ESDEarth System DynamicsESDEarth Syst. Dynam.2190-4987Copernicus PublicationsGöttingen, Germany10.5194/esd-8-477-2017Sensitivity experiments on the response of Vb cyclones to sea surface temperature and soil moisture changesMessmerMartinamessmer@climate.unibe.chhttps://orcid.org/0000-0001-6835-4508Gómez-NavarroJuan Joséhttps://orcid.org/0000-0001-5488-775XRaibleChristoph C.Climate and Environmental Physics, Physics Institute, University of Bern, Bern, SwitzerlandOeschger Centre for Climate Change Research, University of Bern, Bern, Switzerlandnow at Department of Physics, University of Murcia, Murcia, SpainMartina Messmer (messmer@climate.unibe.ch)3July2017834774931December201615December20165May201716May2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://esd.copernicus.org/articles/8/477/2017/esd-8-477-2017.htmlThe full text article is available as a PDF file from https://esd.copernicus.org/articles/8/477/2017/esd-8-477-2017.pdf
Extratropical cyclones of type Vb, which develop over the western
Mediterranean and move northeastward, are major natural hazards that are
responsible for heavy precipitation over central Europe. To gain further
understanding in the governing processes of these Vb cyclones, the study
explores the role of soil moisture and sea surface temperature (SST) and
their contribution to the atmospheric moisture content. Thereby, recent Vb
events identified in the ERA-Interim reanalysis are dynamically downscaled
with the Weather Research and Forecasting (WRF) model. Results indicate that
a mean high-impact summer Vb event is mostly sensitive to an increase in the
Mediterranean SSTs and rather insensitive to Atlantic SSTs and soil moisture
changes. Hence, an increase of +5 K in Mediterranean SSTs leads to an average
increase of 24 % in precipitation over central Europe. This increase in
precipitation is mainly induced by larger mean upward moisture flux over the
Mediterranean with increasing Mediterranean SSTs. This further invokes an
increase in latent energy release, which leads to an increase in atmospheric
instability, i.e. in convective available potential energy. Both the
increased availability of atmospheric moisture and the increased instability
of the atmosphere, which is able to remove extra moisture from the atmosphere
due to convective processes, are responsible for the strong increase in
precipitation over the entire region influenced by Vb events. Precipitation
patterns further indicate that a strong increase in precipitation is found at
the eastern coast of the Adriatic Sea for increased Mediterranean SSTs. This
premature loss in atmospheric moisture leads to a significant decrease in
atmospheric moisture transport to central Europe and the northeastern flanks
of the Alpine mountain chain. This leads to a reduction in precipitation in
this high-impact region of the Vb event for an increase in Mediterranean SSTs
of +5 K. Furthermore, the intensity of the Vb cyclones, measured as a
gradient in the 850 hPa geopotential height field around the cyclone centre,
indicates that an upper bound for intensity might be reached for the most
intense Vb event.
Introduction
The frequency and intensity of extreme events are highly vulnerable to
climate change , e.g. heavy precipitation events in the midlatitudes exhibit
an increase with ongoing climate change . Since it is
difficult to predict changes in extreme weather events, in particular at
regional scales in a possible future climate , it is of
great importance to understand the triggering mechanisms and the involved
processes of high-impact weather events, e.g. cyclonic systems with their
associated wind gusts and heavy precipitation.
A prominent phenomenon of regional high-impact weather in central Europe, and
especially over the northern ridge of the Alps and the adjacent flatlands and
low mountain ranges, is the so-called Vb cyclone. Vb events are known as
cyclones that typically develop over the Mediterranean Sea (Gulf of Genoa)
and travel along the southern side of the
Alps during their intensification phase. As they reach the eastern edge of the Alpine mountain chain, they turn
northeastward towards St. Petersburg . These cyclones
transport large amounts of atmospheric moisture to the northern side of the
Alps and central Europe, thus triggering extreme precipitation events
, and they exhibit a great potential for floods in the Elbe,
Danube, or the Rhine catchments and the Alpine area,
including adjacent flatlands and low mountain ranges e.g. chap. 5
in.
Several studies record that often cut-off lows, including the Vb pathway, are
responsible for extreme precipitation and discharge events in the Alps and
central Europe, e.g. the prominent European flood that occurred in August
2002 . The potential of transporting extreme precipitation to central
Europe is especially high if these cut-off low systems are positioned in the
northern or eastern parts of the Alps . These studies above
demonstrate that there seems to be a wide agreement on the large-scale
dynamics of Vb events. Furthermore, the large-scale dynamics seem to
determine whether a Vb cyclone delivers high precipitation or not
. Despite this fact, an important moisture source needs
to supply the atmosphere with the required moisture. In fact, these
thermodynamical processes, and especially the moisture sources, remain
unclear, as described in the following.
To identify the main moisture sources during Vb events, the case study
approach is widely used in the literature . The most intensively studied Vb cyclone is the
once-in-a-century event that occurred in August 2002 and led to a major
flooding of the Oder and Elbe catchments. Some studies have identified
evaporation from land, together with moisture from the Mediterranean Sea and
the Atlantic, as important moisture sources during the 2002 Vb event
. This is in line with the study performed
by , who suggested that water vapour from separated
moisture sources contributes to the extreme precipitation in the most
affected area during the August 2002 Vb event. These moisture sources include
the Atlantic Ocean and Mediterranean Sea areas inside the model domain, the
evapotranspiration from land areas, and long-range advection from subtropical
areas outside the model domain. However, some more general studies on
precipitation events in Europe suggest that the Mediterranean Sea plays an
important role in such events. , for example, identified
the Mediterranean Sea as the main oceanic moisture source for precipitation
over central Europe. focused on the August 2002 Vb
event and identified evaporation in the western Mediterranean Basin 6 to 2
days prior to the actual event as its most prominent source of moisture.
further supported the fact that the Mediterranean Sea
is not the only moisture source during various heavy precipitation events in
central Europe. They found that additional moisture sources with high
event-to-event variability are needed to trigger such events. These moisture
sources include, in addition to others, the evaporation from European land masses
especially in summer or evaporation from the North Atlantic Ocean in winter.
The fact that evaporation from land, and thus soil moisture recycling, might
play an important role in extreme precipitation events has been further
highlighted in recent studies . Both studies
analyse a rather atypical Vb event in 2013, which was nevertheless associated
with widespread flooding in the Danube and Elbe catchments. Even though there
have been several case studies devoted to identifying the moisture sources
during high-impact Vb events, the results seem to be diverse as the moisture
sources include the Mediterranean Sea, the Atlantic Ocean, and soil moisture.
Therefore, identifying the main moisture source during Vb events in general
and independent of single cases, still remains a challenge.
A one-at-a-time sensitivity experiment can help identifying the main moisture
sources as it allows the diagnosis of the processes that contribute most to the
model parametric sensitivity . Thus, sensitivity
analyses enable the analysis of the impact of several factors on a certain process
. Consequently, the effect on precipitation, for example, can
be determined according to changes in the input variable, e.g. sea surface
temperatures (SSTs).
The present work aims at shedding light on the sensitivity of extreme summer
Vb events and their impact on precipitation over central Europe to several
moisture sources. Hence, a number of idealized sensitivity experiments are
designed and carried out with the regional Weather Research and Forecasting
(WRF) model to disentangle the contribution of these moisture sources during
the five most intense summer Vb events recorded in the
period 1979–2013. Thereby, and according to the variables considered by
previous studies, we test the sensitivity of Vb events to changes in soil
moisture in Europe and SSTs of the Atlantic Ocean and the Mediterranean Sea.
The structure of the study is as follows. Details on the model set-up, data
set, and applied methods are presented in Sect. .
Section provides a short evaluation of the control
simulation, while the results of the sensitivity experiments are discussed in
Sect. . In Sect. , we focus
on the Mediterranean sensitivity experiments, including an analysis of
changes in cyclone tracks and characteristics. Finally, a summary of the main
conclusions and a short outlook is presented (Sect. ).
Data and methodsReanalysis data set
The ERA-Interim data set is used to provide the initial conditions and
6-hourly lateral boundaries for the regional model. This data set is produced
by the European Centre for Medium-Range Weather Forecasts (ECMWF) in a
spectral resolution of T255, which corresponds to a spatial resolution of
approximately 80 km, and 60 vertical levels up to 0.1 hPa .
The 6-hourly estimates of three-dimensional meteorological variables and the
3-hourly estimates for surface variables are generated with the Integrated
Forecast System model version 2006 of the ECMWF assimilating various sources
of observational data, e.g. satellite data, surface pressure observations,
and radiosonde profiles Sect. 4 in.
Observations used in the model evaluation
For evaluation, simulated daily accumulated precipitation and multi-day sums
of daily accumulated precipitation over the five precipitation-intense summer
Vb events are compared to two observational data sets. The first one is the
E-OBS data set version 10.0 . It consists of weather
station data, which are interpolated to a regular 25 km grid over the
European land, i.e. it does not provide data over the ocean. The variables
included in this product are precipitation; sea level pressure; and mean,
minimum, and maximum temperature. All variables have daily resolution and span
the period 1950–2013 . For our analysis we will only use
the daily accumulated precipitation.
The second data set is the EURO4m-APGD precipitation data. It contains the
daily accumulated precipitation distribution over the European Alps and the
adjacent flatland regions for the period 1971–2008 . In
contrast to E-OBS, the data are based on measurements from high-resolution
rain-gauge stations and thus provide 5 km resolution on a regular grid in
the ETRS89-LAEA coordinate system .
Selection of Vb events
For this analysis, five precipitation-intense summer Vb events are selected
in the period between 1979 and 2013 that triggered extreme precipitation over
the region of the northern slope of the Alps and northern central Europe. For
that analysis the ERA-Interim period between 1979 and 2013 is used to identify several
Vb events by applying a tracking tool developed by to
the geopotential height field at 850 hPa . The Vb tracks
are then filtered with a technique adapted from . The
filtered Vb events are classified and sorted according to the accumulated
precipitation delivered over the region of the northern Alps, including parts
of Switzerland, Austria, Germany, and the Czech Republic. More details on the
method of Vb event selection are presented in .
The five most precipitation-intense summer Vb cyclones that we selected include two
events that are of historic importance. One event is the so-called European
Flood, which happened in August 2002 and especially affected the catchment
areas of two rivers: the Elbe and the Oder . The other event took place in August 2005 and caused severe
floods on the northern side of the Alps, especially in Switzerland
. The other three events occurred in July 1981,
August 1985, and June 1979. These three events are not related to historic
flooding events. All five events were initialized by a cold air
outbreak located northeast of the Alps. As this trough moves westwards
lee cyclogenesis is induced at the southeastern flanks of the Alps and hence
in the region of the Gulf of Genoa. From this starting point all five
Vb cyclones move along the Vb track described by
, showing some individual behaviour along the path.
Important parameterizations used to run the WRF sensitivity experiments.
ParameterizationParameter nameChosen parameterizationApplied toMicrophysicsmp_physicsWRF single-moment six-class schemeDomain 1–3Longwave radiationra_lw_physiscsRRTM schemeDomain 1–3Shortwave radiationra_sw_physicsDudhia schemeDomain 1–3Surface layersf_sfclay_pysicsMM5 similarityDomain 1–3Land and water surfacesf_surface_physicsNoah land surface modelDomain 1–3Planetary boundary layerbl_pbl_physicsYonsei University schemeDomain 1–3Cumuluscu_physicsKain–Fritsch schemeDomain 1Grell–Freitas schemeDomain 2No parameterizationDomain 3
The three nested domains (D1 to D3) with their actual resolution are depicted as black boxes.
The box labelled “Alps” denotes the area used for measuring the precipitation intensity of the Vb events.
The shading shows the topographical elevation implemented in the simulations in metres above sea level.
Model set-up and sensitivity experiments
The simulations for the sensitivity experiments are carried out with the
WRF version 3.5.1. WRF is run with
a three nested-domain set-up with a nest ratio of 1:3. The domains have a
spatial resolution of 27, 9, and 3 km and are two-way nested, which allows
feedbacks from the higher- to the lower-resolution domains (Fig. ). The outermost domain covers all of the Mediterranean Sea
and a large part of the Atlantic Ocean. The design of the domains considers a
large area of water masses to be included in the outermost domain in order to
allow strong water vapour signals in the inner domains. Hence, although the
innermost domain does not include the Atlantic, the outer domains allow WRF
to consistently integrate the moisture flux provided by the physical
mechanisms outside the smallest domain. This flux is advected towards central
Europe through the various domain boundaries. The innermost domain targets
central Europe, showing the Alpine mountain chain, and thus the region of
interest, in the middle of the domain (Fig. ). Vertically,
all simulations implement 50 eta levels. The 3 km resolution in the innermost
domain allows the explicit simulation of convective processes; thus, no
additional parameterization is needed. Other important parameterizations
chosen to run the WRF simulations are listed in Table .
Nudging techniques are avoided (except for the pre-simulations for the
moisture sensitivity simulations; see details below), so that Vb cyclones can
freely develop their path and intensity according to the new boundary
conditions imposed by the sensitivity experiments. However, the fact that
nudging is not admitted renders the starting time of the simulation
critical since initializations that are too early may lead to situations where the Vb
cyclone is very different to the one reproduced in ERA-Interim, or even
completely missing. After testing several initiation times (not shown), we
found that starting the simulation 6 h before the corresponding event
is first observed allows the reproduction of the events. This means the simulated
trajectory of the cyclone mimics the corresponding track of the events found
in the original ERA-Interim data set . However, this
relatively short spin-up period of 6 h can be a drawback as the model
might not be in full equilibrium. Note that the spin-up time is equal for all
three domains, which means that there is no additional time lag for the
nested domains.
To assure that this short spin-up period does not affect the performance of
the simulation in the sensitivity studies, a set of experiments was performed
with a spin-up time of 1 week. The set of experiments consists of
sensitivity simulations where SST changes of -5 and +5 K in the Atlantic
Ocean and the Mediterranean Sea are applied (not shown). These tests are aimed
at
assessing to what extent longer simulations can achieve a better equilibrium
state, leading to different results. To force the model to reproduce the Vb
event and circumvent the problem stated above, the wind fields (U and V) and
the geopotential height (GPH) are spectrally nudged (wavelengths larger than
roughly 600 km) above the planetary boundary layer and in domain 1 only. Note
that nudging has only been applied to this 1-week spin-up set-up. We found
hardly any change in the thermodynamic variables when using this longer
spin-up period (not shown). We thus conclude that the length of the spin-up
period is suitable to reach an equilibrium during the whole life of the Vb
event.
Sensitivity experiment for soil moisture
To test the sensitivity of Vb events to soil moisture, three different
experiments are carried out. They enclose a complete desaturation to the
minimum possible soil moisture content of 2 %, homogeneously fixing a soil moisture
content typical of southern Spain across the whole domain (i.e.
17.5 %, which corresponds to the average value of the Vb events in
the region of Ciudad Real, this being one of the driest regions in the
Iberian Peninsula), and a complete saturation of the soil moisture content.
The second set-up is an unrealistic, yet physically plausible, scenario
and can therefore be regarded as a more realistic version of the complete
desaturation of soil. Note that all of the three experiments performed are
rather unrealistic and highly idealized, and are aimed at exploring physical
mechanisms rather than obtaining accurate climate change projections. A
complete desaturation of all the soil moisture throughout all of Europe is
probably the most unrealistic experiment, since it comprises soil water contents
over all of Europe that do not even occur in the Sahara. It is
possible that central Europe could see a general drying to a more
Mediterranean climate , but nevertheless it is quite
a strong reduction in soil water content for most of the land area covered by
domain 1. In this sense, the second experiment is a slightly less unrealistic
version, although still very unlikely given current climate change
projections in the Mediterranean Sea. The projections for central Europe
indicate a robust 5–15 % reduction in soil moisture for the end of the
century, with a tendency for wetter soils in the northern parts of Europe
. This projected reduction is still higher than the
southern Spain experiments, as a reduction of 15 % in the central European
soil moisture would result in our cases in 25 % soil water content.
Similarly, the full saturation experiment is also rather unrealistic even in
a possible moistening scenario of Europe.
Since the evaporation from soil moisture can influence the moisture content
in the atmosphere before the actual Vb event takes place, we have carefully
designed the initialization of these simulations. For all three
experiments, as well as the control simulation, the WRF model is started 5 days before the actual Vb event is initialized and the model terminated after these
5 days. During this pre-simulation, we use the same spectral nudging as
described for the SST test simulations, and the soil moisture is constantly
overruled to impose a fixed value of soil moisture according to each of the
three sensitivity experiments in all four model layers of the Noah model. The
atmospheric water vapour content after these 5 days of the pre-simulation
is then used to overwrite the water vapour present in the initial conditions
taken from the driving data set and used in the actual Vb simulation. The
actual Vb event simulations are started at the same time as the SST
experiments in order to obtain similar cyclone tracks throughout the
different types of experiments and therefore minimize side effects arising
from changes in the Vb dynamics.
Across all soil moisture sensitivity experiments, the initial conditions for
soil moisture in the actual simulation are set to the corresponding value
according to each of the three families described above. In this regard, it is
important to note that just the initial conditions are set, i.e. the model is
free to adjust the soil moisture afterwards due to precipitation and
evaporation processes, for example. For this reason, we did not use spin-up times longer
than 6 h since otherwise the model would use the longer spin-up period
to refill the soil moisture volume until the equilibrium was recovered.
Furthermore, such a short spin-up time precludes obtaining a realistic initial condition
of the water atmosphere content in equilibrium with the perturbed soil, which
is the reason for running the pre-simulations described above. The care taken
in the initialization of the soil experiments pertains especially to the first
model soil layer, which is the most weather-relevant layer and the one with
the shortest response time. It is important to remark that unlike in the SST
experiments, where we change a given boundary condition, in the case of soil
the variables are simulated together with the atmosphere model, and therefore
the soil experiments shall be regarded as perturbation in the initial
conditions. To change the soil moisture content for the actual Vb event
simulation, the original ERA-Interim initial file is modified and the land
values are set to either 0, 0.175, or 0.5 m3 m-3. The latter value
is selected because the soil moisture content of all soil types listed in
the WRF model is always lower than 0.5 m3 m-3.
The full saturation soil experiment described above, represents an averaged
increase in land soil moisture by 21 % compared to the control simulation
for the first soil layer, which is the most relevant for weather. In
contrast, the complete drainage experimental setting and the southern Spain
soil experiment reduces soil moisture by 68.5 and 24 %, respectively,
when temporally and spatially averaging domain 3.
Sensitivity experiment for the Atlantic SST
In order to gain insight into the moisture impact of the Atlantic SSTs on Vb
events, the Atlantic SSTs are increased and decreased by 5 K. The two most
extreme sensitivity experiments are performed to obtain a strong signal in
the results. Since this large change in the Atlantic SSTs does not strongly
impact precipitation (Sect. for more
details) other sensitivity experiments with lower SST amplitudes are not
performed.
The increase in the Atlantic SSTs in our experiment has been chosen according
to the increase in SSTs in the sensitivity experiments of the Mediterranean
SSTs described in Sect. . This is to obtain some consistency
within the two families of the SST sensitivity experiments.
Sensitivity experiments for the Mediterranean SST
For the sensitivity experiments within the Mediterranean Sea, 10 sensitivity
simulations plus a control simulation are performed for each of the five Vb
events. This corresponds to homogeneous SST changes within the Mediterranean
Sea between -5 and +5 K, in 1 K intervals (0 K is the control
simulation). The ERA-Interim SST field is used to calculate the horizontally
interpolated SST field for the input file used by WRF. The homogeneous
increase in SSTs is added to the horizontally interpolated WRF grid obtained
according to the WRF Preprocessing System (WPS) and not to the original ERA-Interim data set itself. This is done
to avoid any inconsistencies in the increased Mediterranean SSTs at grid
points close to the coast lines, related to differences in the land–sea mask
of the ERA-Interim and WRF domains.
Compared to the reference period 1961–1990, chap. 16.3 projected for the fossil intensive A1 scenario a
maximal warming of the Mediterranean open ocean surface air by up to 2.19,
3.85, and 7.07 K for the time periods 2010–2039, 2040–2069, and 2070–2099,
respectively. Additionally, expected an annual warming
of the Mediterranean Sea by 2.6 K and for the summer season a warming of 2.9 K by the end of the 21st century under the RCP8.5 scenario,
this being a worst-case scenario that involves very pessimistic scenario
emissions and leads to severe climate change projections. Hence, the warming
implied in the sensitivity experiments is in line with the spread of
projected scenarios for several periods of the 21st
century.
Model evaluation of the control simulations
The control simulations of the five Vb events are used in the following as reference
for the different sensitivity experiments. As this analysis
shall show the ability of WRF to realistically reproduce such events, key
variables of these control simulations are compared to observational data
sets and ERA-Interim data. The analysis focuses on precipitation and the
trajectories of the Vb events.
The left column shows the daily accumulated precipitation (mm) obtained by observations plotted
against the precipitation obtained by WRF for (a) domain 3 and (c) the Alps (“Alps” box in Fig. )
for each of the 5 days of the five different Vb events. The right column depicts the multi-day sums
of daily accumulated precipitation (mm) for 1 to 5 days for the observations against the multi-day sums obtained by the
WRF simulations for (b) domain 3 and (d) the Alps for each of the five Vb events. The upper row
uses E-OBS as an observational data set, while the bottom row depicts E-OBS (blue icons) and EURO4m-APGD
(red icons) as observational data sets.
Precipitation
To show the performance of WRF in simulating Vb cyclones and their impact, we
first focus on precipitation. Daily accumulated precipitation and multi-day
sums of daily accumulated precipitation are evaluated in two different areas.
First, both variables are compared to observations for the entire domain 3
using E-OBS. For this the E-OBS data set is bilinearly interpolated onto the
grid of the innermost domain and the ocean grid points are masked since the
E-OBS data are land only. For the comparison, the simulated and observed mean
daily accumulated precipitation for five Vb events is shown in Fig. a. WRF generally simulates higher daily accumulated
precipitation compared to E-OBS across all 5 days of the Vb events. These
differences are mainly caused by an overestimation of the simulated
precipitation during the first 2 days of each event and coincide with the
highest daily accumulated precipitation. As a consequence of this, the
multi-day sums of daily accumulated precipitation in domain 3 are
systematically higher for WRF throughout all the selected Vb events than
E-OBS in domain 3 (Fig. b). This mismatch can be
attributed to some extent to deficiencies in the E-OBS data since it is
known that precipitation is underestimated in the E-OBS data, especially over
mountainous areas and during summer . The reason is that
precipitation is mainly driven by convection during summer, and thus it is
very local, making it difficult to capture these phenomena with the sparse
observation network that is available over the Alps .
Additionally, some of the overestimation by WRF can be attributed to the
finer resolution compared to E-OBS. Hence, lower values are expected for the
coarser E-OBS grid, as each grid point represents an average over a larger
area compared to the WRF grid . Furthermore, possible
positive biases in the average precipitation of the regional model
additionally increase the differences between E-OBS and WRF.
Second, the same variables are compared in a smaller area focusing over the
Alps, which is depicted by the “Alps” box in Fig. . In this
case, the simulated extreme daily accumulated precipitation compared to E-OBS
and EURO4m-APGD tends to be around a one-to-one relationship (second
row in Fig. ), indicating a close resemblance between
the observed and simulated daily accumulated precipitation during the
different Vb events. The same is also true for the multi-day sums of daily
accumulated precipitation during the complete event. Note that as indicated
before, WRF overestimates daily accumulated precipitation compared to E-OBS,
whereas it generally underestimates precipitation compared to EURO4m-APGD
data (Fig. d). This opposite behaviour of E-OBS and
EURO4m-APGD compared to WRF underlines the argument about the uncertainties
in the E-OBS data set as an explanation for the mismatch between simulated and
observed precipitation for domain 3. Indeed, the EURO4m-APGD data set
includes a denser spatial network of the rain-gauge stations. This renders it
more suitable to capture the local convective systems that predominantly
occur during summer and that lead to the high amounts of precipitation that
are simulated by WRF but are not captured by E-OBS.
The evaluation indicates that WRF is able to realistically capture the daily
accumulated precipitation and thus also the multi-day sums of daily
accumulated precipitation during the five precipitation-intense summer Vb
events of interest. Furthermore, the fact that WRF overestimates precipitation
compared to E-OBS underlines the ability of WRF to accurately simulate
convective processes over the Alpine area.
Tracks for the five different Vb events. The black line depicts the tracks that are obtained using
the ERA-Interim data set. The light green line shows the tracks detected in the control simulation. The stippled lines
show the tracks of the different Mediterranean SST experiments. The green and black diamonds represent the point of
the cyclone at which it reaches the strongest gradient during its lifetime for the control simulation and ERA-Interim, respectively.
Cyclone track
To evaluate the cyclone trajectories obtained by WRF, the tracks are compared
to the ones observed in ERA-Interim data. The latter are detected by a
tracking tool applied to the
1.5∘× 1.5∘ resolved 850 hPa geopotential height
field see. Since the downscaled geopotential height
field is affected by high-frequency noise, which is introduced by the fact
that the domains are located over the Alps, the track detection is applied to
the outermost domain only. The 850 hPa geopotential height field is
bilinearly interpolated onto a regular latitude–longitude grid with
0.5∘× 0.5∘ resolution to smooth the field and
remove the high-frequency noise. Nevertheless, the resolution is still
somewhat finer than the ERA-Interim grid.
The tracks of the control simulation (light green line in Fig. ) agree well with the ones obtained by ERA-Interim (black
line in Fig. ) in all of the five Vb events. In
particular during the intensification phase of a cyclone, i.e. the first
time steps, the alignment with the ERA-Interim tracks is obvious, even though
a slight displacement towards the south is noticeable. In the decaying phase
of the cyclone more deviations from the ERA-Interim path are found. Note that
the precipitation-intense time steps happen during the intensification phase
of the cyclone and therefore a deviation from the ERA-Interim at the end of
the cyclones' lifetime does not strongly influence the precipitation
amounts, i.e. the key variable in our analysis.
Sensitivity of Vb cyclones to soil moisture and Atlantic Ocean and Mediterranean SSTs
In the following we present the analysis of the different idealized
sensitivity experiments focusing on daily mean precipitation, moisture flux
over land and the Mediterranean Sea, precipitable water, and convective
available potential energy (CAPE). These variables are able to provide
insight into the processes that take place within the moisture exchange from
its sources to the atmosphere. Therefore, all variables are averaged over
domain 3 (tests with areas encircled around the cyclone centre by ≥ 500 km show similar results). Since most of the ocean grid points are located
over the Mediterranean Sea and only a few over the Atlantic (see domain 3 in
Fig. ), these few grid points have been masked to obtain
only the moisture flux over the Mediterranean Sea.
The time steps that are included in the analysis are defined by the time when
95 % of the total precipitation of the event has fallen over the “Alps” box
depicted in Fig. . This allows the study of the impact of the Vb
event itself and avoids a potential contamination of the analysis due to the
development of other weather phenomena, such as frontal systems, in the
decaying phase of the Vb cyclone. Domain 3 represents the influence area of
the different Vb cyclones and it is therefore the region of main interest.
The statistical confidence of the differences between the sensitivity
experiments and the control simulations is established with the
non-parametric Mann–Whitney U test at the 5 % significance level.
The panels in the first row show the mean over 5 Vb events for the soil experiments with a bar for drainage
(0 %, red), southern Spanish soil water conditions (17.5 %, orange), the control simulation (ctrl, black), and full
saturation (SMAX, blue). The second row shows the mean over 5 Vb events for the Atlantic SST experiments, with bars
depicting a decrease in SSTs of 5 K on the left (blue), the control simulation in the middle, and an increase in SSTs
of 5 K on the right (red). The third row shows the mean over 5 Vb events for the Mediterranean SST experiments, with
bars depicting a decrease in SSTs of 5 K on the left and an increase in SSTs of 5 K on the right, with increments of 1 K.
The five columns show the daily mean precipitation, upward moisture flux over land and over the ocean, and mean convective
available potential energy (CAPE) for D3 from the left to the right. Stars above the bars denote significant changes
compared to the control simulation using a non-parametric Mann–Whitney U test and the 5 % significance level. The
units for the y axis are given in the header of each column, whereas the x axis denotes the performed sensitivity studies.
Soil moisture
The idealized soil moisture experiment reveals that a complete drainage of
the soil moisture volume in the initial conditions leads to an average
reduction of 32 % in the daily mean precipitation over the five studied Vb
events in the area of domain 3 (Fig. a). The sensitivity
experiments corresponding to soil moisture as in southern Spain show a small
decrease of around 6 % in daily mean precipitation. In contrast, a fully
saturated soil moisture volume in the initial conditions leads to a
relatively small increase of 7 % with respect to the control simulation. The
daily mean upward moisture flux over land decreases by approximately 81 %
and 14 % for a complete drainage of the soil moisture volume and the
southern Spanish soil conditions, respectively. At the same time the daily mean
upward moisture flux over land shows an increase of 11 % for full saturation
(Fig. b). As expected, the daily mean upward moisture
flux over the Mediterranean Sea, precipitable water, and CAPE reveal only
small changes for the two experiments with the soil moisture volume and
consequently, they do not show significant changes (Fig. c–e). Therefore, the reduction in precipitation as
well as in precipitable water with a complete drainage can be attributed to a
reduction in moisture flux from the land (Fig. b), which
is in turn a direct consequence of the complete removal of the soil moisture
volume.
Panel (a) shows the accumulated precipitation (mm day-1) for the control simulation of the soil moisture
experiments averaged over the five Vb events. Panel (b) shows the same as panel (a) but for the SST experiments.
The second to fourth rows show the differences between the mean daily precipitation obtained by the different
sensitivity experiments and the control simulation (mm day-1). Panel (c) shows the complete drainage soil experiment,
panel (d) the full saturation soil experiment, panels (e) and (f) the -5 and +5 K Atlantic SST experiments, respectively,
and panels (g) and (h) the -5 and +5 K Mediterranean SST experiments. The hatched area denotes significant changes at the
5 % significance level using a non-parametric Mann–Whitney U test.
The reason is that a reduction (increase) in soil moisture volume leads to a
reduction (increase) in latent heat flux and therefore to an increase
(reduction) in sensible heat flux. This further decreases (increases)
precipitation since relative humidity over land is strongly modified during
these experiments (not shown). There is a slight reduction (increase) in the
mean upward moisture flux over the Mediterranean Sea. These changes are not
significant and hence their changes are not analysed in more detail here.
The average spatial precipitation patterns obtained within the soil
experiment show a strong reduction in the continental precipitation for the
complete drainage experiment compared to the control simulation (Fig. a). Especially higher elevated regions such as the Alpine mountain ridge or the
Dinaric Alps are affected
by the decrease in precipitation. In contrast, the differences in the spatial precipitation
patterns between the full saturation experiment and the control simulation
are small (Fig. d). This is also true for the
southern Spanish soil condition experiments (not shown). Furthermore, only a few
small areas of the differences between the two most extreme sensitivity
experiments are significant at the 5 % level using a non-parametric
Mann–Whitney U test, also indicating a high variability in the exact
location of the precipitation changes within the five cases.
Atlantic SSTs
The sensitivity experiment with increased and decreased SSTs in the Atlantic
Ocean reveals only moderate changes in all variables (Fig. f to j), and none of the variables show significant
changes compared to the control simulation. For mean daily precipitation in
domain 3, there is almost no change detectable with changing Atlantic SSTs.
Daily mean moisture flux over land, precipitable water, and CAPE show a very
small change with decreasing and also increasing Atlantic SSTs compared to
the control simulation (Fig. g, i, j). The daily mean
moisture flux over the Mediterranean Sea shows an inverse behaviour compared
to the rest of the variables, i.e. an increase (decrease) in Atlantic SSTs
results in a decrease (increase) of 9 % (7 %) compared to the control
experiment (Fig. h). This is because the impact of the
Atlantic SSTs is only indirectly captured. The surface moisture flux over the
Atlantic Ocean increases (decreases) with increasing (decreasing) SSTs and
thus the atmospheric moisture content becomes more (less) saturated when the
air reaches the Mediterranean Sea. Hence, the Mediterranean Sea behaves in
the opposite direction as the Atlantic, i.e. a reduced moisture flux over
the Mediterranean Sea is observed as long as the Atlantic Ocean supplies the
atmosphere with moisture, and vice versa. Note that the changes in moisture
flux over the Mediterranean Sea are still relatively small and indeed
insignificant.
This lack of sensitivity to Atlantic SSTs means that precipitation of
high-impact summer Vb events hardly changes with changing SSTs. Therefore,
the precipitable water in domain 3 also only increases slightly. As moisture
content in the atmosphere increases marginally, the latent energy remains
almost unchanged and thus CAPE does not vary between these experiments.
The small observable sensitivity in the mean (Fig. ) is
also evident in the precipitation patterns of the Atlantic SST experiment.
The two most extreme sensitivity experiments show on average over the five
precipitation-intense summer Vb events in both cases a patchy pattern with
insignificant anomalies of both signs throughout domain 3 (Fig. e and f). The
insignificance can be explained by a large case-to-case variability in the
precipitation changes for the five Vb events selected.
Mediterranean SSTs
An increase (decrease) in the SSTs of the Mediterranean Sea leads on average
over the five Vb events to an increase (decrease) in daily mean
precipitation, daily mean upward moisture flux over the Mediterranean Sea,
precipitable water, and mean CAPE (Fig. k–o).
Particularly, an increase of 5 K in the Mediterranean SSTs leads to a
significant increase in precipitation of 24 % on average, while a reduction
in Mediterranean SSTs induces a reduction in precipitation of only 9 %
compared to the control simulation (Fig. k). This indicates a
non-linear relationship that is further discussed below. The daily mean upward
moisture flux over land shows no change over the different Mediterranean Sea
sensitivity experiments (Fig. l). As expected, changes in
the Mediterranean SSTs have the strongest impact on the daily mean moisture
flux over the Mediterranean Sea compared to the other variables shown in Fig. . This is because an increase (a reduction) in SSTs of 5 K
results in a change of 124 % (-65 %) in the mean moisture flux over the
Mediterranean Sea compared to the control simulation (Fig. m). In addition to the daily mean upward moisture flux over
land, precipitable water also shows small deviations due to changes in the
Mediterranean SSTs compared to the control simulation. Hence, precipitable
water increases (decreases) insignificantly by 8 % (4 %) with an increase
(a decrease) of 5 K in the Mediterranean SSTs (Fig. n).
As indicated above, the Mediterranean SST sensitivity experiments exhibit a
non-linear increase in precipitation amounts in domain 3 with increasing SSTs
(Fig. k). This can be due to two different mechanisms.
One is the increased moisture flux induced by increased SSTs. This increased
moisture flux leads to a mostly linear increase in the average atmospheric
moisture, as demonstrated by the amount of precipitable water in Fig. n. Nevertheless, the non-linear behaviour observed in the
average precipitation is driven by an increase in atmospheric instability,
i.e. CAPE. Hence, an increase in atmospheric water vapour goes along with an
increase in latent heat and leads to additional convection, which is capable
of removing an even larger portion of water than expected from the single
increase in atmospheric moisture.
As expected from the distinct changes described above, Mediterranean SST
variability leads to significant anomalies in the average precipitation
pattern for the +5 K experiment (Fig. h). The
experiments with +1 to +3 K show almost no significance, whereas the +4 K
experiments show similar significance patterns as the +5 K experiment, but
with a smaller amplitude (not shown). The cooling experiments, including the
-5 K experiment, do not generate significant changes on the 5 % significance
level (Fig. g) compared to the control simulation
(Fig. b). For the sensitivity experiments with the
Mediterranean SSTs, an increase in SSTs leads to a strong increase in
precipitation over coastal areas, together with a reduction in precipitation
over the Alpine areas. This is explained by the loss of moisture over the
coastal areas in the sensitivity experiments induced by the destabilization
of the atmosphere pointed out above. Note that the changes over the coastal
areas are not significant since the exact location and amount of
precipitation varies across the five high-impact summer Vb events. This
increased precipitation is responsible for the removal of great amounts of
atmospheric moisture so that the precipitation over central Europe, and
especially the Alps, is reduced as a side effect. The significant pattern in
precipitation reduction nicely resembles the water transport towards the Alps
that is significantly reduced for the +5 K Mediterranean SST experiment. In
case of a cooling, there is a reduced precipitation over coastal areas
because of an increased stability of the atmosphere. Since the precipitation
is reduced in coastal areas, the air is more likely saturated when it hits
the Alps during the Vb event. Hence, more precipitation can fall in the
Alpine region during the event with decreased SSTs in the Mediterranean Sea.
However, such changes for a decrease in Mediterranean SSTs are not
significant on the 5 % significance level.
Discussion
The three families of sensitivity experiments suggest that the Vb
events are mostly sensitive to changes in the Mediterranean Sea and seem to
be rather insensitive to changes in the Atlantic SSTs and the soil moisture
content. This is because an increase of 5 K in the Mediterranean SSTs leads
to a rise in precipitation of up to 24 % over central Europe. This high
number can otherwise only be exceeded by an initialized and complete
desaturation of the soil moisture in all of domain 1 and all four layers of
the Noah soil model implemented within WRF. However, the latter experiment is
an unrealistic extreme and more realistic situations are not likely to
provoke an appreciable impact on the severity of precipitation-intense summer
Vb events, as the southern Spanish soil condition experiment confirms.
Furthermore, the insensitivity of the Vb events to Atlantic SST
changes might also be due to the fact that they are all observed during
summer. This is consistent with the argument that the North Atlantic might
influence the atmospheric moisture more strongly in winter
. Still this does not mean that the Atlantic has no
influence on the Vb cyclones throughout a season as seasonal SST change might
change the atmospheric circulation, stimulating the generation of a Vb cyclone.
Nevertheless, such responses cannot be assessed with the experimental
design selected and are thus beyond the scope of the study.
Our results are in line with the case studies of and
as they identified the Mediterranean Basin as a key
area for the massive amount of precipitation over Europe during the Vb event in August
2002. additionally suggested that the moisture sources
during this event include the Atlantic Ocean, evapotranspiration from land
areas, and long-range advection from subtropical areas outside the model
domain. However, the latter results can only partially be confirmed in our
study since we found only marginal contributions of soil moisture and
Atlantic SST changes to precipitation amounts. Still, our study cannot be
directly compared to the results found by , since we
summarize the main moisture source from various high-impact summer Vb events
instead of one isolated case study. Additionally, our analyses are also in
line with results obtained from general circulation model simulations, showing an amplification of
extreme summer precipitation with rising Mediterranean SSTs from the period
1970–1999 to the period 2000–2012 . Furthermore, our
study and seem to agree on the reduction in
precipitation over eastern Switzerland and western Austria. In addition,
evaporation from land is frequently identified as an important moisture
source during Vb events, as found by and
for the Vb event in 2002, and by and
for the Vb event in 2013. The 2013 Vb event is not
included in our study because it follows a rather untypical Vb trajectory.
This might be one reason for the different result in this study and the ones
carried out by and . Furthermore, only
the soil moisture volume at the beginning of the event is artificially
removed, thus allowing moisture recycling during the event. This might be an
additional reason for the divergence in the results on moisture evaporation
from land. Nevertheless, it is important to emphasize that the main
difference between this study and the studies mentioned above is that we
analyse the main driving moisture source of different Vb events instead of a
single case study. Thus, it cannot be expected that the average behaviour of
several Vb events fully agrees with single case studies. Even though the
agreement between these events is relatively large, there is still
case-to-case variability. Additionally, it is noteworthy that the fact that
the Mediterranean Sea seems to be the main contributor to heavy precipitation
events independent of case studies is in line with .
Furthermore, the increase in precipitation in coastal areas as they were
found for the Mediterranean SST experiment is confirmed by the study of
. In their study they attributed a strong increase in precipitation at
the eastern coast of the Black Sea to increases in the SSTs of the Black Sea.
also argued that the strong increase in precipitation
is connected to an enhancement of the instability in the lower troposphere
that allows the triggering of deep convection.
The gradient within an area of 1000 × 1000 km2 for the geopotential height at 850 hPa is shown
for two different Vb events. The coloured lines indicate changes in the gradient over the time of the Mediterranean
SST experiments. The black line shows the evolution in the gradient in the ERA-Interim data for the same event.
In panel (a) the most intense Vb event (18–20 July 1981)
is shown. In panel (b), the least intense of the Vb events (20–24 August 2005)
is shown. The data based on WRF show an hourly resolution and a spatial resolution of 0.5∘× 0.5∘.
The ERA-Interim data are based on 6-hourly temporal resolution, which has been linearly interpolated on 1-hourly temporal
resolution, while the spatial resolution is 1.5∘× 1.5∘.
Analysis and discussion of changes in cyclone (and) characteristics
Since the Mediterranean Sea seems to be the most important factor for the
high-impact summer Vb events, this section focuses on the
sensitivity of the dynamics of the cyclones in the experiments with the
Mediterranean SSTs.
The 10 tracks (stippled lines in Fig. ) obtained by the
sensitivity experiments with the Mediterranean SSTs for each of the five
Vb events line up with the tracks obtained in the control simulation
(light green line in Fig. ). Especially the first time steps
of each of the events show a good agreement between the 10 sensitivity
experiments and the control simulation. Only during the mature and decaying
phases of the cyclones do the tracks within the sensitivity experiment start to
diverge (Fig. ). This indicates that deviations in the track
cannot be made responsible for changes in the precipitation within the
sensitivity experiments. A strong latitudinal displacement of the tracks
might have influenced and changed the moisture advection to the impact area
over central Europe and hence precipitation amounts. Since only very small
deviations within the tracks are found, this effect can be excluded.
Another important variable for the dynamics of a cyclone is the mean gradient
within an area of 1000 × 1000 km2 at 850 hPa, which is a measure of
the wind intensity around a cyclone assuming the geostrophic approximation.
The analysis shows that the cyclone with the steepest gradient during its
lifetime is almost insensitive to changes in the Mediterranean SSTs (Fig. a). In contrast, the cyclone that has the weakest
gradient of the five studied Vb events shows a much stronger sensitivity to
changes in the Mediterranean SSTs (Fig. b). Thus, a
warming of the Mediterranean SSTs has the potential to intensify Vb cyclones,
while a slight reduction in intensity can be obtained by cooling the
Mediterranean SSTs. The three other cyclones (not shown) obtain
maximum gradients located in between the ones depicted in Fig. . Therefore, it seems that the five summer Vb
cyclones show an increasing sensitivity towards changes in the Mediterranean
SSTs with decreasing maximum gradient. This is especially true during the
first 30 to 50 h of the lifetime of a cyclone, i.e. during the
intensification phase. These results may indicate that a maximal threshold of
the cyclone is reached in the most intense Vb cyclone; thus, only weaker cyclones are
able to intensify with warmer Mediterranean SSTs. This threshold can be
interpreted as an energy threshold as the gradient in 850 hPa geopotential
height around a cyclone is related to the wind speed (via the geostrophic
approximation) and thus the kinetic energy. Therefore, our results indicate
that warmer Mediterranean SSTs lead in a non-linear way to stronger kinetic
energy, whereas the growth of the strongest cyclones might be capped by a
possible upper energy limit. This result is in line with the work of
on southern hemispheric cyclones. They investigated the
influence of eastern Australian coastal SSTs on extratropical cyclone
intensification and results suggest that SSTs play only a minor role in the
intensification of the most intense cyclones, as they are more strongly
influenced by the prevailing atmospheric conditions. Also, the work of
, who analysed extreme values in vorticity and
GPH fields during the winter, support that extremes in
the GPH might be limited by an upper bound.
Summary
In this study, we identify the main moisture source for a composite of five
different high-impact summer Vb events. For this, three different families of
idealized sensitivity experiments are carried out over five
precipitation-intense summer Vb events that occurred in the period between
1979 and 2013. The three sensitivity experiments include artificial removal
and supply of soil moisture as well as changes in the SSTs of the Atlantic
Ocean and the Mediterranean Sea. The experiments are conducted with the
regional model WRF, driven with the ERA-Interim reanalysis data set.
The validation of WRF with two observational data sets, E-OBS and
EURO4m-APGD, reveals that WRF is generally able to reproduce precipitation
amounts in Vb events over the Alpine region. There is however a slightly
better agreement with EURO4m-APGD, which suggests that the convective
processes largely responsible for summer precipitation in the Alps are
reasonably reproduced by the model. Hence, the latter database seems to be
more suitable than E-OBS for recording the precipitation in this area of
complex topography.
Additionally, the track characteristics of the high-impact summer Vb events
in the control simulations exhibit good agreement with the ones obtained in
the ERA-Interim data set. This supports the model's ability to
simulate the relevant physical processes in a reasonable way.
Various sensitivity experiments are carried out, which allow one to draw the
following conclusions: a complete removal of the soil moisture content over
great parts of Europe and in all four layers of the soil model in the initial
conditions leads to a notable reduction in daily mean upward moisture flux
over land, which leads to an increase in sensible heat flux and a reduction
in latent heat flux. The increase in sensible heat conversely drives a
reduction in relative humidity. The reduction in daily mean upward moisture
flux and relative humidity lead to a reduction of approximately 32 % in
precipitation over central Europe. For the southern Spanish soil condition
sensitivity experiment the processes just described are valid but in a
smaller extent such that the reduction in precipitation only reaches 7 %.
Conversely, for an increase in soil moisture content the same processes hold
but in the inverse and also in a reduced way and hence leads to a small
increase of around 7 % in precipitation.
Nevertheless, these soil moisture experiments, but especially the complete
drainage experiment, are very unrealistic and extreme. Still, it seems
unlikely that a considerable impact on the severity of precipitation-intense
summer Vb events, i.e. on precipitation amounts, can be obtained in more
realistic scenarios.
The changes in precipitation patterns for the soil moisture experiment generally show
a decrease (increase) over domain 3 for a full drainage
(saturation) of the soil moisture content. Nevertheless, the case-to-case
variability for the location of the precipitation changes is high and
inconsistent, and thus no significant changes are found (5 % significance level).
Similarly, the sensitivity experiments varying the Atlantic SSTs show almost
no change in precipitation over domain 3, indicating that on average the
Vb events are hardly sensitive to changes in the Atlantic SSTs. The
same holds true for the precipitation pattern changes for the Atlantic Ocean.
In these experiments the sign and location of changes varies between single
Vb events, and hence no significant change can be found, either for
increasing or for decreasing Atlantic SSTs.
A 5 K increase in the Mediterranean SSTs leads to a more similar absolute change
in precipitation than a complete removal of the soil moisture content. Hence,
an increase in Mediterranean SSTs of 5 K leads to an increase in
precipitation of approximately 24 %. The larger precipitation rates for
warmer Mediterranean SSTs are induced by a strong increase in daily mean
upward moisture flux over the Mediterranean Sea, together with a decrease in
the atmospheric stability induced by the release of more latent heat. While
the increase in mean upward moisture flux feeds a linear increase in
precipitable water, i.e. the water content in the atmosphere, a non-linear
increase in CAPE, i.e. the atmospheric instability, leads to convection that
is able to remove more moisture from the atmosphere than expected by a single
increase in water vapour. Hence, a non-linear behaviour is found in the
precipitation sensitivities. This is attributable to an increase in atmospheric
instability with increasing Mediterranean SSTs due to a strong significant
increase in moisture flux over the Mediterranean Sea. Conversely, a decrease
in Mediterranean SSTs leads to processes that are inverted from those described before,
and thus produces a slight reduction in precipitation over central Europe.
The increase in Mediterranean SSTs by 5 K generates changes in the Balkan
coastal areas together with significant decreases in precipitation amounts
over the eastern ridge of the Alps. This indicates that the air contains
enough moisture to precipitate out while it is lifted over the Dinaric Alps.
Note that the exact location and amount of precipitation does change within
the different Vb events, and consequently no significant change can be
obtained here. This topographic-induced precipitation leaves the air drier
than in the control experiment when it reaches the Alpine area and explains
the significant reduction in precipitation over the whole expected air
advection path of a Vb event. The same mechanism, but reversed, happens in a
cooled Mediterranean SSTs scenario. Still, unlike in the former case, the
changes induced by a cooling of the Mediterranean SSTs do not reach a
significant level (5 % significance level).
The above-mentioned changes in precipitation amounts and patterns indicate,
from all the sensitivities analysed, that these five
precipitation-intense summer Vb events are mostly sensitive to changes in the
Mediterranean SSTs.
The Mediterranean SST experiments allow further interesting findings. While
there is a good agreement in the trajectories of Vb events across sensitivity
experiments, the intensity measured by gradient within an area of 1000×1000 km2 around the cyclone centre is generally different in the
various sensitivity experiments carried out. In particular, we found that a
warming of the Mediterranean SSTs can lead to an increase in the gradient,
and thus to a more intense cyclone during its intensification period within
the first 30 to 50 h. Similarly, a decrease in the cyclone intensity is
found for a decrease in Mediterranean SSTs. Interestingly, the change in
intensity of the cyclone is inversely proportional to the maximal intensity
that is obtained during a cyclone's lifetime in the control experiment. That
is, the most intense cyclone shows little to no change in intensity, either
for decreasing or for increasing Mediterranean SSTs. This may indicate that
strong cyclones are limited in growth of kinetic energy since they might be
capped by an upper bound. Conversely, there seems to be the possibility
for weaker cyclones to grow in kinetic energy with increasing Mediterranean
SSTs in a non-linear way. A possible reason for the limited sensitivity of
strong cyclones to changes in the intensity might be that these cyclones are
more strongly steered by the large-scale atmospheric conditions, as described
by .
As a final remark, these results shall not be understood as climate change
projections. An important drawback in this type of sensitivity studies is
that to some extent the physical consistency cannot be granted. In our set-up,
the most non-physical problem is the heating of the ocean surface alone. This
has the effect that a strong and artificial temperature gradient is
introduced near the coastal areas, which does not correspond to a natural
behaviour. Although in these experiments the model seems to bring this
disturbance back to a physically plausible situation after a few hours, this
introduces artefacts in the simulation, which are difficult to isolate.
Therefore, obtaining more physically consistent and thus reliable results
would require running transient simulations driven by comprehensive Earth
system models under realistic climate change scenarios.
The data are available upon request from the corresponding
author Martina Messmer (messmer@climate.unibe.ch).
MM, JJG-N, and CCR contributed to the design of the
experiments. Martina Messmer ran the simulations and wrote the first draft.
All authors contributed to the internal review of the text previous to the
submission.
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors are grateful for the funding provided by the Alfred Bretscher-Fonds für Klima- und Luftverschmutzungsforschung. Thanks are
also due to the support provided by the Oeschger Centre for Climate Change
Research and the Mobiliar Lab for climate risks and natural hazards
(Mobilab). Juan José Gómez-Navarro acknowledges the funding provided
through the contract for the return of experienced researchers, resolution
R-735/2015 of the University of Murcia and the CARM for the funding provided
through the Seneca Foundation (project 20022/SF/16). The ERA-Interim
reanalysis data were provided by the ECMWF. Furthermore, we acknowledge the
E-OBS data set from the EU-FP6 project ENSEMBLES
(http://ensembles-eu.metoffice.com) and the data providers in the ECA&D
project (http://www.ecad.eu). Thanks are due to European Reanalysis and
Observations for Monitoring for providing us with the APGD data set. The
simulations are all run at the Swiss National Supercomputing Centre (CSCS).
Thanks are due to the two anonymous referees and the editor Rui A. P. Perdigão for their constructive comments that helped to improve the
paper.
Edited by: Rui A. P. Perdigão
Reviewed by: two anonymous referees
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