ACCORD
University of Berne
Partner:
University of Berne (07)
Responsible Scientist: Dr Evi Schuepbach
Scientific Staff: Dr Evi Schuepbach, Marut Doctor
Address:
CABO, Physical Geography, University of Berne, Hallerstrasse 12,
CH-3012 Berne, Switzerland
Telephone: (41) (0) 31-631-88-43
Fax:
(41) (0) 31-631-85-11
Email:
[email protected]
1.
Original Objectives and Extent to Which They Have Been Achieved ...................................................... 285
1.1 Original objectives ................................................................................................................................ 285
1.2 Extent to which they have been achieved ............................................................................................. 285
2.
Precipitation Regimes in Switzerland (Task 3.1) ...................................................................................... 285
2.1 Western Plateau, Jura............................................................................................................................ 286
2.2 Eastern Plateau, North of the Alps........................................................................................................ 286
2.3 South of the Alps (Ticino) .................................................................................................................... 286
2.4 Valais .................................................................................................................................................... 286
3.
Seasonal Variation of Precipitation in Switzerland (Task 3.7) .................................................................. 287
3.1 Daily mean precipitation exceeding 10 mm (strong precipitation) ....................................................... 287
3.2 Daily mean precipitation exceeding 40 mm (heavy precipitation) ....................................................... 287
4.
Circulation Patterns Associated With Heavy Precipitation Events in Southern Switzerland (Tasks 3.5 and
3.7)... ................................................................................................................................................................... 288
4.1 Composites of 300/700 hPa thickness................................................................................................... 288
4.2 Anomalies of the 700 hPa geopotential height and comparison with the Alpes Maritimes................ 288
4.2.1
Data and method.......................................................................................................................... 288
4.2.2
700 hPa geopotential anomaly dZ ............................................................................................... 289
4.3 Sub-period analysis of the links between surface weather and upper-air circulation............................ 289
5.
Conclusions and Relevance to ACCORD Objectives................................................................................ 289
6.
Direction of Future Research..................................................................................................................... 290
7.
References ................................................................................................................................................. 290
8.
List of ACCORD Publications .................................................................................................................. 290
Appendix A: Doctor et al. (2000), draft paper for submission to Climate Research .......................................... 298
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Original Objectives and Extent to Which They Have Been Achieved
1.1
Original objectives
It is known that southerly air flow both in May and at the beginning and middle of autumn is
frequent over Switzerland. This flow may produce heavy rain, and sometimes strong
thunderstorms on the southern slopes of the Alps due to ascent forced by the mountains.
Hence, it is during these periods of the year that strong precipitation events are most likely to
occur in the southern Swiss Alps. The circulation patterns likely to cause severe weather
(heavy rainfall) in the southern Alps/northern Italy region are also thought to be important in
instigating (and maintaining) lee cyclones which are capable of producing severe weather
over the northern Italy/Alps region. Hence, the main original scientific objectives were the
isolation of those atmospheric circulation patterns producing heavy precipitation, and
establishing links between the atmospheric circulation and surface weather over southern
Switzerland/northern Italy. These links were to be examined over time. The work was carried
out in collaboration with Partner 04 (CNRS).
Partner 07 also aimed at a sub-period analysis to validate the effectiveness of the
classification of circulation patterns producing southerly flow in the Alps. Finally, the Alpine
circulation was to be linked with hemispheric circulation modes (together with Partner 01),
using the Dzerdzeevskii classification as a starting point for this analysis.
1.2
Extent to which they have been achieved
Partner 07 has emphasised efforts on surface weather/upper-air circulation links for a number
of reasons. Strong and heavy precipitation events in the southern Alps/northern Italy were
isolated using both station data (1901-1997) and gridded data (1961-1990 and 1971-1995),
see Section 2. Both composites of 300/700 hPa thickness and clusters of 700 hPa geopotential
height over the Atlantic/European region were linked with the surface variables (see Section 2
and Doctor et al., 2000), thus contributing to the achievement of the original scientific
objectives. The issue of spatial domain was addressed by comparing clusters of upper-air
circulation in two Alpine sub-regions (southern Alps in Switzerland and the Alpes Maritimes
in France). The sub-period analysis (1991-1996) of surface weather/upper-air circulation
relationships using high-resolution ECMWF T213 data is still under way. Similarly,
investigation of the the linkage between the behaviour of the hemispheric-scale circulation
and the Alpine circulation over time has not been fully completed yet, although annual and
seasonal analyses of the frequency of the 13 Dzerdzeevskii circulation classes have been
completed (not presented here).
2.
Precipitation Regimes in Switzerland (Task 3.1)
The Swiss Institute of Meteorology (SMI) has measured meteorological parameters since
1755 in Basel and since 1768 in Geneva. However, most of the measurements at a very large
number of sites in Switzerland started in 1864. In about 1981, the meteorological
measurements were automated at many stations (ANETZ = Automated Network), and
recordings are now available every 10 minutes summing up to 144 observations a day.
However, conventional measurements at some stations have continued. The stations where
only precipitation is measured continue to be operated manually.
Sixteen stations were selected for analysis in ACCORD. Table 1 shows the numbers of these
stations in the SMI databank, the record length, the altitude of the site, the Swiss coordinates
of the station and, in the last column, the region in which the station is located. Figure 1
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shows a map of the location of the sixteen stations. The selection represents a combination of
stations from the automatic and conventional network, and rain-gauge only stations and also
represents different climate regions, altitude levels, and environments in Switzerland.
The monthly precipitation data (corrected but not homogenized) from SMI were used to
analyse the precipitation regimes of the selected Swiss stations for four regions. The seasonal
precipitation cycles of stations in these four regions are described in the following sections.
2.1
Western Plateau, Jura
The stations in western Switzerland (Geneva, Le Sentier and La Dôle) display only small
variations during the year. They represent the transition from the oceanic type with a
pronounced precipitation maximum in winter to the continental type with a pronounced
maximum in summer. The station at La Dôle, a mountain-top site in the Jura Mountains, has a
tendency to oceanic type with a small maximum in December and January. All three stations
show a slight minimum in April.
2.2
Eastern Plateau, North of the Alps
The eastern Plateau (Zurich) and all selected stations to the North of the Alps, located south
and to the east of Zurich (Guttannen, Altdorf, Kronberg, Chur and Arosa) have generally a
moderate seasonal cycle in precipitation. The precipitation regime is of a continental type
with a maximum in summer and a minimum in winter. The heavy rainfall due to
thunderstorms explains the maximum in summer. The particularly frequent thunderstorms in
summer in the Alpstein region (Kronberg in northeastern Switzerland) give an even stronger
seasonal cycle with a pronounced maximum in summer. In contrast, the annual amplitude is
relatively small at Guttannen (Bernese Oberland) which is nearer Sion in the region of the
Valais (see Section 2.4).
2.3
South of the Alps (Ticino)
The selected stations in this part of Switzerland are located in the Lago Maggiore region
(Camedo, Bosco-Gurin, Lugano), and Airolo a little further north. The Ticino, located south
of the Alps, has a bimodal precipitation regime. It is characterised by a strong annual cycle
with two pronounced maxima in May and October, and a marked minimum in winter. The
times of maxima are when frequent southerly currents produce heavy rain and sometimes
strong thunderstorms on the southern slopes of the Alps. Hence, it is during this period of the
year that strong precipitation events are most likely to happen. In spite of some
thunderstorms, a relative minimum is found in July because the southerly airflows are
relatively rare.
2.4
Valais
The stations selected in the Valais are located in the vicinity of Sion (Brig, Binn and BourgSt-Pierre). They show generally little variation over the seasonal cycle. Brig has a
precipitation climate which is typical of the interior valleys of the Alps with slightly more
precipitation in the winter than in the summer months. In winter, the winds are generally
strong at this altitude, and consequently, rain or snowfall can more easily cross the Alpine
barrier in the cold season than in summer. This phenomenon explains the slightly greater
amount of precipitation in winter. Also, in summer, thunderstorms occur much less frequently
in the interior valleys of the Alps than in the rest of the Alpine region. With a small
precipitation maximum in summer, Bourg-St-Pierre has a few more characteristics of the
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northern Alpine climate. Binn has a moderate seasonal cycle of precipitation with two
maxima in October and May, similar to the Ticino stations.
3.
Seasonal Variation of Precipitation in Switzerland (Task 3.7)
Precipitation categories of 0 mm, 0.1-1 mm, 1.1-10 mm and > 10 mm were selected. Daily
precipitation > 10 mm was defined as strong precipitation, and daily precipitation > 40 mm as
heavy precipitation.
3.1
Daily mean precipitation exceeding 10 mm (strong precipitation)
Figure 2 shows the months with a maximum and minimum of strong precipitation (greater or
equal to 10 mm) and dry days over the period 1901-1997. The stations in the Ticino and Binn
have the strongest seasonal cycles with the maximum of days with strong precipitation in May
and a second maximum in October. As mentioned in Section 2, the high frequency of
southerly flow during these two months explains these two peaks.
Both the months with the lowest number of days with strong precipitation and the months
with the greatest number of dry days are found in December or January at the Ticino stations.
At Binn, however, the month with the greatest number of dry days is October.
The precipitation regime and the number of days with strong precipitation are also strongly
related at several stations to the north of the Alps (Altdorf, Zurich, Kronberg and Arosa).
These stations also show a rather strong seasonal cycle. The maximum of days with strong
precipitation occurs in June or July. However, the greatest number of dry days occurs in
September or October, which differs from the months with the lowest amount of rainfall
(December or January).
Another station with quite a strong seasonal cycle is La Dôle (Western Jura). The month with
the greatest number of strong precipitation events is December, followed by January.
September is the month with the greatest number of dry days.
It is interesting to note that the months with the lowest number of dry days are either May or
June at all sixteen Swiss stations. The short duration of anticyclones during this time of the
year and the importance of convection may explain this phenomenon.
3.2
Daily mean precipitation exceeding 40 mm (heavy precipitation)
According to Plaut and Vautard (see Final Report of Partner 4 and INLN), days with heavy
precipitation events were defined as days when precipitation is greater or equal to 40 mm
(Figure 3). Only heavy precipitation events in the southern Alps were linked with atmospheric
circulation. For this reason, the number of stations was reduced to eight. The period of
reference was 1961-1990 (NCEP period).
The main difference compared with the threshold value of 10 mm is that the greatest number
of days with heavy precipitation occurs in October at the Ticino stations (and in September at
Lugano) and at Binn instead of May. This means that precipitation variability is greater in
autumn than late spring. Blocking highs can last for weeks during some autumns, with no
significant precipitation measured in the whole of Switzerland. As mentioned in Section 3.1,
the month with the greatest number of dry days is September or October at many Swiss
stations. On the other hand, during some years, southerly flow may be persistent. As the water
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of the Mediterranean sea is warm at this time of the year, the southerly flow may contain a lot
of water vapour. Therefore, extreme precipitation events can be expected in Ticino. With a
mean annual precipitation amount of 2200 mm, Camedo receives the greatest amount of
precipitation of all Ticino stations, and even of all stations at low altitude in the whole of
Switzerland.
4.
Circulation Patterns Associated With Heavy Precipitation Events in Southern
Switzerland (Tasks 3.5 and 3.7)
4.1
Composites of 300/700 hPa thickness
The linking of heavy precipitation events in the northern Italy/Alpine region with atmospheric
circulation patterns over the European/Atlantic region was initially carried out with
compositing. Compositing is an ‘environment-to-circulation’ approach, i.e. the discrimination
of surface (environmental) variables (such as heavy precipitation events) and their links with
atmospheric circulation patterns. A tool to compute composite pressure patterns
(compositing) and their standard deviations has been developed (Burkard, 1998) together with
a routine to compute ‘subtraction maps’ for information on the differences between
composites, and ‘anomaly maps’ showing the deviations from a (long-term) mean.
Mathematically speaking, the ‘subtraction map’ is a subtraction of one map from another, and
the ‘anomaly map’ is a subtraction of the period of reference from the period of interest.
Composite maps of 300/700 hPa thickness (12 UTC) were plotted for all days with > 40 mm
precipitation measured at surface stations in the southern Alps (1961-1990). Figure 4 shows a
composite for May and displays a tropospheric thermal field showing a cold anomaly to the
west of the Alps and advection from southwest to the Alps. The composite for October is very
similar (not shown).
4.2
Anomalies of the 700 hPa geopotential height and comparison with the Alpes
Maritimes
4.2.1 Data and method
The data used for this analysis, which is described in detail in a journal paper produced as part
of the ACCORD work (Doctor et al., 2000, attached to this report as Appendix A), was the
gridded rainfall data set of the Alpine Precipitation Climatology (APC) from ETH Zurich
(Frei and Schär, 1998), which is based on observations of one of the densest rain-gauge
networks in the Alps. The APC is determined from daily analyses of bias-uncorrected, quality
controlled data over the 25 year period 1971-1995, and is held in the Mesoscale Alpine
Programme (MAP) data bank.
Principal components of the circulation fields over the Lago Maggiore region and the Alpes
Maritimes were calculated using the methodology developed by Plaut and Vautard (see the
Final Report of Partner 4 and INLN). The geopotential 700 hPa anomaly was calculated for
every day from 1971 to 1995. The annual cycle of this 700 hPa geopotential field and the
mean monthly trend were removed in order to find the geopotential anomalies (dZ). In order
to cover the Lago Maggiore region, 10 gridpoints were selected from the MAP databank.
Their location is shown in Figure 5.
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4.2.2 700 hPa geopotential anomaly dZ
Figure 6 shows the three clusters associated with heavy precipitation events in the Lago
Maggiore and in the Alpes Maritimes regions. dZ of Cluster 1 is very similar to the Greenland
High-Sole cyclone (GASC) of the Alpes Maritimes in Plaut and Vautard (see the Final Report
of Partner 04 and INLN), and dZ in Cluster 2 resembles the Quadrupole (QP) in the Alps
Maritimes. Cluster 3 with a low over Ireland (IL = Ireland Low) is also very similar to that of
the Alpes Maritimes. The slight difference is that the low is more pronounced for the Alpes
Maritimes. For IL, the high over Scandinavia and Russia is also stronger. It shows the
importance of blocking highs for the Alps Maritimes.
Figure 7b shows the relative occurrence of heavy precipitation events for the Lago Maggiore
region associated with the three clusters. It shows a relative occurrence of 38% for GASC,
26% for IL and 17% for QP. Figure 7a provides evidence that the circulation patterns on most
days over the 25 years are not correlated with any pattern of the three clusters. Hence,
circulation patterns of most days are independent of these three clusters associated with
heavy precipitation in southern France or southern Switzerland.
4.3
Sub-period analysis of the links between surface weather and upper-air
circulation
T213/L31 data (0.5° x 0.5° resolution) 1991-1996 from the European Centre for MediumRange Weather Forecasts (ECMWF), Reading, UK, were retrieved for the assessment of the
classification of the atmospheric circulation which is likely to cause heavy precipitation
events in the southern Alps/northern Italy region. The analysis of these data is still under way.
5.
Conclusions and Relevance to ACCORD Objectives
Over the year, the number of days with heavy precipitation events (daily mean precipitation >
40 mm) in southern Switzerland has not increased over the 1971-1995 period. Disaggregated
by season, however, positive changes have occurred in summer (April-September), while
heavy precipitation events in winter (October-March) have decreased. The behaviour of heavy
precipitation events in the southern Swiss Alps is thus different from the Alpes Maritimes
where heavy precipitation events only occur in winter, and the number of events in winter has
decreased. This is evidence for regional differences in the temporal behaviour of heavy
precipitation events in the Alps.
A slight increase in the amount of precipitation for each heavy precipitation event has
occurred in the southern Swiss Alps; this is similar to results from the Scandinavia/UK region
(see the Final Report of Partner 02).
Heavy precipitation events in southern Switzerland over the period 1971-1995 are associated
with three main circulation regimes (700 hPa geopotential height) over the Atlantic/European
region steering southerly flow to the Alps:
(i)
(ii)
(iii)
anticyclone over Greenland steering southerly flow to the Alps from a low centred
over the UK;
low pressure over Spain and high pressure over northern Europe (with advection of
southeasterly winds to the Alps); and,
low pressure over Ireland with southwesterly flow over the Alps.
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6.
Direction of Future Research
The concentration on extreme events will continue. A focus of research will be the analysis of
the seasonality of occurrence of heavy precipitation events in the southern Swiss Alps, i.e.
both the tendency of heavy precipitation events to increase in summer over the last 25 years
and to decrease in winter. Both the role of the upper-air circulation over the Atlantic/European
region and of surface variables (such as water vapour) will be addressed. A similar analysis
will be carried out for dry days.
7.
References
Burkhard, R., 1998: A Tool for the Computation of Composite Pressure Patterns, Including
Anomaly Maps, using IDL. CABO, Physical Geography, University of Berne, Switzerland,
10 pp.
Doctor, M., Plaut, G. and Schuepbach, E., 2000: ‘Atmospheric circulation associated with
heavy precipitation events in the southern Alps in Switzerland and comparison with the
Alps in southern France‘, Climate Research, in preparation (attached as Appendix A).
Frei, C. and Schär, C., 1998: ‘A precipitation climatology of the Alps from high resolution
rain gauge observations‘, International Journal of Climatology, 18, 873-900.
8.
List of ACCORD Publications
Doctor, M., Plaut, G. and Schuepbach, E., 2000: ‘Atmospheric circulation associated with
heavy precipitation events in the southern Alps in Switzerland and comparison with the
Alps in southern France‘, Climate Research, in preparation (attached as Appendix A).
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Table 1: Summary of the seasonal cycle of daily strong precipitation events in Switzerland.
Station
Length
record
of Height
(meters)
Brig
Binn
Bourg-st-Pierre
Camedo
Bosco-Gurin
Lugano
Airolo
Guttannen
1961-97
1961-97
1901-97
1961-97
1961-97
1901-97
1901-97
1901-97
617
1400
1620
570
1505
276
1149
1055
Genève
3.62-97
430
La Dôle
Le Sentier
Zuerich
1973-97
1901-6.96
1901-97
1672
1020
475
Kronberg
Chur
Arosa
Altdorf
1973-97
1901-97
1901-97
1901-97
2250
640
1850
451
Region
Latitude
longitude (°E)
Valais
Valais
Valais
Ticino
Ticino
Ticino
Ticino
Bernese
Oberland
Western
Plateau
Jura
Jura
Eastern
Plateau
NE Alps
Grisons
Grisons
N Alps
46°19’/ 7°59’
46°22’/ 8°11’
45°57’/7°12’
46°09’/ 8°37’
46°19’/ 8°30’
46°00’/ 8°58’
46°32’ /8°32’
46°39’ /8°18’
(°N)/ Month(s) with Month(s) with
maximum
minimum
precipitation
precipitation
11,12
7
5,10
1
8
2
5,10
12
5,10
12
5,10
1,12
5,10
1
6
1
Amplitude
of seasonal
cycle
weak
medium
weak
distinct
distinct
distinct
medium
weak
46°15’ /6°08’
9
4
weak
46°30’ /6°06’
46°36’ /6°13’
47°23’ /8°24’
12
6
6
4
4
1
medium
weak
medium
47°17’ /9°21’
46°52’ /9°32’
46°47’ /9°41’
46°52 /8°38’
6
8
6
7
2
3
2
3
distinct
medium
medium
medium
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i n
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n
u
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Berne
u
a
t e
a
l Z urich
s
P
Altdorf
T h e
La D ôle
Brig
C hur
Arosa
G uttannen
Le Sentier
s
A l p
Airolo
Bosco-G urin
Binn
Sion
G eneva
Kronberg
C am edo
Lago
M aggiore
Locarno
Lugano
Bourg-St-Pierre
Figure 1: Map of Switzerland and the Lago Maggiore area.
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Genève
La Dôle
Le Sentier
Brig
12
Binn
10
Bourg-st-Pierre
Camedo
months
8
Bosco-Gurin
Lugano
6
Airolo
Guttannen
4
Altdorf
2
Zürich-SMA
Kronberg
0
max strong prec.
min strong prec.
max dry days
min dry days
Chur
Arosa
Figure 2: Monthly distribution of highest/lowest number of days with mean daily precipitation
> 10 mm (strong precipitation) and highest/lowest number of days with no precipitations (dry
days), period of study 1901-97 (except for Genève, La Dôle, Brig, Binn, Camedo, BoscoGurin and Kronberg).
70
Cam edo
60
B o s c o -G u rin
50
Lugano
40
A iro lo
30
B in n
B rig
20
G uttannen
10
A lt d o rf
ec
.
.
D
ov
N
Ju
l
Au y
gu
st
Se
pt
.
O
ct
.
M
ay
Ju
ne
ril
Ap
ch
ar
b.
M
Fe
Ja
n.
0
Figure 3: Number of days with heavy precipitations events (> 40 mm mean daily
precipitation) at selected stations in Switzerland from 1961-1990.
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Figure 4: 300/700 hPa thickness composite (1961-1990) in May for days with mean
precipitation > 40 mm.
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Figure 5: The ten gridpoints in the Lago Maggiore area in the gridded APC rainfall dataset.
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Figure 6: Comparison of 700 hPa geopotential anomaly clusters as identified for days (1971-1995) with heavy
precipitation (> 40 mm) in the Ticino (a) and in the Alpes Maritimes (b); (From Doctor et al., 2000).
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Figure 7: a) Histograms of contingency tables of all days against di, the angular distance to
cluster i center;
b) Probability against di of any day being a class i intense event (IE).
(From Doctor et al., 2000).
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Appendix A: Doctor et al. (2000), draft paper for submission to Climate Research
ATMOSPHERIC CIRCULATION ASSOCIATED WITH INTENSE
PRECIPITATION EVENTS IN THE SOUTHERN ALPS IN
SWITERLAND AND COMPARISON WITH THE ALPS IN
SOUTHEASTERN FRANCE
Marut Doctor (1), Guy Plaut (2) and Evi Schuepbach (1)
(1) Climate and Background Ozone (CABO), Physical Geography, University of Berne,
Switzerland
(2) Institut Non Linéaire de Nice, Nice, France
Abstract:
The circulation patterns at 700 hPa causing intense precipitation in southern Switzerland (Lago
Maggiore region) is being analysed. Three clusters were identified using principal component analysis.
The correlation of days with intense precipitation events (>40 mm/day) with the 3 clusters is computed
to see what circulation pattern causes the most frequent intense precipitation. Rain composites for a
given grid point are also calculated to isolate he rain intensity for a given cluster. It was found that the
cluster “Greenland High” and “Bretagne Low” (GASC) produce the most intense and most frequent
precipitation. These three clusters are very similar to the cluster conducive to frequent and intense
rainfall in the Alpes Maritimes region (southeastern France).
1) Introduction
Among all natural hazards, floods cause the greatest material damage in the world (W.M.O.,
1994). They represent 32% of the damage caused by all natural hazards. Due to orographic
effects, the European Alps are particulary sensitive to floods - essentially the ones following
intense precipitation events. The reason for intense precipitation in the Julian and Carnic Alps
(northeastern Italy and Slovania) is the development of a Mediterranean cyclone. These
surface lows can be sometimes secondary lows, moving from the western Mediterranean to
the Alps, generated by an upper low over western France or over the Golf of Biscay (SMI,
1993). Associated with such a Mediterranean cyclone at the western edge of the Alps is
southerly flow of moist air from the Adriatic Sea to the southeastern Alps (Rakovec et al.,
1998). This advection of warm and moist air from the Mediterranean explains a considerable
amount of the intense rain floods in the Alpine region (Jansà et al., 1995). A Mediterranean
surface low was also important for the two Alpine floods at Vaison-la-Romaine in 1992 and
at Brig in 1993, whereas its role is not as clear for the Piedmont flood in 1994. By the Brig
floods there was a persistant low at ground level and in altitude as well extending from
western Mediterranean to Central France (Grebner, 1994). It gave a continuous moist south to
southeasterly flow. Grebner and Richter (1991) explain the importance of the secondary low
south of the Alps for the flood of July and August 1987.The intense precipitation event of the
6th/7th November 1997 in northern Italy is also explained by an intense southerly flow from
the western Mediterranean to the Alps, and even to the North Sea. It was triggered by an
upper through moving eastward, due to the Alpine and Apenine lifting, persistant
precipitation fell in Lombardia and in the Apenine area (Frontero et al., 1998).
In Switzerland, the region around the Lago Maggiore, especially the site at Camedo (Figure 1)
is most likely to experience intense precipitation events (Zeller et al., 1990). With a return
period of 10 years, the mean precipitation intensity by hour reaches 13 mm at Camedo, 8 mm
at Locarno, 4 mm at Stalden and only 2 mm at Sion (see Figure 1).
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The Lago Maggiore region (northwestern Italy and southern swiss Alps) is also one of the
areas, behind the Julian and Carnic Alps with the highest precipitation values in an Alpine
wide climatology direved from high-resolution rain gauge data (1931-60) of more than 1000
stations between Mont Blanc and Hoher Tauern (Frei and Schär, 1998; Fliri and Schüepp,
1983). Yearly maximum values are 3000 mm in the northeastern Italy region and 2500 mm in
the Lago Maggiore region (Widmann, 1996). In the northern Alps (Glarner Alpen, Alpstein,
and Allgäu), maximum precipitation reaches only 2000 mm. Yet, the frequency of days with
at least 1 mm of precipitation is significantly higher in the northern Alps than in the south of
the Alps. This indicates that more intense precipitation are likely to occur in the Julian and
Carnic Alps or in the Lago Maggiore region. Comparatively, the mean precipitation and the
frequency of days with precipitation are relatively small in the Alpes Maritimes region
(southeastern France). The highest values for this region are 1500 mm. (Kessler et al., 1990).
In the Lago Maggiore region, the precipitation maxima occurs in May and October, while the
peak is in June and November in the Julian and Carnic Alps. Heavy precipitation events are
likely to occur during these two months. In the Alpes Maritimes, the maximum occurs only
for the high ridges in May and October, like for the Lago Maggiore region. For the coast and
the middland, secondary maximum would appear in January (sometimes in April) instead of
May for the high ridges. It is interesting to note that the precipitation minimum is in winter in
the Lago Maggiore region and Julian Alps, but in the summer in the Alpes Maritimes.
In this paper, we present a comparative analysis of those circulation patterns (700 hPa level)
which are associated with intense precipitation events in the Laggo Maggiore region and the
Alpes Maritimes in France.
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Fig.1. Map of the Laggo Maggiore region.
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2) Data and Methods
2.1 Data
The data used for this study was the gridded rainfall data set of the Alpine Precipitation
Climatology (APC) at ETH Zurich (Frei et al., 1998), which is based on observations of one
of the densest rain-gauge networks in the Alps. The APC covers the entire alpine region
including the adjacent foreland up to 300 km around the mountain range, encompassed by a
longitude and latitude window of 2°-17°E and 43° - 49°N and extending west-east from
central France across Switzerland to eastern Austria, and north-south from southern Germany
to the Ligurian and the Adriatic coasts. The gridded data sets displays a horizontal resolution
of about 25 km with a grid spacing of 0.3° in the west-east direction, and 0.22° in the northsouth direction. The APC is determined from daily analyses of bias-uncorrected, quality
controlled data over the 25 year period 1971-1995, and is held in the Mesoscale Alpine
Programme (MAP) data bank.
Input data for determining the circulation fields were the NCEP (National Centers for
Environmental Prediction) reanalysed 700 hPa geopotential heigt data. The goal of reanalysis
data is to produce new atmospheric analysis using historical data (1958 onwards) and as well
to produce analysis of the current atmospheric state. This geopotential reanalysis data is
available over an Atlantic-European window, which extends from 80°N to 30°N and from
60°W to 70°E. Since precipitations heights were daily, we arbitratily selected 12 UTC
geopotential heights.
Figure 2. Location of the 10 gridpoints over the Lago Maggiore region
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2.2 Method
The annual cycle and a seasonal linear trend where substracted from the Z700 height field;
then we were left with Z‘, the so-called anomaly height field. A prinicpal component analysis
was performed, and the 10 leading PC’s were kept for classification purpose of the Large
Scale Circulation (LSC hereafter) (the higher the order of a PC, the smaller the scale of the
corresponding pattern).
In order to cover the Lago Maggiore region, 10 gridpoints were selected from the MAP data
bank; their location is shown in Figure 2. The coordinates of these gridpoints extend from
southwest (7.8° / 45.92°) to northeast (9.0° / 46.36°). Intense Precipitation Events (IE) are
defined as days when at least oneamong these 10 gridpoints collected 40 mm or more. Once
IE have been selected, we take the corresponding Z‘ fields, and proceed to the classification
of these fields within their leading PC space, using Dynamical Cluster Algorythme
(Michelangeli et al., 1995). The number of cluster is unknown „a priori“, and several values
are tried; the significance checks are performed (Plaut and Vautard, 1998) and most often, the
90% significance level is reached for only one value of this number; for instance there are
definitely 2 Large Scale Circulation clusters for the Alpes Maritimes IE (Plaut and Vautard,
1998).
Once the clusters are obtained, the Z‘ of each cluster center may be computed. Let us call it
Z’i for cluster i. Afterwards, a pattern correlation corr(Z‘,Z’i) =cos((Z‘,Z’i) may be computed
between Z‘, the geopotential anomaly of ANY day within the 25 years 1971-1995, and that of
the ith cluster center. We define the angular distance di(Z‘) as:
Di(Z‘) = 1 - corr(Z‘,Z’i)
If di(Z‘) = 0, Z‘ and Z’I are perfectly correlated; if di=2, they are inversely correlated,
whereas they are independent for di=1.
In order to check the way in which rainy days LSC are correlared with the clusters, several
quantities were stratified against di interval (0<di<0.2, 0.2<di<0.4 …). Taking all 25 years
days into account, we compute the relative frquency of IE whithin each interval, which
provides the probability that any day can be an IE as a function of di(Z‘). We also computed
the rain composite for gridpoint 6 located near Camedo, taking again into account ALL the 25
years days, whether wet or dry, for each di intervals. Finally, the number of days with IE (>40
mm) and very IE (>60mm) WITHIN each class were computed for each di interval.
3. Results
The 90% significance level could not be reached when IE were selected using a 17 gridpoints
domain around Lago Maggiore with gridpoints lying on both sides of the mountain ridge
between the Rhone valley and Ticino. That’s why we reduced the number of gridpoints to 10
in order to encompass the region south of the Alps only (figure 2). Only classifications within
3 clusters were significant at the 90% conficence level.
3.1 Geopotential anomalies of cluster center
Left pannels of figure 3 show the average Z‘ of the 3 classes of LSC for IE over the Lago
Maggiore region; for comparison the corresponding patterns for IE over the Alpes Maritimes
are displayed ont the right pannels: although 2 classes were prefered for this later sub-region,
it could be observed that the cluster centers for classifications into 3 classes were also quite
robust (despite the significance level fell below 90%, see Vautard and Plaut, 1998). Cluster 1
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is very similar to the Greenland High (GASC) of the Maritimes diplayed next (the same
pattern as with 2 classes). Both cluster 2 (Quadrupole, QP) and cluster 3 (Ireland Low, IL) can
unambiguously be associated with rather similar patterns corresponding to IE over the Alpes
Maritimes. The most striking differences between the left and right pannels of figure 3 are
following:
i) the GASC cluster for the Alpes Maritimes (right) diplays a much more promounced Low
south of Ireland;
ii) although cluster 3 share quite similar a minimum close to Ireland, the Eastern Europe high
is much more pronounced for the Alpes Maritimes; indeed, the High seems to play an
important role in blocking the perturbation over the Western Mediteerranean in the later case;
iii) for QP, the center of High is over Scandinavia for the Lago Maggiore, but over Atlantic
for the Alpes Maritimes.
Figure 3. Panel a) Z700 anomaly field, Z‘, for the 3 cluster centers of large scale circulations
corresponding to IE over Ticino;
panel b) The same for classification into 3 clusters of LSC corresponding to IE over the
Alpes Maritimes
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Note that almost the 3 same clusters were found using the 17 gridpoints of the larger domain
extending both sides of the mointain ridge between Ticino and the Rhone valley, altough the
significance level remained well below 90%. With a „Rhone“ domain made up to the 7
gridpoints north of the Alps ridge alone, it’s interesting to note the emergence of a new cluster
with a pronounced Low over southern Scandinavia (not shown); such a cluster was shown to
supply most intense snowfalls (winter IE) over the Savoy-Mont Blanc domain in (Plaut,
1999).
3.2 Contingency tables and relative occurrence of IE
Figure 4 upper pannels show that most days Z‘ fields within the 25 years don’t correlate to Z’i, the ith
cluster center anomaly field. Repartition are nearly normal with a light maxima right (interval d=1 to
1.2); the same was true for the Alpes Maritimes.
Figures 4 lower pannels display the relative frequency of IE for each di interval; like for the
Alpes Maritimes, cluster 1 (GASC) exhibits the highest discriminating power, since 38% of
the days with d1<0.2 (55% for the Alpes Maritimes) are actually IE. The percentage drops
down to 26% for IL, and 17% for QP.
Figure 4. a) Histograms of contingency tables of all 25 years days against di, the angular
distance to cluster I center ( see text for definition of di);
b) Probability against di od any day being a class I IE
3.3 Rain composites
Figure 5a show the rain composites for gridpoint 6 near Camedo; remember the computation
takes again into account ALL the 25 years days. The results are consistent with the relative
occurences of IE displayed on figure 4 right pannels: the biggest amount of rain is observed
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for GASC (30 mm), whereas it is 23 mm for IL and 11 mm for QP. In comparison to the
Alpes Maritimes, two features may be underlined:
i) the amount is higher for this gridpoint that for gridpoint 9 of the Alpes Maritimes; in the
same way, the annual amount of rain is much greater in Ticino than for any Alpes Maritimes
gridpoint (2400 mm versus less than 1100 mm).
ii) GASC brings three times more rain than QP. For the Alpes Maritimes gridpoint 9, GASC
brought only twice more rain than QP.
We also computed rain composite patterns for IE belonging to each cluster (not shown). Let
just quote that the shapes of the patterns are almost undistinguishable, with a maximum height
near Camedo for the 3 clusters; however the amounts are highest for GASC everywhere.
3.4 Contingency tables of intense and very intense events
The histograms of figure 5b show, for each cluster population, the number of intense (shadded
bars) and very intense events (VIE) (dark bars) against di, the correlation distance to their
own cluster center. The theshold value for a very strong event is 60 mm. Unlike for the IE, the
greater number of VIE isn’t located at 0<di<0.2. The poor correlation of VIE with di may be
due to a bias in the stations (Frei and Schär, 1998), especially in northern Italy where the
network of stations is rather sparse. Furthermore, the majority of stations are located in the
valleys. It means that the topographical surroundings are unequally represented. For example,
there aren’t enough of stations on mountain summits.
Cluster 1 again benefits from the best discriminating power, The number of IE and VIE for
the WHOLE IE population are displayed against d1 on figure 5c; one should notice that these
histogramms are much wider than those of pannel a or b, which means there is a true
collimation of IE histograms are displayed agains d2 or d3 (not shown), or even against the
direction of the composite of LSC of all IE (fig. 5d); it is amazing that in this last case one
finds NO EVENT with d<0.2: the composite pattern of all LSC of IE trul appears as pointing
towards a sparsely populated phase space direction, unlike the cluster center patterns.
3.5 Trends
Unlike for the Alpes Maritimes where a strong negative trend was found for IE (the average
height amount from IE decreased by almost 150 mm during the APC period), and the MontBlanc massif, where a 50% increase occurred, trends are much less significant of IE over
Ticino. In this region, the number of IE remains more or less stationnary. We note however a
light increase in the precipitation amount by IE event.
If we consider the trend for the months from April to September only, we find a increase of
the IE for the Lago Maggiore region. On the other hand, if we consider the months from
October to March only, we note a decrease of the IE. It is consistent with what was found for
the Alpes Maritimes, where IE occurs only in the winter half year.
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Figure 5: a) All 25 years days gridpoint 6 rain composite against di;
b) Histograms of contingency tables of class I IE against di;
c) Histograms of contingency table of ALL IE against d1, the angular distance to cluster 1
(GASC)center. Vertical scale has been changed by a factor 2 with respect to b).
d) Same as c), but for the angular distance to all IE composit Z‘.
4. Conclusion
Intense precipitation event occur in the Lago Maggiore and the Alpes Maritimes above all
when Greenland High and Low over Bretagne appears at 700 hPa level (cluster 1). Both the
relative occurrence of an IE being in cluster 1 and the rainfall amount are the greater for the
cluster 1 for both regions. The amount of rain is however greater in the Lago Maggiore
region. The proportion of very intense events is also only correlated with the cluster 1. Unlike
the decrease of days with intense events in the Alpes Maritimes, no significant trend is being
noted for the Lago Maggiore region if the whole year is taken into acount. However, we note
an increase in summer IE (April-September) and a decrease in winter IE (October-March).
As the IE occurs only in winter in the Alpes Maritimes, we can suppose a diminution of
circulation patterns bringing IE in the south of the Alps in winter.
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5. Bibliography
- Ceschia, M., Micheletti, S. and Carniel, R., 1991. Rainfall over Friuli-Venezia Giulia: High amounts
and strong geostrophical gradients, Theoretical and Applied Climatology, 43, 175-180.
- Fliri, F. and Schüepp, M., 1983. Synoptische Klimatographie der Alpen zwischen Mont-Blanc und
Hohen Tauern, wissenschaftliche Alpenvereinshefte, 29, 686 pp.
- Frei, C. and Schär, C, 1998. A precipitation climatology of the Alps from high resolution rain gauge
observations, International Journal of Climatology, 18, 873-900.
- Frontero, P., Pugnaghi, S., Lombroso, L., Santangelo, R., Quintavalla, C. and Selvini, A., 1998.
Forecasting intense precipitation events on southeast slopes of Alps, MAP Newsletter, 9, 18-19.
- Grebner, D., 1994. Meteorologische Analyse des Unwetters von Brig und Saas Almagell vom 24.
September 1993, Wasser, Energie, Luft, 86. Jahrgang, Heft 1/2, 41-44, Baden.
- Grebner, D. and Richter, F.G., 1991. Ursache der Hochwasser 1987. Ergebnisse der
Untersuchungen. Mitteilung des Bundesamtes für Wasserwirtschaft Nr.4, Mitteilung der
Landeshydrologie und –geologie Nr. 14, 23-40, Bern.
- Jansà, A., Genovés, A., Campins, J. and Picornell M.A., 1995. Mediterrannean cyclones and Alpine
intense-rain floods events, MAP Newsletter, 3, 35-36.
- Kessler, J. and Chambraud, A., 1990. Météo de la France. Tous les climats localité par localité,
éditions J.C. Lattès, 391 pp.
- Michelangeli, P-A., Vautard, R. andLegras B., 1995. Weather regimes: Recurrence and quasi
stationnarity, Journal of Atmospheric Sciences, 52, 1237-1256.
- OMM/WMO, 1995. rapport annuel 1994, No 824, 61 pp.Genève.
- Plaut, G. and Vautard, R., 1998. Classification of z700 field for intense precipitation days over the
Alpes Maritimes, Accord first annual report, unpublished.
- Rakovec, J., Gregoric, G., Vrhovec, T., Zagar, M. and Strajmat, U., 1998. Forecasting intense
precipitation events on southeast slopes of Alps, MAP Newsletter, 9, 18-19.
- SMA/ISM/SMI, 1995, Annalen 1993 der Schweizerischen Meteorologischen Anstalt, Zürich.
- Widman, M.L., 1996. Mesoscale variability and long term trends of Alpine precipitation and their
relation to the synoptic-scale flow, Dissertation ETH No. 11769, 185 pp., Zürich.
- Zeller, J., Geiger, H. and Röthlisberger, G., 1990. Starkniederschläge des schweizerischen Alpenund Alpenrandgebietes, Eidg. Forschungsanstalt für Wald, Schnee und Landschaft, WSL, Nr. 5,
Birmensdorf
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