Strawman scenario for MAP-SOP events

Introduction

This strawman scenario is based on an evaluation of the MAP-seasons 1989-1998. The evaluation was carried out with the aid of the daily weather map of the University of Berlin (Berliner Wetterkarte, 1989-1998), which contains an excellent overview of the large-scale weather situation, the central European weather situation including the Alpine region, and an extensive overview of local observations focusing on extreme events.

The evaluation covered MAP-relevant weather situations leading to Foehn events in the Rhine valley and in the Wipp valley over the Brenner-pass (gap flow), to heavy precipitation events in the southwestern part of the Alps (especially in the western Po valley and Ticino area) and in the southeastern part of the Alps (especially in the Friuli and Julian Alps region) and to pronounced deep southerly or northerly flows over the Alps, favourable to potential vorticity banner generation and to gravity wave breaking.

Figure 1.Schematic depiction of the time sequence of a major MAP event. Continuous lines are surface isobars with say 4hPa spacing, dashed are 500hPa contour lines with say 40gpm interval. Thick arrows indicate Foehn flow in the Rhine valley and gap flow over the Brenner pass, other symbols are conventional. It should be noted that with slowly moving, intensifying troughs the surface front may very often consist of more than one line which may modulate the Foehn flow considerably. Panel a) denotes day i + 0.5 (onset of shallow Foehn in the Wipp valley), panel b) approaching trough over France, panel c) onset of heavy precipitation in the SW-part, and panel d) end of Foehn in the Wipp valley, onset of precipitation in SE-part.

Table 1 Major event strawman scenario. See text and Fig. 1 for a more detailed description.

South Foehn events were selected if a significant (roughly 5 hPa or more at sea level) cross-Alpine pressure difference was present in the high-resolution weather map, without (shallow Foehn) or with (deep Foehn) a pronounced southerly flow at crest height and above. Heavy precipitation events were selected if at least one station in the relevant regions reported a 12-hourly precipitation of roughly 30 mm or more, associated with a southerly low-level flow component impinging on the Alps. If a heavy precipitation event was obviously associated with a pronounced upper-level trough (indicating high upper tropospheric potential vorticity) approaching the Alps via France, it was selected as `upper-level feature' event. The lack of upper level PV-maps for the present evaluation may have considerably underestimated such events. This should be kept in mind when interpreting the statistics. Both potential vorticity banner and gravity wave breaking events were pragmatically declared to take place when a southerly or northerly flow across the Alps with approximately 30 knots or more at crest height was present. It has to be considered that very often this criterion is fulfilled only over part of the Alpine crest. In principle, gravity wave breaking is also possible with an arbitrary flow direction, e.g. a westerly flow over the Alps. Due to the lack of experience on that matter and to avoid confusion, no such cases have been selected.

Statistics of the SOP periods of the last ten years

MAP events that occurred during the SOP period of the last ten years have been singled out and displayed in See Statistics of MAP events. For explanation of acronyms see legend of Table 2.

Table 2 A ten-years statistics of MAP events in the SOP period (see Table 3). The table headings are explained in the following: phenomena: according to the MAP Science Plan, 1998; mean number of days: number of 24 hour periods, not necessarily calendar days; mean number of events: an event is a continuous period of the occurrence of one or more MAP-related phenomena; mean duration of events: ratio between mean number of days and mean number of events; the number in brackets are based on the total number of days of a two- or multi-phenomenon event irrespective of a simultaneous occurrence of phenomena; distribution ratio: indicates the ratio between the mean maximum number of events within an arbitrary five week period and the mean number of events within the rest of the MAP season (the total length of SOP is exactly 10 weeks); mean maximum interval: mean value for the ten years 1989 to 1998 of the maximum observed interval between two successive occurrence of a phenomenon within the SOP period; ULF: Upper level feature event; FOR: Foehn event in the Rhine valley; GAP: Gap flow (Foehn) event in the Wipp valley (Brenner pass); ORP: Orographic precipitation event (W: In the western section, E: in the eastern section only); GWB: Gravity wave breaking event (N: north of the Alps with a southerly flow, S: south of the Alps with a northerly flow); PVB: potential vorticity banner event (N: north of the Alps with a southerly flow, S: south of the Alps with a southerly flow).

Conclusion

The very positive result is that during each of the MAP seasons from the most recent 10 years all of the MAP-relevant phenomena occurred at least once within the period September 7 to November 15. Even if we count major events, i.e. time windows of a few days where several (three or more) of the MAP-relevant phenomena occur, at least one such event was present in each of the 10 years. The duration of a major event may vary between 2 and 7 days with a mean of 3.5 days. A typical major event has a temporal characteristic as displayed in See Schematic depiction of the time sequence of a major MAP event. Continuous lines are surface isobars with say 4 hPa spacing, dashed are 500 hPa contour lines with say 40 gpm interval. Thick arrows indicate Foehn flow in the Rhine valley and gap flow over the Brenner pass, other symbols are conventional. It should be noted that with slowly moving, intensifying troughs the surface front may very often consist of more than one line which may modulate the Foehn flow considerably. Panel a) denotes day i + 0.5 (onset of shallow Foehn in the Wipp valley), panel b) approaching trough over France, panel c) onset of heavy precipitation in the SW-part, and panel d) end of Foehn in the Wipp valley, onset of precipitation in SE-part. and in See Major event strawman scenario. See text and Fig. 1 for a more detailed description. (major event strawman scenario).

The less positive result is that there is a strong tendency for repeated major events within a short period and hence quite an unbalanced distribution of expected actions within the MAP SOP. The median of the interval between the onset of two events is 8 days, with a modal value of only 5 days. The distribution ratio tells us that we have to expect two thirds or more of all events within one half of SOP. Looking at special phenomena, the distribution becomes even more unbalanced. For orographic precipitation events we must expect three quarters of the events within one half of SOP and even much worse, nearly all pronounced northerly or southerly flow regimes leading to gravity wave breaking and vorticity banners have to be expected in one half of SOP. This may be explained by the persistence of the large-scale flow regimes over the Alpine region.

Minor events (only one or two phenomena occurring simultaneously or overlapping) usually either concern periods with only shallow Foehn (westerly upper level flow) or with potential vorticity banner generation and wave breaking south of the Alps with a deep and pronounced northerly flow. The latter sometimes occurs after a major event, i.e. after the passage of the upper- level trough.

In most of the 10 most recent MAP seasons long periods (14 days mean maximum interval) without any of MAP relevant phenomena occurred. This is usually caused by a stationary/blocking high pressure cell over or close to Central Europe, a phenomenon called "Altweibersommer" in German (corresponding to the American "Indian summer"). For some of the phenomena the maximum period without an event may be as long as 4 to 5 weeks for gravity wave breaking, potential vorticity banners or orographic precipitation and even 6 weeks for upper-level features. Also for the more frequent Foehn events (FOR and GAP) a period of roughly 3 weeks has to be expected without any occurrence.

Table 3 Statistics of MAP events. For explanation of acronyms see legend of Table 2.

References:

Berliner Wetterkarte, 1989-1998: FU Berlin
MAP Science Plan, 1998: Zurich