The Papal Front of 3 May 1987: A remarkable example of frontogenesis near the Alps

Abstract

All available, routinely collected surface data are used to document the development of the 'Papal Front', which crossed southern Germany on 3 May 1987 causing casualities and damage. Objective analyses on isentropic surfaces, initialized with the European aerological data, indicate that the development of the violent mesoscale front, which exhibited squall-line characteristics, began when a distinct mid-troposopheric anomaly of potential vorticity reached the north-western rim of the Alpine arc. The forced uplift ahead of the associated shortwave trough combined with ageostrophic motions due to a jet streak at higher levels apparently provided a trigger for deep convection as was observed over the Alpine foreland. Numerical simulations with a dry hydrostatic model indicate the relative importance of the synoptic-scale forcing and the Alpine orography for the generation of the 'Papal Front'. A westerly 'orographic jet' over the foreland below crest height only develops in the simulation with full Alps and is corroborated by the few mountain wind observations that are available.A comparison with studies of frontal propagation along other mountain ranges reveals that the routine network in the Alpine region and its surroundings is at least of similar density, but so far hardly used for frontal case-studies. It is concluded that a series of 'routine-data case-studies' could help to determine the variability of frontal progression along the Alps and, thus, provide guidance when assessing the significance of single events, particularly those sampled during special observation programmes. * Retired, formerly with Deutscher Wetterdienst, Offenbach, F.R.G. t This term is used with the meaning of 'land in front' as in the German word Vorland 125 * The semantic problem arising from geographic names with English and German versions is dealt with in the following way. In the text the English version is used (e.g. Munich, Constance), whereas the German form of the names appear in the tables and figure captions (e.g. Munchen, Konstanz).

Increase in the number of extremely strong fronts over Europe? A study based on ERA‐Interim reanalysis (1979–2014)

“…High-impact, small-scale weather events, such as hail storms, wind extremes, or intense precipitation, are frequently associated with the passage of large-scale fronts [James and Browning, 1979;Volkert et al, 1987;Mills, 2005;Catto et al, 2012;Catto and Pfahl, 2013;Reeder et al, 2015;Schemm et al, 2016]. Hence, trends in the frequency, intensity, and spatial variability of fronts potentially are an important agent of trends in extreme weather phenomena and drivers behind their high spatiotemporal variability.…”

Gravity currents in rotating channels. Part 1. Steady-state theory

“…In the atmosphere, large-scale gravity currents also occur propagating along major mountain ranges (e.g. Volkert, Weikman & Tafferner 1991, and references contained therein), and while usually only affected by one lateral boundary, may experience effectively finite values of W 0 if subject to lateral blocking effects due to the synoptic-scale flow. The same is also true of gravity currents in the oceans flowing predominantly along only one lateral boundary, such as a coastline, since these can be affected by blocking effects due to the potential vorticity gradients set up by the presence of strong topographic features, such as the shelf-break.…”

The Papal front Part 3: Findings from research‐type analyses

“…Since 1993, operational numerical shortrange forecasts have been made for the Alpine region in Germany and Switzerland with a horizontal grid resolution of 15km. The new mesoscale model of the Meteorological Office has similar capabilities, but is focused on the British Isles for its routine operations (research-type hindcasts of the Papal front case have been presented by Volkert et al 1991). These tools have the potential to resolve multiple frontal structures, such as occurred on 3 May 1987.…”