Introduction
High-impact weather events may have dramatic impacts on society, being a
problem that could be potentially enhanced under a changing climate
. Such events not only lead to great economical damage, but also to
personal damage . Tropical but also
extra-tropical cyclones can certainly be classified as high-impact weather
events, as they are associated with extremely strong winds and heavy
precipitation that eventually can lead to floodings .
In Central Europe, and more precisely in the
northern Alpine region, a source of high-impact events is associated with the
occurrence of Vb cyclones. Such a type of cyclone was first mentioned by
, and later defined by in a cyclone
classification based on its characteristic pathway (Fig. ). The categories were labelled according to Roman
numeration from one to five. The Vb track (a subcategory within the fifth
category, and the only one still in use) is associated with extreme
precipitation and flash floods over Germany, Austria, Switzerland, the Czech
Republic and Poland. The origin of Vb cyclones is either the Bay of Biscay,
the Balearic Sea or the Ligurian Sea, where moisture uptake occurs. The
cyclone moves eastward via Italy and the Adriatic Sea, before it turns
northward to the Black Sea or Saint Petersburg. Along this track,
orographically induced rainfall takes place on the northern side of the Alps
and its foothills. Note that a Vb cyclone is normally not associated with
heavy precipitation events on the southern slope of the Alps. Heavy
precipitation events on the southern side of the Alps can already be
triggered by deep upper-level troughs over the Western Mediterranean and by
Genova cyclogenesis .
Hereinafter the expression Vb cyclone is used for cyclones that follow van
Bebber's track.
Despite the destructive potential of Vb cyclones, literature provides only
little information about its characterisation. Most studies on Vb cyclones
focus on case studies. For instance the one-in-a-century flood in August
2002, induced by a Vb cyclone, has been analysed extensively
.
A main focus in these studies is the moisture source. Although
the Mediterranean Sea is an important source for precipitable water, other
studies suggested that the evaporation from land contributes to the
precipitation amounts .
Additionally, the Atlantic Ocean and long-range advection of moisture cannot
be despised, as demonstrated by using a tracer for
water vapour. Another focal point is set on the synoptic-scale conditions
leading to the extreme event in August 2002 .
suggested a positive interference of
several factors that occurred in August 2002. These factors include advection
of humidity, a quasi-stationary tropospheric trough, inducing upper-level
divergence, and orographic lifting of the surface low.
concluded that the anomalous large-scale situation in the summer of 2002 and the
associated increased cyclone-activity is the main reason for the devastating
floods in August 2002. Other Vb cyclones that appear as case studies in
literature is cyclone “Axel”, in July 2001, which is related to the
Vistula flood . More important for Switzerland is the
Vb cyclone in August 2005, which led to a very severe flood on the northern
flanks of the Alps . The author found that the peak
rainfall occurs in August, and is due to a combination of warm ground
surfaces and moisture convergence into the Alpine region. Another event
happening in June 2013 is associated with cyclones that are non-standard
Vb systems, according to their paths, but nevertheless caused floods
affecting the Danube and Elbe catchment . The authors
recognised equatorward ascending warm conveyor belts as key processes for the
heavy precipitation, which are fed by evapotranspiration from soil moisture.
Fewer studies go beyond case studies and analyse a Vb cyclone climatology.
provided an objective catalogue of Vb cyclones, which
allowed the authors to infer that these are only rare events (3.5/year) with
a peak occurrence during April. focused on the summer
half year of a future climate. They projected a decrease in the total number
of Vb cyclones, although their related mean precipitation increases. An
extended study, considering the last 500 years of flood history, only found a
weak relation between Vb pathways and flood occurrence .
Although there is only little literature specifically focused on
climatological Vb cyclone characteristics, there are several studies devoted
to the cyclones in the Mediterranean region, a more general category to which
the Vb cyclones belong to. performed an objective
climatology of cyclones in the Mediterranean region, concluding that the
Genoa region, which is also the main origin of Vb cyclones, generates most of
the cyclones in the Mediterranean region. The authors also stated that
topography-controlled cyclogenesis regions account for the most intense
events. showed that the period from 1881 to 2001, which
includes the most extreme precipitation events, reveals a more frequent
appearance of the weather pattern trough over Middle Europe. This pattern also
encloses the Vb cyclones. analysed a similar,
although shorter period of time (1995–2002), focusing on precipitation over
the Alps. They determined a seasonal change in the moisture source for both
sides of the Alps, i.e., the Mediterranean Sea and the Atlantic Ocean
are the major sources in winter for the southern and northern slopes of the
Alps, respectively, whereas soil moisture content is predominant in summer.
Hence, Vb cyclones show all the prerequisites needed to trigger high-impact
weather events.
Cyclones in the Mediterranean region are frequently analysed under future
climate change, as the Mediterranean Basin is a key hotspot for societal
vulnerability . Unfortunately, there is still no
clear agreement on the trend in cyclone numbers in a future climate. Some
studies concluded that there will be a general decrease in the number of cyclones
during the entire year or just in winter
, due to either a polar shift of cyclones, a
positive shift of the NAO, changes in baroclinicity or static stability.
However, other analyses reported an increase in the total number of cyclones
over the Mediterranean . The studies also
disagreed with respect to extreme events, where and
found an increase, whereas and
found a decrease in extreme cyclones in the
Mediterranean. Additionally, indicated no significant
changes concerning the track properties and precipitation. This is in
contrast to , who found a pronounced decrease, especially in
summer precipitation, due to an intensified anticyclonic ridge. Nonetheless
an increase in precipitation events, especially over and around the Alpine
region, can be expected especially in winter according to the study of
due to the fine scale structures in the Alps. Furthermore
identified two opposing factors in the Mediterranean
region, which might be responsible for the large inter-model spread of the
CMIP5 models. On one hand these are a general increase in atmospheric
moisture content and thus, an increased cyclone precipitation intensity. On
the other hand found a reduction in precipitation
intensity due to a dynamical weakening of the cyclones. The lack of
consistency that emerges from these studies illustrates the difficulties
involved in climate projections, and point out that a deeper process
understanding is important to reduce uncertainties.
As outlined above, only basic climatologies of Vb cyclones in the past
and the future have been
performed so far. However, a comprehensive analysis concerning the triggering
mechanisms driving the extreme precipitation associated with Vb cyclones is
still missing. Thus, in this study we provide a climatology of Vb cyclones
and also explore the physical mechanisms that impact precipitation amounts of
Vb cyclones. The climatology is performed for a 35-yr period using the
ERA-Interim reanalysis, whereas the physical mechanisms are studied in more
detail for Vb cyclone subcategories.
The structure of this paper is as follows: Sect. provides
an overview of the data sets and methods used in the study. Section
describes the basic climatology of Vb cyclones, gives
insight into the Vb cyclone variability, and investigates their underlying
physical mechanisms. Finally, Sect. provides a summary
and discussion of the results, presenting also a short outlook.
Data and methods
Reanalysis and observational data sets
This study is based on ERA-Interim, a global atmospheric reanalysis data set
produced by the European Centre for Medium-Range Weather
Forecasts (ECMWF). ERA-Interim provides 6-hourly estimates of
three-dimensional meteorological parameters and 3-hourly estimates for
surface variables for the time period between 1979 to present. The
ERA-Interim data set is generated by running the 2006 version of the
Integrated Forecast System model of the ECMWF with a resolution of T255
(approximately 80 km) and 60 vertical levels up to 0.1 Pa. The system
assimilates observational data with a 4-dimensional variational analysis
(4D-Var) in a 12 h analysis window. A number of observational data sets are
assimilated in the final product ranging from satellite data to surface
pressure observations and radiosonde profiles see Sect. 4 in.
In this study we analyse Vb cyclones for the 35-year period 1979–2013 at
6-hourly resolution. To facilitate the comparison, the 3-hourly forecasted
precipitation data are accumulated to 6-hourly data. Furthermore, a vertical
integration of moisture in the atmosphere is computed to obtain the total
amount of precipitable water. As precipitation data in ERA-Interim are
predicted using a forecast model, they are subject to spin-up and spin-down
effects that need to be kept in mind .
Since some important results in this study are based on precipitation
amounts, and given that ERA-Interim reanalysis suffers from the
aforementioned spin-up and spin-down effects, the precipitation data are
compared to the purely observational gridded product E-OBS
. The E-OBS data are based on weather station data and
are interpolated to a 25 km grid. It is a European land-only, daily, gridded
data set. Several variables are available: precipitation, sea level pressure
and mean, minimum and maximum temperature for the period 1950 to 2013
. As this data set is based on observations only, there
are a number of limitations that need to be mentioned. On the one hand the
observations show several inhomogeneities in space and time that populate
observational products with uncertainties rendering them mutually inconstant
even in areas properly covered by observations . The
spacial inhomogeneity is due to a different density of the observational
network of each country, whereas the temporal heterogeneity is based on the
varying number of stations maintained by the countries .
On the other hand precipitation is especially affected by uncertainties over
mountain areas, such as the Alps, our region of interest. Note that the
uncertainties are maximal in the summer, as precipitation is driven by
convection . This form of precipitation is very local,
and thus difficult to capture with the sparse network covering the Alps
.
Detection and tracking methods
In this study we use a tool developed by that
automatically detects and tracks all types of cyclones within a certain area.
Thus, in a first step all cyclones that occur over Western and Central Europe
during the 35-year time period are retained. Note that different tracking and
detection methods identify comparable characteristics of midlatitude cyclones
. For the Mediterranean, different methods show
some disagreement in particular with respect to the identified number of
cyclone, but agree in terms of location, annual cycle, and trends of cyclone
tracks . The technique has a number of free parameters
that allow to adjust the search according to specific purposes. In our case,
these parameters are fit to special characteristics of Mediterranean
cyclones. In particular, since they develop as shallow low-level cyclones,
weak gradients must be chosen in order to be able to detect these at the
beginning of their life cycle.
Since the Alps introduce disturbances into the geopotential height field,
and to remove small-scale and secondary low pressure centres, the input data z850
from ERA-Interim is previously low-pass filtered using a weighted
average of 5 × 5 grid points prior to the analysis of the tracks. The
weights are defined according to a Hann-window function. The 5 × 5
window has been chosen by analysing the robustness of the obtained
trajectories after different window sizes are applied in the smoothing (not
shown). Similarly, several levels of the geopotential height fields are
tested within the detection and tracking technique, using finally the 850 hPa
level. The reason is on the one hand to have a balance between the shallow
character of Mediterranean cyclones and therefore a level close to the
ground. On the other hand, it is hardly possible to find meaningful tracks at
the surface, as mountains introduce substantial artefacts into the
geopotential height field that render the cyclone tracking more difficult.
Tests with several levels and filters have shown that the prominent
Vb cyclones (Alpine flood 2005, Elbe flood 2002, Axel 2001) can be identified
in the 850 hPa level in combination with a 5 × 5 grid point weighted
average low-pass filter, so this is the configuration employed in the manuscript.
The position of the cyclones is identified by local minima in the
geopotential height at 850 hPa (z850), taking the eight neighbouring grid
points into account. The minimum gradient around a cyclone centre in an area
of 1000 × 1000 km2 is used to focus on cyclones, thus filtering
out polar and weak minima such as heat lows. For this study we applied a
minimum gradient of 25 m/1000 km. Additionally, a maximum gradient of
50 m/1000 km must be reached at least once during the life cycle of a cyclone.
The minimum lifetime of each cyclone is set to 24 h. The minima are
combined by a next neighbourhood search within a distance of 1000 km. This
threshold is chosen as it resembles roughly the Rossby deformation radius.
The three boxes used to automatically filter out the Vb cyclones
from the total number of cyclones found by the tracking technique
. O, E and R denote the origin, the end and the
restriction box, respectively. The black box over the alpine region defines
the area of interest for precipitation amounts. The two stippled black lines
indicate the position of the two cross-sections used to calculate the
moisture flux. The topography corresponds to the one implemented in
ERA-Interim.
As the tracking tool detects all cyclones, not only Vb cyclones, the output
of this tool has to be further filtered. For this task, we define areas
(boxes) where a potential Vb cyclone shall pass (or not pass) at least once
in its lifetime in order to be retained. The origin box
(42–46∘ N, 4–13∘ E) accounts for the
fact that Vb cyclones, per definition, either develop or intensify over the
Mediterranean Sea close to Genoa, while the end box
(46–52∘ N, 12–19∘ E) assures the sudden
turnaround northward at the eastern edge of the Alps. Note that the cyclone
can leave the end box towards the east at the end of its life cycle. The
purpose of the boxes is to guide the cyclones around the Alps. As soon as
this task is fulfilled the cyclones can move freely. A third restrictive box
(46.5–55∘ N, 5–11.5∘ E) covering the
Alps and the eastern part of Germany is introduced to avoid that cyclones
stay at any time on the northern side of the Alps before they move around the
Alpine range on its eastern side. These three boxes are displayed in
Fig. and are labelled with O for the origin, E for the end and
R for the restriction box. Note that these simple criteria are similar to that
described by .
Composite analysis of midlatitude cyclones
A prominent problem when analysing the structure and physical processes
related to cyclones is that they do not occur at a fixed location, but they are
moving objects. Hence, a simple temporal mean becomes misleading due to the
different location of the storm. A simple approach to overcome this problem
is to use a moving grid whose centre coincides with the storm at each time
step. However, this still has a problem when the analysis is performed on a
regular latitude-longitude grid (as is the case of ERA-Interim). This is so
because the area of each grid box relative to the centre of the cyclone
decreases with higher latitudes, so each grid point might be representative
of a different area in different time steps if the storm moves northward.
This is not a major problem in tropical cyclones, since the effect becomes
insignificant near the equator, but it is a matter of concern in the
midlatitudes, precisely where Vb cyclones evolve. Since this study aims at
analysing temporal composites of several variables, or the most precipitation
intense time step in different cyclones, this becomes a technical challenge
that has to be addressed.
This study applies a composite tool based on the projection described by
. The method works as follows. The variable of
interest, defined on a latitude-longitude grid, is first remapped onto
spherical coordinates in a 0.5∘ × 0.5∘ resolution grid, where
the cyclone's centre is set in the pole of the grid. This grid extends with a
radius of 23∘ around the cyclone's centre. Hereby, a spherical cap is
obtained for each time step, which is always directed towards the north. Once
the variable of interest is remapped onto this grid (which is different for
each time step), all regular statistics can be calculated over this variable
(means, maximum, etc.), and the results are fully comparable among different
time steps and storms.
Selecting the precipitation influencing time steps in the life cycle of Vb cyclones
In the following analysis we focus on precipitation triggered by Vb cyclones.
The area of interest for precipitation covers the northern slopes of the
Alps, the southern part of Germany, Austria and the southern part of the
Czech Republic. This area is depicted in Fig. as a black
rectangle. The beginning and the end of the tracks of the Vb cyclones can be
located, e.g. in the Atlantic Ocean and far up in the north of Poland or
even Russia, respectively. If the centre of the cyclone is placed that far
away from the black box described above it must be assumed that the cyclone
no longer influences the falling precipitation in this area. Furthermore,
it is more likely that a different weather feature, like a frontal system,
disconnected from the Vb cyclone, produces precipitation in that area. To
omit this we make use of the composite tool described in
Sect. in a slightly adapted way, such that only time
steps are included in the analysis which exhibit a cyclone centre close
enough to the region of interest. Thus, the radius of the grid area around
the cyclone's centre in the composite tool, which is influenced by the
cyclone, depends on the gradient of the cyclone, and thus on its intensity.
Hence, Vb cyclones ascribing a gradient within plus or minus 1 standard
deviation obtain a radius of 6∘. Gradients that exceed (fall behind)
one standard deviation, 75 (25) percentile or 95 (5) percentile obtain a
radius expansion (decrease) of 0.5, 1 or 1.5∘,
respectively. Only if this radius is able to reach the precipitation box
depicted in Fig. , the time step is considered as
precipitation contributing. Note that the flexible radius in combination with
the composite tool is only used to define the precipitation influencing time
steps. For all other variables the composite tool is applied with a fixed
radius of 23∘ as described in Sect. .
Results
Basic climatology of Vb cyclones
Vb cyclones are relatively rare events compared to the frequency of cyclones
detected over the Mediterranean and Europe. Applying the tracking approach of
to the smoothed z850 surface, 3448 cyclones are detected
over Europe between 1979 and 2013. After filtering out the cyclones with the
boxes described in Sect. , a total of only
82 cyclones is classified as Vb cyclones, i.e. only 2.4 % of all cyclones in
Western and Central Europe. Due to their rareness, the average appearance of
Vb cyclones per year is 2.3, with a mean duration of 3.1 days. Still, the
occurrence of these events is irregularly distributed over the 35-year period
(for instance up to five Vb cyclones are tracked in 1979 and 1984, while none
is found in the years 1989, 1993 and 2011). However, considering ERA-Interim
(E-OBS) data set in this period of time the rare Vb events are responsible for
almost 15 % (14 %) of extreme precipitation days in the northern Alpine
region and Central Europe. Here, extreme precipitation days are defined as
exceedance of the 99 percentile. Hence, we note that even though Vb events
are relatively rare, they have a great potential to trigger high-impact
weather events.
Using a similar approach, reported an annual average
of 3.5 Vb cyclones per year. The climatological probability of Vb cyclones to
appear on any day is 3.8% in their analysis, compared to the 2.0 % found in
the present study. The potential reason for this discrepancy is that their
method substantially differs from the one used here: different input data,
tracking tool, and posterior filtering. Still, 62 % of the Vb cyclones
tracked in this study coincide with those found by ,
considering only the overlapping period from 1979 to 2002 of the two studies.
Probability density field of all detected Vb cyclone centres (top
panel), of the heavy precipitation events (HPEs) (bottom left panel) and of
the weak precipitation events (WPEs) (bottom right panel). The shading shows
how probable it is that a Vb cyclone centre is located at the according grid
point at any time step in the 1979–2013 period.
Beyond the irregular distribution of the 82 Vb cyclones over the analysed
period, they are also not evenly distributed within the annual cycle.
Considering the standard seasons winter (December, January, February), spring
(March, April, May), summer (June, July, August) and autumn (September,
October, November) there are less Vb cyclones in winter than expected from a
homogeneous distribution over the annual cycle (p level < 0.1) and an excess
in spring (p level < 0.05). The fact that Vb cyclones emerge more frequently
in spring has already been pointed out by and confirmed
by , which reported a maximum in April.
The Vb cyclones are also characterised by their trajectories. The probability
density of all Vb cyclone centres is depicted in the upper panel of
Fig. and estimated using a Gaussian Kernel Density Estimator
with a bandwidth of 0.6∘. Thus, the shaded area indicates how likely
it is that a Vb cyclone centre passes through this grid point at any time
step. The density illustrates the most common pathways followed by
Vb cyclones, and fits very well the track described by ,
illustrating why this categorisation is still in use. As expected the Genoa
region is most frequently passed by the cyclones, as this region coincides
with the origin box. Even a rare Vb cyclone, which develops in the lee of the
Atlas Mountains, is detected once. Note that the end of the Vb cyclone track
is more diverse, given that the last time steps of the Vb cyclones are no
longer bound to any box. The effect of the restriction box is clearly visible
as a sharp bend of the orange-coloured contours at the western flanks of the Alps.
Variability within Vb cyclones
In this section we build upon the analysis of Vb cyclone characteristics
focusing on remarkable differences between these events, particularly with
respect to precipitation. Thereby, Vb events are classified by using a
precipitation criterion. For this, the mean precipitation over a box covering
the Alpine region is calculated for each time step. The box is depicted in
Fig. as a black rectangle. In the following, the
precipitation of the precipitation contributing time steps (as described in
Sect. ) is accumulated over each Vb event.
Note that this has the disadvantage that the accumulation period differs
among various Vb cyclones, which has to be taken into account in the analysis hereafter.
Probability density function of the accumulated precipitation from
ERA-Interim of Vb events (black line) for extended winter (left panel) and
extended summer (right panel). The red lines indicate the 25, 75 and
95 percentile from left to right. The vertical, black lines indicate the
accumulated precipitation of all Vb cyclones occurring during each season,
respectively.
Average precipitation during the time step with maximum
precipitation for the 10 HPEs (left panel) and the 10 WPEs (right panel). The
black box indicates the precipitation region of interest, depicted also in
Fig. as a black rectangle.
To compare the precipitation triggered by Vb cyclones to regular
precipitation days, the distribution of such precipitation needs to be
estimated. As the Vb cyclone duration differs from one another, the
estimation of the precipitation distribution becomes challenging. Hence, a
bootstrap method is applied to estimate such precipitation distribution over
this particular box that has to mimic the real distribution of Vb cyclone
duration. The estimation is based on 3 million bootstrap samples of observed
precipitation accumulated during a period whose length is selected randomly
according to the observed distribution of length of Vb cyclones. The same
method is applied to the ERA-Interim and daily E-OBS data set to confirm
consistency between observational data and forecasted reanalysis data.
Figure displays the estimated distribution of precipitation in
the box covering the Alpine region for ERA-Interim. Using ERA-Interim (E-OBS,
not shown) data set, only two (none) of the Vb cyclones exceed the
95 percentile in winter, whereas in summer, 24 (19) Vb cyclones produce extreme
precipitation over the region of interest. Note that there is a wide
variability in accumulated summer precipitation within Vb cyclones, ranging
between almost no precipitation and extreme events. Furthermore, most of the
winter events show a narrow variability. For consistency reasons and the sake
of brevity only the results for ERA-Interim data are shown hereafter,
although it has to be noted that the results for the E-OBS data set resemble
the findings based on ERA-Interim discussed here in more detail. The striking
difference between summer and winter is most probably due to the effect of
the Clausius-Claperyon equation, which relates air temperature to its ability
to carry water vapour.
To gain further insight in the characteristics of Vb cyclones, those related
to heavy precipitation events (HPE) and weak precipitation events (WPE) in
the extended summer season are analysed in detail. Thereby, the 10 most
extreme events with respect to precipitation are selected.
Figure displays a composite of the most intense precipitation time
step of the 10 HPE on the left and WPE on the right. As expected, the HPEs
generate much more precipitation and affect a wider area. Locally, the
precipitation amounts are even doubled compared to the WPEs. More interesting
is the fact that the HPEs composite shows a precipitation pattern that is
expected from Vb events. The main precipitation falls on the northern flanks
of the Alps and extends towards the east as far as the catchment of the river
Elbe and Oder, which were the main contributors of the floods in August 2002
. In contrary the WPEs show maximum precipitation more to the east
of the Alps. Additionally, there seems to be frontal behaviour that
interferes with Vb cyclones in some of the WPEs which is illustrated by the
long precipitation band in Eastern Europe. It is important to note that even
though precipitation patterns are different, the associated PDFs of the
trajectories of HPEs and WPEs, depicted in the lower panels of
Fig. , do not show significant differences. Thus, the
trajectory of the Vb cyclone does not seem to play an important role on
deciding whether an event will cluster into the HPE or the WPE subcategories.
Composites of precipitable water content [kg m-2, shading] and
precipitation amounts (black contours) for the HPEs (left panel) and for the
WPEs (centre panel). Additionally the moisture flux is shown by arrows
integrated over the vertical structure of the atmosphere (reference vector:
300 kg m-1 s-1). The time step with maximum precipitation is
shown for all variables. The distance from the centre of the composite
cyclone to the edge is 23∘. The difference in precipitable water
between HPEs and WPEs is shown (right panel).
Physical mechanisms driving Vb cyclones variability
As precipitation amounts are linked to available moisture content in the
atmosphere, it makes sense to investigate whether the state of the atmosphere
plays a prominent role on the rainfall associated to a Vb cyclone
. The precipitable water at the most
precipitation intense time step shows a much higher amount for the HPEs, than
for WPEs (Fig. ). Still, the differences around the centre
of the cyclone are small compared to the high moisture band further off the
centre. The moisture fluxes depicted in Fig. imply that
the major part of this moisture is transported straight to the northeast, and
thus away from the northern Alpine region for the WPEs and the HPEs. Note
that some of this precipitable water can lead to precipitation further north
over, e.g. Eastern Germany. The additional precipitation in the northeast of
the box can easily be detected in Fig. . This is especially
true for the HPEs. Furthermore, the case-to-case variability in precipitable
water is relatively large, and indeed some HPE cases contain even less
precipitable water than certain WPEs. Hence, precipitable water in the
atmosphere is not an unambiguous variable suitable to predict whether a
Vb cyclone would potentially lead to severe precipitation.
Probability density function of the accumulated moisture flux of all
Vb events (black line). The vertical top (bottom) lines indicate the
accumulated moisture flux of the HPEs (WPEs) through the stippled black lines
depicted in Fig. labelled with “MED” (left panel) and
labelled with “ATL” (right panel). The red lines indicate the 5, 25, 50,
75 and 95 percentile from left to right.
A related variable that is in principle more accurate to characterise the
differences observed between HPEs and WPEs is the moisture flux through
certain latitude sections of interest. This allows testing the hypothesis
generally assumed stating that Vb cyclones receive most of their precipitable
water from the Mediterranean Sea. Therefore, a latitudinal section over the
Adriatic Sea is selected as depicted in Fig. with a
stippled black line labelled MED. This section is chosen because it is
located over a region of high vertical moisture transport. Results of the
moisture flux through this section indicate however that no clear separation
of the moisture flux across this line between the HPEs and WPEs is possible
(left panel of Fig. ). Thus, this criterion is not suitable
to characterise the high-impact related Vb cyclones. Similarly, a second
cross-section over France and Switzerland labelled ATL in
Fig. allows to analyse the southward moisture flux from the
North Atlantic Ocean. As the right panel of Fig.
demonstrates, this cross-section enables a slightly clearer separation
between HPEs and WPEs than the moisture fluxes across MED line. The
increased transport of HPEs suggests that a large part of moisture is
transported from the North Atlantic, instead of the Mediterranean Sea. Still,
no clear separation between the HPEs and the WPEs is found, especially
because it is very sensitive to the exact location of this cross-section (not
shown). Hence, these results indicate that moisture variables alone do not
allow explaining the different behaviour observed between HPEs and WPEs.
Difference between the minimum and maximum pressure during the
entire life cycle for the HPEs (left panel) and WPEs (right panel). The
events are sorted according to precipitation produced in the target area
(Fig. ), with #1 being the most extreme precipitation
event. The three horizontal black lines indicate the mean and the standard
deviation, respectively.
Average geopotential height at 850 hPa (left column panels),
500 hPa (centre column panels) and 300 hPa (right column) for the time step
with maximum precipitation of all HPEs (upper row panels) and WPEs (bottom
row panels). The distance from the centre of the composite cyclone to the
edge is 23∘.
Since the thermodynamic state of the atmosphere cannot present a distinct
explanation for the within-Vb variability, we turn our attention to the
dynamical mechanisms. One reason for the different amounts in precipitation
between the HPE and WPE might be the speed of the cyclone. Fast moving Mediterranean
cyclones may not be able to pump up as much water from the ocean than slow
moving cyclones. Nevertheless a clear separation in cyclone propagation speed
between the HPE and WPE is not successful. Reasons for the deviations in
precipitation amounts and pattern among Vb cyclones are found more clearly in
the geopotential height field. Figure reveals that
the WPEs overall show a smaller intensification rate than HPEs (p level < 0.01).
Further differences in the geopotential height are found in the
spatial structure of the average state (Fig. ). The
HPEs and WPEs show similar features at first glance in the z850 field during
the most precipitation intense time step. In both cases a low pressure area
is localised in the centre of the storm. This is expected, as this is indeed
the criterion used for the detection tool applied in the first step.
Nevertheless, there are some important differences. The composites of HPEs
exhibit a strong cyclone, as a steep gradient in combination with a deep
depression is observed. The fact that the HPEs are triggered by a distinct
cyclone is more obvious when considering higher levels, because the
depression from the ground extends through the 500 and 300 hPa levels.
Another indication for a strong developing cyclone is the westward tilting of
the system. This is in contrast with the WPEs, which are induced by a shallow
depression. Even though the z850 shows an isolated isobar, this feature is
lost at the 500 hPa and appears as a weakened trough at 300 hPa again.
Another important feature is detected in the northwest of the WPEs in terms
of a depression. This system seems to coalesce with the original depression,
which leads to an asymmetric geopotential height gradient on the southern
side of the cyclone centre.
The hypothesis of associating a stronger cyclone with HPEs is underlined when
analysing the potential vorticity (PV) at the 325 K potential temperature
surface (Fig. ). For the HPEs, it presents a PV-streamer
that is close to a cut-off. This feature is absent in the WPE cases, which
show instead a PV maximum in the northwest of the cyclone centre, similar to
the situation of the 300 hPa geopotential height field.
Composites of potential vorticity [PVU, shading] on the 325 K
potential temperature surface at the time step with maximum precipitation of
the HPEs (left panel) and the WPEs (right panel). Wind fields are also shown
for the 325 K potential temperature surface (reference vector:
10 m s-1). The distance from the centre of the composite cyclone to
the edge is 23∘.
The aforementioned differences in the geopotential height fields between the
HPEs and WPEs trigger important differences in the wind field in different
elevations. Note that the wind fields in different elevations are in good
agreement with the vertically integrated moisture flux in regard to direction
and relative strength. Figure shows that the HPEs
experience strong winds on the southern side of the cyclone centre,
transporting air masses directly towards the northwest of the cyclone and out
of its influence region. More importantly, rotation around the centre of the
cyclone becomes apparent. The same features are also visible on the 325 K
potential temperature surface in Fig. , where air masses are
transported towards the northeast due to its U-shape, whereas the wind system
close to the centre rotates. This rotation is highly important for
high-impact Vb cyclones (HPEs), as moisture needs to be transported around
the Alps to produce orographic lifting along the northern side of the Alps.
Hence, orographic precipitation is generated on the northern side of the
Alps. This is supported by the fact that a major part of precipitation
amounts is actually found on the northeastern side of the Alps (not shown)
and thus in the region of interest during the most precipitation intense time
step. In contrast, WPEs do not exhibit such a rotating wind field. This is
mainly due to the influence of the deeper depression in the northwest of the
cyclones appearing at the same time as the WPEs. The strong gradient, which
is maintained at the southern side of the actual cyclone centre, results in
strong U-shaped wind fields that preclude rotation. Nevertheless, a certain
amount of rotation is found above the ground (not shown), which explains the
modest amounts of precipitation found in the region of interest. However, the
main part of the precipitation can still be detected on the southern side of
the Alps (not shown), as orographic lifting occurs there. Thus, the
precipitation on the southern side of the Alps is not able to influence the
main precipitation area of Vb cyclones during the most precipitation intense
time step.
Discussion and conclusion
The results concerning the basic climatology of Vb cyclones show a good
agreement with the findings previously reported by ,
i.e. the rareness of Vb cyclones (2.3 Vb cyclone appearances per year), the
peak of Vb cyclones in spring and a general agreement of the exact appearance
of 65 % of all Vb cyclones compared to . As our
findings seem to be robust with respect to the applied method, our study goes
beyond the statistical climatology introduced by and
deepens on the physical mechanisms in order to understand the large
variability within the Vb-cyclone-triggered precipitation.
The analysis of the precipitation distribution associated with Vb cyclones
reveals that the cases identified in the extended winter are not able to
trigger extreme precipitation. This fact can be explained through the
application of the Clausius-Claperyon equation. However, summer cases exhibit
larger variability, leading to a number of extreme situations. This motivates
a further subclassification of the summer cases using accumulated
precipitation over the northern Alpine region and Central Europe as a
classification criterion. Although the moisture content in the atmosphere
provides a first separation between the extended summer and winter
Vb cyclones through the Clausius-Claperyon equation, it fails to serve as a
criterion to separate the 10 WPEs and HPEs in the extended summer, since the
inter-case variability is too large. Thus, the moisture content in the
atmosphere cannot unambiguously separate the HPEs and WPEs. Also neither the
northward moisture flux from the Mediterranean Sea nor the southward flux
from the Atlantic can succeed in disentangling the different behaviour of
WPEs and HPEs. The fact that neither the Mediterranean nor the Atlantic Sea
are exclusively responsible for the precipitation brings us to the
conclusion that various moisture sources contribute to precipitation in
Vb events. This result is consistent with previous findings reported by
, and for the
one-in-a-century flood in August 2002. The large amount of possible moisture
source combinations and the various moisture patterns that are associated to
the Vb cyclones enable us to conclude that the moisture content and source
strongly depend on a case-to-case basis and preclude obtaining general conclusions.
In contrast, the variables associated to the large-scale dynamics,
i.e. geopotential height and PV at the potential temperature level 325 K, allow a
meaningful categorisation of the HPEs and WPEs. The average geopotential
height field in HPEs shows a distinct cut-off low pressure system extending over the whole
atmosphere. Additionally, PV shows a PV-streamer close to cut-off. These
two features trigger a vortex that can be traced in the wind fields. These
fields suggest that precipitation is triggered by a northerly Alpine inflow.
Thus, most of the precipitation falls on the northern to northeastern side of
the Alps. Similar situations (pivoting cut-off) have been found by
in the context of past extreme floods in Switzerland.
Hereby the cut-off low is located over the Adriatic Sea and is
near-stationary due to blocking surface highs, located over western and
eastern Europe. Also identified the orographic enhancement
as an important trigger for the high precipitation records in August 2002. The
WPEs in contrary are only associated with weak low pressure systems that do
not elongate through various atmospheric layers. Also PV reveals only an
initial state of a PV-streamer. Hence, there is no vortex visible in the wind
fields. In the case of the WPEs, we conclude that the Alpine inflow takes
place at the southern or southeastern side of the Alps, which is supported by
the mostly southerly located precipitation amounts. These features are
similar to the “Canarian Trough” described by in
association with past extreme floods in Switzerland. These cyclones are
strongly influenced by a low over Brittany and thus show a southwesterly flow
. The same is true for the WPEs, which are strongly
influenced by a low in the northeast of their cyclone centres. Additionally
it must be kept in mind that cyclone development in the vicinity of the Alps
is a very complex problem. Due to strong non-geostrophic secondary flows, the
appearance of near-surface closed isobars is not sufficient to produce
cyclonic vorticity . Thus, although WPE can be detected
as closed isobars, i.e. cyclones, these cyclones are shallow and show no real
meteorological impact, which stands in clear contrast to the cyclones
associated with HPE.
The fact that unlike humidity, the large-scale dynamic behaviour of the
atmosphere allows a clear differentiation between the HPEs and WPEs, leads us
to the conclusion that the thermodynamic state of the atmosphere only plays a
secondary role in triggering heavy precipitation associated to Vb events.
These findings have important implications for a future climate change. On
the one hand, an increased moisture amount is projected in the atmosphere as
a response to the increase in temperature with a changing climate (again
associated to the Clausius-Claperyon equation). Hence, an increase in
precipitation amounts can be expected in principle in the future. This
argument is supported by stating that an increase in
atmospheric moisture content is responsible for an increased cyclone
precipitation intensity in the northern Mediterranean. On the other hand,
, , and
argue that shifts in the cyclone track, and thus changes
in the more important dynamical part of a Vb cyclone, are expected under a
future climate. In particular, the former studies project a poleward shift of
the storm track in a future climate, while expect an
eastward extension of the storm track towards Europe. These projections about
the changing behaviour of the storm track in the future suggests that the
phenomenon Vb cyclone could become even rarer if either of the two shifts
occur. Combining these two arguments, it can be expected that Vb cyclones
would happen more seldom, but with an increased intensity in precipitation.
point in the same direction in a study on Vb cyclones.
However, this hypothesis is associated with a large amount of uncertainty,
and a more precise assessment of the future behaviour of Vb events and their
related impacts cannot be done with the evidence exposed in this analysis.
Thus, more research on the large-scale dynamic changes of different
Vb cyclone subcategories under a future climate is needed to fully understand
the changes in precipitation amounts and frequency of Vb cyclones.
Even though it is possible to find a reason for the high variability in
Vb cyclone triggered precipitation amounts, the exact triggering mechanism
for precipitation cannot be found using the coarse resolution of ERA-Interim.
This is especially true in the Alpine region, where the coarse resolution is
a strong limiting factor of ERA-Interim. As the Vb cyclones are phenomena,
which strongly depend on mountains, more insights could be gained using
regional modelling. Dynamical downscaling will not only improve the spatial
resolution, but also the temporal resolution. Such a higher resolved data set
will allow a closer look into thermodynamics, while an increased temporal
resolution can provide additional information on dynamics
. Planned sensitivity studies on SST over different
locations and soil moisture in the regional model framework will allow us to
gain a deeper insight on the moisture source and thus on thermodynamics in
several single Vb events. Furthermore, the fine resolution allows
distinguishing the mechanisms that trigger a Vb cyclone with and without
meteorological impact. Additionally the regional model framework can simulate
the diabatic heating processes and thus PV development or the presence of
warm conveyor belts during the cyclones life cycle. Thus, future studies will
consider the re-evaluation of the Vb cyclone climatology based on
high-regional downscaling products, as well as direct assessments of the
evolution Vb events through climate simulations.