Los reanálisis arrojan luz sobre el desastre de los aludes de 1916

Uno de los peores desastres meteorológicos de la historia tuvo lugar en el sureste de los Alpes durante el infame invierno de 1916/17. Los aludes ocurridos después de un episodio de grandes nevadas mataron a miles de soldados y civiles. Las técnicas numéricas actuales abren nuevas posibilidades para estudiar este episodio histórico. La combinación de las mediciones históricas con los reanálisis y la regionalización dinámica (dinamical downscaling) hace posible reconstruir el tiempo atmosférico descendiendo incluso hasta la escala local y, por lo tanto, a la escala captada por documentos históricos de los impactos meteorológicos

Simulations of the 2005, 1910, and 1876 Vb cyclones over the Alps - sensitivity to model physics and cyclonic moisture flux

In June 1876, June 1910, and August 2005, northern Switzerland was severely impacted by heavy precipitation and extreme floods. Although occurring in different centuries, all three events featured very similar precipitation patterns and an extratropical storm following a cyclonic, so-called Vb (five b of the van Bebber trajectories) trajectory around the Alps. Going back in time from the recent to the historical cases, we explore the potential of dynamical downscaling of a global reanalysis product from a grid size of 220 to 3 km. We investigate sensitivities of the simulated precipitation amounts to a set of differing configurations in the regional weather model. The best-performing model configuration in the evaluation, featuring a 1 d initialization period, is then applied to assess the sensitivity of simulated precipitation totals to cyclonic moisture flux along the downscaling steps. The analyses show that cyclone fields (closed pressure contours) and tracks (minimum pressure trajectories) are well defined in the reanalysis ensemble for the 2005 and 1910 cases, while deviations from the ensemble mean increase for the 1876 case. In the downscaled ensemble, the accuracy of simulated precipitation totals is closely linked to the exact trajectory and stalling position of the cyclone, with slight shifts producing erroneous precipitation, e.g., due to a break-up of the vortex if simulated too close to the Alpine topography. Simulated precipitation totals only reach the observed ones if the simulation includes continuous moisture fluxes of >200 kg m−1 s−1 from northerly directions and high contributions of (embedded) convection. Misplacement of the vortex and concurrent uncertainties in simulating convection, in particular for the 1876 case, point to limitations of downscaling from coarse input for such complex weather situations and for the more distant past. On the upside, single (contrasting) members of the historical cases are well capable of illustrating variants of Vb cyclone dynamics and features along the downscaling steps

Synoptic and Mesoscale atmospheric features associated with an extreme Snowstorm over the Central Andes in August 2013

We study the synoptic and mesoscale characteristics of a snowfall event over the Bolivian Altiplano in August 2013 that caused severe damage to people, infrastructure and livestock. This event was associated with a cold front episode following the eastern slope of the Andes-Amazon interface and a cut-off low pressure system (COL) over the Pacific Ocean. Large scale analyses suggest a two-stage mechanism: The first phase consisted of a strong cold surge to the east of the Andes inducing low level blocking of southward moisture transport over the SW Amazon basin due to post-frontal high-pressure up to 500 hPa synchronized to a Rossby wave train. The second stage was initiated by the displacement of 500 hPa anticyclone over the Andes due to a Rossby wave passage and a subsequent increase in north-easterly moisture transport, while another cold front along the eastern Andes provided additional lifting. We analyse an analog event (July 2010) to confirm the influence of these large-scale features on snow formation. We conduct a mesoscale analysis using the Weather Research and Forecasting (WRF-ARW) model. For this purpose, we perform a series of high-resolution numerical experiments that include sensitivity studies where we apply orographic and lake Titicaca temperature modifications. We compare our findings to MODIS snow cover estimates and in-situ measurements. The control simulation is able to capture the snow cover spatial distribution and sheds light over several aspects of the snowfall dynamics. In our WRF simulations, daytime snowfall mainly occurs around complex orography whereas nocturnal snowfall is concentrated over the plateau due to a combination of nocturnal winds and complex orography inside the plateau. The sensitivity experiments indicate the importance of the lake and mountain for thermal wind circulation affecting the spatial distribution of snowfall by shifting the position of the convergence zones. The influence of the lake's thermal effect is not evident around the regions surrounding the lake

Reanalysis sheds lights on 1916 avalanche disaster

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. Today’s numerical techniques open up new possibilities to study this historical event. Combining historical measurements with reanalyses and dynamical downscaling makes it possible to reconstruct weather even down to local scales and thus to the scale captured by historical documents on weather impacts