and allow a physically meaningful interpretation. Now that the main ingredients of the weather situation (persistent blocking, warm Mediterranean Sea, moisture transport, and temperature increase) and of the societal vulnerability are identified, the behavior of these factors in a future climate or a future society can be studied.
In fact, reanalyses demonstrate how almost forgotten historical observations can once again become valuable, requiring scientists to go back to the archives – work that is best performed jointly by climatologists and historians. This helps to better understand worst-case weather events in the past and future and their societal impacts.
(Source: „December 1916: Deadly Wartime Weather“, Geographica Bernensia, Geographisches Institut der Universität Bern)