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Extreme hydrometeorological events and climate change predictions in Europe. Journal of Hydrology 518 (2014) 206-224


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s u m m a r y Field meteorological data collected in several European Commission projects (from 1974 to 2011) were re-analysed in the context of a perceived reduction in summer storms around the Western Mediterranean Basin (WMB). The findings reveal some hitherto overlooked processes that raise questions about direct impacts on European hydrological cycles, e.g., extreme hydrometeorological events, and about the role of feedbacks on climate models and climate predictions. For instance, the summer storms are affected by land-use changes along the coasts and mountain slopes. Their loss triggers a chain of events that leads to an Accumulation Mode (AM) where water vapour and air pollutants (ozone) become stacked in layers, up to 4000(+) m, over the WMB. The AM cycle can last 3–5 consecutive days, and recur several times each month from mid May to late August. At the end of each cycle the accumulated water vapour can feed Vb track events and generate intense rainfall and summer floods in Central Europe. Venting out of the water vapour that should have precipitated within the WMB increases the salinity of the sea and affects the Atlantic-Mediterranean Salinity valve at Gibraltar. This, in turn, can alter the tracks of Atlantic Depressions and their frontal systems over Atlantic Europe. Another effect is the greenhouse heating by water vapour and photo-oxidants (e.g., O3) when layered over the Basin during the AM cycle. This increases the Sea Surface Temperature (SST), and the higher SST intensifies torrential rain events over the Mediterranean coasts in autumn. All these processes raise research questions that must be addressed to improve the meteorological forecasting of extreme events, as well as climate model predictions. � 2014 Elsevier B.V. All rights reserved.
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Extreme hydrometeorological events and climate change predictions
in Europe
Millán M. Millán
Fundación CEAM, Valencia, Spain
article info
Article history:
Available online 8 January 2014
Hydrological cycle
Torrential rains in Europe
Mediterranean mesometeorology
Climate feedbacks in Europe
Field meteorological data collected in several European Commission projects (from 1974 to 2011) were
re-analysed in the context of a perceived reduction in summer storms around the Western Mediterranean
Basin (WMB). The findings reveal some hitherto overlooked processes that raise questions about direct
impacts on European hydrological cycles, e.g., extreme hydrometeorological events, and about the role
of feedbacks on climate models and climate predictions. For instance, the summer storms are affected
by land-use changes along the coasts and mountain slopes. Their loss triggers a chain of events that leads
to an Accumulation Mode (AM) where water vapour and air pollutants (ozone) become stacked in layers,
up to 4000(+) m, over the WMB. The AM cycle can last 3–5 consecutive days, and recur several times each
month from mid May to late August. At the end of each cycle the accumulated water vapour can feed V
track events and generate intense rainfall and summer floods in Central Europe. Venting out of the water
vapour that should have precipitated within the WMB increases the salinity of the sea and affects the
Atlantic-Mediterranean Salinity valve at Gibraltar. This, in turn, can alter the tracks of Atlantic Depres-
sions and their frontal systems over Atlantic Europe. Another effect is the greenhouse heating by water
vapour and photo-oxidants (e.g., O
) when layered over the Basin during the AM cycle. This increases the
Sea Surface Temperature (SST), and the higher SST intensifies torrential rain events over the Mediterra-
nean coasts in autumn. All these processes raise research questions that must be addressed to improve
the meteorological forecasting of extreme events, as well as climate model predictions.
Ó2014 Elsevier B.V. All rights reserved.
1. Introduction
After the 1972 UN Stockholm Conference on the Environment, a
number of actions were initiated worldwide following the recom-
mendations of the Conference (MIT, 1970, 1971). The European
Commission initiated its programme in Environment and Climate
in 1973, by launching several large projects, with intensive field
campaigns in the areas of Atmospheric Chemistry, i.e., Air Pollution
(Guillot, 1985) and Desertification (Mairota et al., 1998). While
searching for measurement sites, and during instrumental deploy-
ments, scientists were alerted by locals regarding a decrease in the
number of summer storms. An oddity at the time was that the
comments came mainly from areas surrounding the Western Med-
iterranean Basin (WMB), except from Central Italy.
In December 1991, the EC’s Unit Head for Environment and
Climate (Dr. Heinrich Ott) and I met to review data from the MECA-
PIP and RECAPMA projects (Appendix A). The subject of the
summer storms came up, and he inquired about using the field
information and meso-meteorological data collected in the EC pro-
jects to find an answer to 18 years of comments about storm loss
around the Mediterranean. Moreover, the query of the storms, as
in this Special Issue, offered a chance to challenge conventional
wisdom, raise questions, and postulate mechanisms that could be
used to select topics for EC research programmes. This paper, fol-
lowing the initial objective, describes the analysis sequence,
including a review of the available field data, the interpretations,
the questions arising, and the likely answers and hypotheses de-
rived at each stage.
2. The precipitation types
In 1992 we talked again about initiating the precipitation study
in areas where large field campaigns had taken place, and the com-
ments about the storms had arisen (Appendix A). For example, in
Marseille-Fos Berre (France), in the Po Valley (Italy), in the Mijares
Valley, Valencia-Teruel (Spain). We also considered the availability
of the data and, above all, its spatial extent. In the MECAPIP the
measurements covered the north-eastern quadrant of the Iberian
Peninsula. They included meteorological towers, tethered balloons
and instrumented aircraft measurements, all the way from the
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Journal of Hydrology 518 (2014) 206–224
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Mediterranean coast to the headwaters of the Mijares Valley, and
beyond, i.e., to Madrid, and Bilbao in North-Atlantic Spain, and
along the Ebro Valley (Millán et al., 1991, 1992). The RECAPMA
project had further extended the experimental coverage from the
Atlantic coast of Portugal, to southern France and Italy. It included
instrumented flights over the WMB in July 1991. In fact, we were
reviewing those results when the query about the storms came up.
From 1992 to 1994, other projects (SECAP, BEMA I) were
launched, extending the spatial scale from Portugal to Greece, Tur-
key and Israel (Millán et al., 1997). In all, the Mijares Valley had
been used in five campaigns, which put this area at the head of
the list for the precipitation study. The issue of the storms was fi-
nally addressed in 1993 – at first, riding piggy-back on the budget
of on-going EC projects and funds from the Spanish National Re-
search Programme (SNRP). It became a full project in 1995, after
funding was procured from the SNRP to purchase high spatial res-
olution, daily precipitation data. The data series from 497 stations
for the period (1950–1996) in the area outlined in Fig. 1, cost us
Fig. 1. Spatial and temporal disaggregation of the precipitation types for the Valencia Region and neighbouring areas in Spain (domain maps) for the period 1950–2000.
Precipitation contours in mm. Yearly average series for the whole domain in mm.
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 207
45,000(+) . This was the final deciding factor for limiting the study
to a domain that includes the Valencia Region, and adjacent areas
in Spain.
Because the comments pertained to summer storms the first
step was to disaggregate the precipitation data by weather types,
using criteria derived from the MECAPIP, RECAPMA, and SECAP
projects. The detailed procedure, including the co-analysis of the
rain series with the daily synoptic maps, and the correlations with
the North Atlantic Oscillation (NAO) index was published by Millán
et al. (2005a,b). At the end of 1997, we had identified three sources
of precipitation in this domain:
(A) Classic Atlantic Fronts. Their most frequent occurrence is
from early autumn to late spring (Fig. 1A, series A). The main
foci occur over the west-facing slopes of the Iberian Cordil-
lera System. There are also weaker, secondary, foci over
some of its ranges extending towards the sea. We also
included in this type the convergences of westerly Atlantic
flows with Mediterranean seabreezes along the Iberian Cor-
dillera System in late summer. These may be accompanied
by a cold trough at the 500 hPa level. In these cases, the rains
can be very intense, the maxima occur directly over the Ibe-
rian System, and the collected amounts taper down to zero
at the coast. This type contributes <20% of the total precipi-
tation in the study domain. Its correlation with the (NAO)
index is negative.
(B) Summer Storms in this area are associated with the diurnal
development of the Iberian Thermal Low in summer (Barry
and Chorley, 1987). This type includes the storms driven
by the combined seabreeze and up-slope winds (Sections 4
and 7), which develop in the afternoon over the mountain
ranges at 60–100 km from the coast. They are more frequent
from late April to September (Fig. 1B, series B). During the
late evening and night the storms can migrate easterly
(i.e., towards the coast) before dissipating. Rain amounts
were attributed to this type whenever the Iberian Thermal
Low was observed on the synoptic maps at 12:00 UTC and/
or at 18:00 UTC on the day of the event. Their main foci
occur over the east-facing slopes of the Iberian Cordillera
System, i.e., facing the Mediterranean Sea. They contribute
11–16% of the total precipitation in the domain and show
no correlation with the NAO index.
(C) Mediterranean Cyclogenesis events, ‘‘Backdoor Cold Fronts’’
(Huschke, 1986) or ‘‘Levanters’’ (Meteorological Office,
1962). These develop during the migratory period of the
anticyclones from the Atlantic to Siberia from early fall until
late winter-spring (Fig. 1C, series C). Thus, they are always
associated with an anticyclone over Central Europe driving
an easterly flow of cold continental air over a warmer Med-
iterranean sea. They can be very intense, and they tend to
occur mainly at night and be localised spatially. They can
last from one to three days. If a cut-off low develops south-
west of the Iberian Peninsula (Gulf of Cadiz), and migrates
via Gibraltar and the Sea of Alboran towards the WMB, the
easterly flow is reinforced. The area affected can be much
larger and the precipitation can occur day and night. They
can also last longer (i.e., of the order of one week). This type
contributes >65% of the total precipitation in the study
domain, and the focal areas are right over the coastal areas.
The correlation with the NAO index is positive.
A first question concerns the generalised use of the NAO index
as a simple climatic indicator for precipitation in this area of com-
plex terrain which extends from the sea to the European Continen-
tal Divide (Figs. 2 and 3). Other findings are that the contribution
from Atlantic depressions (A) has been decreasing for the last
50 years in this region. The summer storms over the mountains
surrounding this part of the Mediterranean Basin have nearly dis-
appeared in the last 50 years, while extreme precipitation events
associated with Mediterranean Cyclogenesis (C) appear to be
increasing rapidly, becoming more torrential in nature, continuing
well into the spring period (Peñarrocha et al., 2002; Millán et al.,
2005b), and also beginning as early as late summer.
Finally, the sum of rain types (B) and (C) amounts to some 75–
80% of the total precipitation in this area. Both occur with easterly
winds and, thus, from water evaporated within the Mediterranean
Basin. This is a recurring theme in this work, since it would suggest
that the rain water from these two types may simply be recycled
within the Mediterranean Basin itself. Thus, the initial questions
regard the influence of orography, and the source(s) of the water
vapour involved in each precipitation type.
3. The lay of the land: the role of the European continental
The spatial distribution of components (A) and (C) in Fig. 1
raises the question of which precipitation is Atlantic, i.e., from
water evaporated from the Atlantic, and which is Mediterranean.
To start with, Fig. 2 shows that the real, and ‘‘whole’’,
Fig. 2. Drainage Basins of the World (Source United Nations, Grey areas are endorheic basins that do not drain to the ocean.
208 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
Mediterranean Catchment Basin extends to the sources of the Nile
River, and includes the Black Sea as well as other parts of Europe
that are not usually considered Mediterranean (e.g., Austria, Hun-
gary, Romania, etc.). A closer look over Europe, in Fig. 3, shows that
the European Continental-Water Divide follows the high ground
and the peaks of the major mountain ranges, including the Iberian
Cordillera System on the western border of the study area. To the
right and south of the Divide all waters drain into the Mediterra-
nean, and to the left and north they drain into the Atlantic.
Depending on the meteorological conditions, e.g., advection
across the Divide, Föhn type effects tend to keep a large fraction
(i.e., up to 1/3) of the water vapour carried by the airmasses on
the upwind side of the Divide. Thus, this mechanism limits the
amount of water vapour transported from one side of the Divide
to the other. In other meteorological conditions, e.g., V
track type
events (van Bebber, 1891), the Divide favours surface convergence
of airmasses from the Atlantic and the Mediterranean (type A,
above), and can become the focus for intense precipitation and
runoff to either side of the Divide (Ulbrich et al., 2003; Gangoiti
et al., 2011a,b).
There are also mountain passes on the Divide that provide di-
rect paths for airmasses into and out of the Mediterranean Basin,
and are notorious for generating intense ‘‘Gap Winds’’, e.g., the Tra-
muntana through the Carcassone Gap, and the Tarifa winds at
Gibraltar (Scorer, 1952). This fact, in turn, raises more questions
regarding the source and transport of water vapour, and the trigger
mechanisms for some Mediterranean Cyclogenesis events. For
example, the formation of Genoa Depressions: are they concurrent
with, or are they a result of, intense cold Mistral winds over warm
waters in the Gulf of Genoa? Thus, we could ask how much of the
runoff on each side of the Divide proceeds from water vapour evap-
orated within, and converging from, the same side? what condi-
tions favour the net transfer of water vapour from one side of
the Divide to the other? and where along the Divide? and how
Finally, if the concept of Atlantic Frontal Systems is to be up-
held, e.g., for rain in places like the Italian Peninsula, the questions
are: how much additional water vapour? and evaporated from
where? is required to re-activate the Atlantic Fronts, or any upper
trough, after they cross the Divide into the Mediterranean Catch-
ment Basin. An answer to these questions may require re-defining
some terms. For example, the southerly migration of an upper cold
trough, or a pool of cold air aloft from higher latitudes can act as a
trigger for intense Mediterranean Cyclogenesis. Though, only if
there is enough moisture in the lower atmosphere, or if the process
is accompanied by a mechanism to recharge moisture, e.g., cold air
advection over warm water, as in a Backdoor Cold Front (Huschke,
Previous work indicates that the water vapour involved in pre-
cipitation types (B) and (C), regardless of the trigger mechanism,
originates from within the Mediterranean Basin (Millán et al.,
1995; Pastor et al., 2001). This will be reconsidered in Section 8
by introducing the concepts of a carrier component and a trigger
component for the water vapour involved in the precipitation. An
extensive validation of this hypothesis and of the recurring ques-
tions raised above requires a combination of in-depth meso-mete-
orological analysis and the disaggregation of precipitation by
weather types across the Basin, rather than just modelling (Sec-
tion 6), as well as extensive studies of isotopic rain composition
along judiciously selected transects across the Divide.
4. Meso-meteorological processes: The combined up-slope and
The comments from locals indicated that summer storms form
(or used to form) in the afternoon over the mountains surrounding
the WMB, at distances ranging from 60 to 100(+) km from the sea.
By 1995, we also knew that the storms developed as the final stage
in a coastal wind system that develops on summer days and com-
bines the up-slope winds and the sea breeze, henceforth called the
‘‘combined breeze’’. The specific characteristics of this wind sys-
tem have been documented in ten EC projects since 1986 (Appen-
dix A,Millán et al., 1987, 1992, 1997, 2000, 2002) and derive from
the nature of the WBM. That is, a large and deep sea totally sur-
rounded by high mountains in the subtropical latitudes. The devel-
opment of the combined breeze is strongly influenced by
orography (Mahrer and Pielke, 1977; Miao et al., 2003), and it is
quite different from a ‘‘classic seabreeze’’ (Munn, 1966; Stull,
1988), since the marine air mass becomes strongly modified along
its path. Fig. 4 illustrates its evolution during the day with model-
ling results (RAMS, Pielke et al., 1992). This is the key to under-
standing the feedback processes described in this work.
Fig. 3. Colour-coded altitude map of Europe (Source USGS and the European Soil Bureau). The dark-blue line marks the European Continental-Water Divide. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 209
(1) Upslope wind cells develop early in the morning, i.e., right
after sunrise, over the east-and-south facing slopes of the
coastal mountains (Munn, 1966; Millán et al., 1991, 1992;
Salvador et al., 1997).
(2) The seabreeze develops over the coastal plains, and hence-
forth its progress inland occurs in a stepwise manner.
Namely, it goes up the slopes by incorporating one after
another the up-slope wind cells formed earlier that morning.
This sequence is illustrated in Fig. 4 (graphs b, c, d, e), dis-
cussed further below.
(3) After advancing each step, the leading edge (or front) of the
combined breeze rests, and remains stationary over that
position, from 1/2 to 1(+) h, see orographic chimneys
(4) During July and August, the combined breeze can last from
12 to 14 h at the coast, and its average wind run can reach
160 km, as shown in Fig. 5 (Millán et al., 2000).
(5) It can take from 4 to 6 (+) h for the front of the combined
breeze to reach all the way from the coast to the top of the
mountains 60–100 km inland (Figs. 4 and 5).
(6) The experimental data and the modelling results also show
that once the front reaches the highest mountain tops, it
tends to become locked onto that position, i.e., this becomes
the last step, and it can remain in that location for 4–6 h, as
shown in Fig. 4 (graphs d, e), i.e., until the end of the breeze
period (Salvador et al., 1997).
(7) During the evening and night, drainage flows develop
towards the sea, i.e., the land breeze (Fig. 4, graphs a and
f), and convective activity develops over the coast-sea inter-
phase as the colder drainage winds meet the warmer waters
(e.g., red
arrow in Fig. 4 graph a).
The graphs in Fig. 4 show that the up-slope wind cells remain in
place until incorporated into the growing (larger) wind system. For
example, by 10:00 UTC the combined breeze has progressed 30 km
inland, while three up-slope wind cells remain further up the
slopes. At 12:00 UTC, it has progressed 40 km inland, and appears
t = 02:00 UTC t = 10:00 UTC
t = 12:00 UTC t = 14:00 UTC
t = 22:00 UTCt = 18:00 UTC
MECAPIP 27 - July -1989 (ω Component)
Fig. 4. RAMS simulated vertical (
) wind component on July 27, 1989, along 40°N Latitude, crossing the coastal city of Castellón towards the Javalambre massif (map in
Fig. 11 and text). Solid lines mark the ascent component of the wind, and dotted lines the descent. Red arrows mark the surface component of the seabreeze and the upslope
wind cells, and blue arrows the compensatory subsidence aloft, and the drainage winds.
For interpretation of color in Fig. 4, the reader is referred to the web version of
this article.
210 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
to be in the process of incorporating another cell (two still remain
in place). By 1400 UTC, it has incorporated all the upslope wind
cells, and reached 80 km inland, while the vertical injection at its
front reaches 3500 m high. At 1800 UTC, the front of the com-
bined breeze is still locked at the mountain ridges 80(+) km inland,
but its injection is weaker (to 3000 m). By 2200 UTC, the com-
bined breeze has ceased and the surface drainage flows are becom-
ing established and flowing towards the sea (see also Fig. 4 graph a,
at 02:00 UTC).
This kind of step-like progress was first documented during the
MECAPIP project (in 1989) by photographically tracking the evolu-
tion of small cumulus clouds forming at the leading edge of the
breeze (Millán et al., 1992). It was then observed that the upslope
wind cells tend to develop over the same orographic features, each
day, under similar weather conditions. This has also been repro-
duced by modelling. Thus, in the ‘‘combined breeze’’ the positions
of its front are conditioned by the orography, and the injection over
each of those points resembles the mechanism creating chimney
clouds (Huschke, 1986). Therefore, the term convective-orographic
‘‘chimney’’ will be used henceforth to refer to ‘‘a sustained vertical
injection of surface air at a specific front position’’. In addition, the
altitude of the injections into the return flows increases at each
chimney, both because: the airmass gains more potential temper-
ature, as it follows a longer path along the heated surface, and the
bases of successive chimneys occur at ever increasing heights up
the mountain slopes.
Since each chimney remains stationary for extended periods of
time, see (3) above, the surface airmass that reaches its base be-
comes injected upwards, directly into the return flows aloft. There,
the injected airmasses move seaward and sink under compensa-
tory subsidence along their way. The total amount of sinking sus-
tained by the return flows seems to be comparable to the
altitude of the feet of the chimneys over which they were injected
(Salvador et al., 1997). Sinking also increases the stability of the re-
turn flows, and favours the formation of layers over the coastal
areas and the sea (Millán et al., 2000).
5. Boundary layer folding and the formation of strata over the
Mediterranean Sea
As a result of the processes described, the part of the surface
boundary layer that arrives at each new chimney is injected into
the return flows aloft, stratified, and folded on top of the (older)
part that had been injected previously at a lower height and closer
to the coast. The length of the strata formed in each step can also
be comparable to the distance between the foot of the current
chimney and that of the previous one, or longer, as the return flows
under increasing compensatory subsidence (Section 6) may ac-
quire jet-like characteristics. Finally, the compensatory subsidence
over the sea completes the atmospheric circulation. Thus, these
mechanisms form part of a ‘‘closed’’ vertical circulation that grows
during the day (see Section 7), reaches its maximum development
in the mid-to-late afternoon, and ceases by evening, only to begin
anew the following day.
An example of the pollutants (ozone) and the water vapour in-
jected into the return flows is shown in Fig. 6. The measurements
were contracted by the JRC-ISPRA and taken by an aircraft instru-
mented by FhG-IFU and Aerodata during the MECAPIP project. At
the time of the measurements, three layers can be seen outlined
by arrow lines. The two vertical arrows in Fig. 6 show how a verti-
cal-looking satellite, i.e., the NASA MODIS-Terra (King et al., 2003),
would see a large column value for water vapour when looking
down the developing orographic chimneys, and through the vari-
ous layers formed over the slopes of the coastal mountains. This
would explain the comment by Gao and Kaufman (2003) about
the MODIS signal over the Mediterranean Basin in summer ‘‘being
largest above the coastal side of the mountains’’. In Section 9the
water vapour signal will be used as a tracer of opportunity to char-
acterise the dynamics of the airmasses recirculated vertically by
the coastal wind systems in the Western Mediterranean Basin.
Examples of the resulting temperature structure over the sea
are shown in Figs. 7, 12 and 13. The inversions mark the different
layers formed over the sea, and Fig. 7 also illustrates how repetitive
the coastal circulations can be over the same part of the Western
Mediterranean Basin in summer. Common features in all the pro-
files are: a nearly adiabatic layer in the upper 1500(+) m of the
soundings, i.e., from 2000 m to 3500(+) m, and a serrated pro-
file with multiple temperature inversions in the lower 2000 m.
The former is indicative of a very well mixed airmass, or of the sus-
tained subsidence of a well-mixed airmass, or both. The upper lay-
ers are formed at the last stage of the combined breeze, over the
mountain tops, after the surface air has become well mixed during
its travel over the heated land, and the layer formation time is also
the longest (4–6 h). The thickness of the upper nearly-adiabatic
layers (suggested in Fig. 4e and c) is comparable to the altitude
of the ridges over which their injection took place (e.g., 1500–
2000 m).
When the MECAPIP flights were being planned (1987), it was
not really known how high the layers could reach, or how far they
would extend over the sea (Millán et al., 1987). The early morning
flights over the sea were intended to document the layers formed
the previous day. The vertical flight plane was over the sea at
40 km from the coast, and the top of the sawtooth flight pattern
was fixed at 3500 m. However, the height of most of the coastal
flights was arbitrarily limited to less than 1200 m by the flight
crew, and to less than 3000 m over land (Fig. 6). Only in the last
part of the project, i.e., after 22 of the 32 contracted flight hours
had been used up, and after studying the lone flight leg that
reached 3000 m over the coast (Fig. 7, black profile), did the flights
reach 3000 m, or up to 3500 m on a few occasions.
This illustrates a problem of credibility between the project’s
Principal Investigator (the author) and the scientists performing
the flights. Above all, it shows the enduring difficulties in convey-
ing processes of this nature to people used to boundary layers of
less than 1000 m high, over flat grassy terrain in higher latitudes,
and under, so-called, prevailing winds that do not change direction
by almost 180°, from day to night, every day. The following RECAP-
MA project was intended to document how far, and how high, the
Fig. 5. Averages of the wind direction and speed at Castellón (CS-SUR, Fig. 11). Data
obtained during the last two weeks of July in the years 1989, 1990, 1991, 1994,
1995. The seabreeze period lasts from 07:00 to 21:00, and its wind run is
160 km each day.
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 211
layers reached over the Western Basin. These flights were carried
out by partner CEA (FR), all of them reached 3500 m high, and
some 4000 m; however, they were still not high enough.
The MECAPIP and RECAPMA results were mentioned in the Re-
search Result Reports of the EC (Le Bras, 1993; Le Bras and Angeletti,
1995), and presented at the 5th and 6th EC Symposia on PHYSICO-
Angeletti, 1990; Angeletti and Restelli, 1994), EUROTRAC Symposia,
NATO ITM meetings, and in the Atmospheric Environment Service of
Canada in Toronto in 1991 and 1993. Following this, the formation of
(ozone) layers was also documented over the Adriatic Sea (Fortezza
et al., 1993; Orciari et al., 1993; Georgiadis et al., 1994), and in the
area of Vancouver, BC, Canada (McKendry and Lundgren, 2000).
More recently, nocturnal temperature soundings (2009–2011) from
cruise ships in the Western Mediterranean, conducted by JRC-ISPRA,
show that the upper nearly adiabatic layers can reach as high as
5000 m.
6. Modelling and the self organisation of the coastal circulations
Fig. 8 shows a vertical section of a RAMS model (Pielke et al.,
1992) simulation of the wind field over the last 180 km of the flight
plane in Fig. 6 (marked by ?MODELLED PART), at 1600 UTC. The
grid size used was 2 km 2 km, and in order to emphasise the
structure, the vertical component of the wind has been multiplied
by ten. At the leading edge of the combined breeze, at 90 km in-
land at that time, it shows an injection to 5000 m, which can be
compared with those in Fig. 4. The lines mark the approximate ver-
tical boundaries of the flight path in Fig. 6, making it obvious (now)
that the flight did not reach high enough to fully capture the depth
of the vertical injections.
If ‘‘closed-loop’’ vertical circulations develop in other coastal
areas of the WMB, they can merge and become self-organised dur-
ing the day to originate a meso-
scale circulation extending to the
whole Basin. In this case, the chimneys at the leading edges of the
combined breezes join and form well-defined convergence lines
along the mountains surrounding the basin, i.e., some 60–100 km
inland from the coasts, as illustrated in Fig. 9 for the Iberian Penin-
sula, and in Fig. 10 for the main Mediterranean Basin.
The upper part of Fig. 10 shows that the surface winds emerge
from the centre of the Western Basin, and increase in speed while
flowing anticyclonically (clockwise) towards convergence lines
over the main mountain ranges surrounding the basin (in grey).
Their resolution can be compared with those in Fig. 9. An outcome
Fig. 6. Ozone and water vapour on a vertical flight plane across 350 km of the Iberian Peninsula at 1449–1559 UTC, July 20, 1989 (transect H ?G, in the map). Note the
column of cleaner and drier air sinking at the left of the combined breeze front (see Fig. 4, graphs d and e), for the ?MODELLED PART see Section 6, and Figs. 8 and 9.
212 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
of the self-organisation is that the compensatory subsidence be-
comes generalised and more intense over the coastal areas and
the sea, as illustrated at the bottom of Fig. 10. It shows that to
replenish the surface air moving towards the coasts, continuity re-
quires sinking of the air, i.e., compensatory subsidence (dotted
lines) over the seas.
This is important because the subsidence can keep the surface
layer confined below a height of 200 m, as illustrated in Fig. 11.
The series show the average height to the first observed tempera-
ture inversion in black, and the average plus one standard devia-
tion (in light grey) to emphasise the amplitude of the
oscillations. In Valbona, the soundings were performed at selected
time periods instead of the half-hour cadence used on the other
sites; hence, the gaps in the series. The confinement and the oscil-
lations can be considered ‘‘unusual’’, or even as faulty data, if inter-
preted in the light of conventional Boundary Layer (BL) theories
(Delbarre et al., 2005).
Fig. 11 shows that the BL at the coast grows initially and then
oscillates around the 200 m level, and that the oscillations occur,
or propagate, along its path up the slopes. Similar behaviour,
including the vertical confinement of the BL, and the oscillations
propagating downwind, had been observed in 1978 during the
Nanticoke Power Plant Experiment on the northern shores of Lake
Erie, Ontario, Canada (Portelli et al., 1982; Kerman et al., 1982). In
the WMB, the strong confinement of the boundary layer plays a
key role in the development of storms (Section 8) but it is not well
simulated by the model which, for the same boundary conditions,
yields BL values in the 900–2000 m range (Salvador et al., 1999).
In the WMB, the shallow vertical confinement of the BL has
been observed to extend all the way from the coast to each of
the orographic chimneys along the path of the combined breeze
and, eventually, to the last chimney that forms in the afternoon
as far as 80–100 km inland. This is shown in Fig. 11 for this study
area, and it was also documented by soundings during the RECAP-
MA project from the Italian coast (Castel Porciano), past Rome to
the mountains. However, being considered anomalous, or faulty,
at the time by the partner in charge (CNRS, IT), the data somehow
vanished. In the large meso-
circulation the confinement of the
BLs due to compensatory subsidences can also occur over inland
areas, and yield surprisingly low mixing heights in the most unex-
pected places. Some of these areas have been outlined with ellipses
in Fig. 9. This could explain the ‘‘highly anomalous’’ temperature
soundings, i.e., yielding less than 350 m mixing heights on summer
afternoons, as observed by Bölle et al. (1993) during the EFEDA
project over the parched dry lands of Albacete (ellipse A in
Fig. 9), data which have also disappeared.
To simulate the development of the sea breeze, mesoscale
meteorological models require the use of grids smaller than
9km9km(Salvador et al., 1999). With larger grids the breezes
are not simulated unless the domain extends to the whole basin
(Fig. 10). But even when using smaller grids (i.e., 5 km 5 km)
and vertical resolutions at the limit of numerical instability (e.g.,
36 m), current models are not able to reproduce all the structures
documented experimentally over the Western Mediterranean
(Figs. 7, 12 and 13). The first recurring and non-trivial problem is
the interplay between the size of the model domain and the grid
size used. For example, the wrong domain can eliminate compen-
satory subsidence over the sea and overland areas (Millán, 2008),
and/or play dramatically with the simulated injection heights, as
indicated in the comments to Figs. 4, 6, 8 and 10. The second prob-
lem is the prevailing tendency to interpret experimental data ‘‘too
locally’’, as if nothing else is happening outside the experimental
area (Bölle et al., 1993; Delbarre et al., 2005). The third problem
is that current BL parametrisations in meteorological models, for
Conc. O3 (ppb).
0 102030405060708090
10 15 20 25 30
Temperature (C).
Fig. 7. Temperature (red) and ozone (blue) profiles over the Gulf of Valencia in the Balearic Basin, obtained by spiral flights during the RECAPMA project (on 16 July 1991)
over the triangles in the inserted maps at right. The thick black temperature profile was obtained over the same area (short trace in the middle map) during the MECAPIP
project (from 0527 to 0541 UTC on July 20, 1989) two years earlier. The black triangle line shows the modelled temperature profile for the MECAPIP flight.
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 213
flat grassy terrains under moderate to strong winds, do not really
apply in complex-coastal-terrains (Stull, 1988).
Although the meso-scale model can reproduce the main fea-
tures of the flows, it does not capture the details, i.e., the fine tem-
perature structure that defines all the layers formed in the return
flows (black triangle line in Fig. 7). Nor does the model simulate
the amount of compensatory subsidence that laminates the layers
and keeps the surface boundary layer so strongly confined verti-
cally. All these issues were first noticed when trying to simulate
the MECAPIP and RECAPMA temperature soundings in 1992–
1993. An example of two extreme cases encountered is shown in
Fig. 12. The conclusions are that modelling requires a modeller
with a very good insight into the possible meteorological processes
involved, i.e., from the local to regional scales (meso-scales) includ-
ing how (very) far their effects can reach, including their compen-
satory mechanisms. It also requires a critical attitude towards the
results, and a judicious iterative play with simulations using differ-
ent grid sizes and domain combinations (Millán, 2008). For
example, by comparing details about stream lines, and conver-
gence lines over the Iberian Peninsula in Figs. 9 and 10.
The observed temperature profiles show increases from 13 K to
19 K in the return flows (Fig. 13) and a higher water content in the
upper layers than in the layers directly above the sea (Fig. 6). That
is, the perturbed Relative Humidity (RH) of the IPCC (2001, 2007).
This is the result of the water vapour added during their path over
land (albeit not enough to trigger storms, as mentioned below).
The observed temperature and moisture profiles can thus be con-
sidered ‘‘anomalous’’ with respect to current advection-dominated
model simulations, which assume that the water vapour is evapo-
rated directly from the surface (or comes from upstream). And be-
cause the models are unable to simulate the systematic folding of
the surface boundary layer over the sea for several consecutive
days, they cannot simulate the resulting accumulation of atmo-
spheric properties (i.e., temperature, pollutants and water vapour)
in layers over the coastal areas and over this large interior sea.
The self-organisation of the local circulations from the meso-
scale (to 20 km), through the meso-bscale (to 200 km), to a
full-fledged meso-
scale (to 2000 km) circulation during the
day can be considered a good example of atmospheric properties
and processes spilling up and down the meteorological scales, typ-
ical of sub-tropical latitudes with complex coastal terrains. The
temperature and humidity profiles observed over the coastal areas
and the sea are the result of the systematic folding of the surface
boundary layers for several consecutive days, along with other pro-
cesses that may dominate during the night. For example, the relax-
ation of the compensatory subsidences, the interaction of the
drainage flows with the layer system near the coasts (Fig. 4 graph
a, at 02:00 UTC), and the vertical re-stacking of the layers accord-
ing to their potential temperatures in a Margulis 2nd type circula-
tion (Iribarne and Godson, 1981). These can modify the structure,
and pump upwards the whole layer system during the night, and
they also require specific experimental research.
7. Open or closed atmospheric circulations in the WMB, and the
first climatic tipping point
The number of steps that the combined breeze takes to reach
the mountain ridges inland and, thus, the number of chimneys,
and layers, formed during its development depend on the lay of
the slopes around the WMB basin (Figs. 4 and 6) During this pro-
cess, storms can develop whenever the Convective (-orographic)
Condensation Level (CCL) of the incoming air mass is reached with-
in the chimney at the leading edge of the combined breeze. This
can trigger deeper (moist) convection and the development of con-
vective showers or a storm. If storms do develop, the air mass be-
comes mixed all the way up to the tropopause, and the closed-loop
coastal wind system becomes ‘‘open’’. That is, it ends up behaving
like a small monsoon. This is illustrated in Fig. 14A.
However, the airmass coming inland from the sea sustains heat-
ing as it moves inland along the warm surface. Thus, the formation
of a storm requires the addition of water vapour to offset its heat-
ing, and keep the CCL of the incoming airmass below its height of
injection into the return flows aloft. For example, by evaporation
from: the surface, coastal wetlands, vegetation, forests, irrigated
crops, etc. Otherwise, heating prevails and the CCL of the incoming
airmass will keep on rising, the combined breeze will keep pro-
gressing inland, and we can consider that a ‘‘first critical threshold,
or tipping point’’ in this system will be crossed when the CCL be-
comes higher than the highest injections over the mountain
tops. This is represented by the first loop in Fig. 15 (EC, 2007)
which presents a synthesis of our results and postulates. The other
loops in this figure will be discussed along the text.
Fig. 8. RAMS simulation of the vertical wind field over the last 180 km of the flight,
marked by ?MODELLED PART in Fig. 6. The lines mark the vertical limits of the
instrumented flight on that day.
Fig. 9. RAMS simulation of the surface wind field (h= 14.8 m) over the Iberian
Peninsula at 1600 h, 20 July 1989 (5 km 5 km grid). The intersect of the flight
plane in Fig. 6, marked ?MODELLED PART, for Fig. 8 is shown by a red line. Grey
outlines mark the lines of convergence of the surface flows, i.e., the locus of the
highest injections (Fig. 8) where summer storms used to develop. The ellipses mark
areas where streamlines appear, signalling strong compensatory subsidence to
maintain the 3D continuity of the flows.
214 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
Under these conditions the coastal circulations remain
‘‘closed’’. That is, if storms do not develop all the return flows
formed will continue moving seaward, under compensatory
subsidence during the entire day, forming layers that contain the
non-precipitated water vapour and the pollutants carried by the
combined breeze, and piling them up to 4000(+) m over the Wes-
tern Mediterranean Basin (Figs. 7, 12 and 13). This was the situa-
tion prevailing during the EC projects when their objective was
to validate the postulated mechanisms (vertical recirculations
and the formation of reservoir layers) responsible for the high lev-
els of surface ozone, and other photo-oxidants, measured in the
WMB. In fact, at the time of preparing the MECAPIP proposal in
1986, the last three weeks of July and the first week of August were
selected by the author, after studying 20 years of synoptic weather
maps, as the best time window for the instrumented aircraft to
avoid storms (Millán et al., 1987, 1992). The temporal histogram
in Fig. 1B, prepared independently, using a different data set (daily
rain values) more than 10 years later, seems to validate the early
meteorological choice.
Moreover, because summer precipitation will decrease over the
mountain slopes if storms do not develop, the conditions that
maintain the combined breeze as a ‘‘closed’’ wind system (viz.,
too much heating and insufficient evaporation along its surface
path) can be considered part of a first feedback loop that tips the
local climate towards increased drought inland, shown in
Fig. 14B. The change from a ‘‘closed’’ circulation in the morning
to an ‘‘open’’ system (storm/or shower) in the afternoon still oc-
curs almost daily over Italy in summer. This is due to the conver-
gence of the combined breezes from the Ionic and the Adriatic
Seas over the Apennine Mountains (Section 8) and is suggested
by the model results in Fig. 10.
In the Western Mediterranean Basin, the ‘‘closed loop’’ condi-
tions last from 3 to 5 consecutive days and recur several times a
month. That is, for a total of 12–24(+) days per month, from late
spring to late summer (i.e., May–September). This was determined
initially from the ozone accumulation cycles recorded by air qual-
ity monitoring networks since 1994 (MIMAM, 2009). During these
periods the Western Mediterranean Basin behaves like a ‘‘large
cauldron’’ (Section 8) where the airmasses ‘‘boil’’ from the edges
towards the centre, favouring photochemical reactions between
new coastal emissions and the ‘‘aged’’ recirculating airmasses.
Using satellite (MODIS-terra) data, it has been estimated that the
coastal winds recirculate vertically each day from 1/4 to 1/2 of
the depth of the layers accumulated over the sea on the previous
days (Millán, 2008).
8. Summer storms and land-use changes
The main problem with the condensation level is that the tem-
perature/moisture relationship is exponential (Iribarne and God-
son, 1981), and the consequences of this can now be explored
Fig. 10. Top: RAMS simulation of the surface wind field (at a height of 14.8 m) over the Mediterranean at 16:00 UTC on 19 July 1991 (40 km 40 km grid). Bottom: The
vertical component of the wind field along the 39.5 North Parallel (grey trace in the upper graph) shows deep orographic-convective injections (solid traces) over Eastern
Spain and, to the right, over Sardinia and the west-facing coasts of Italy, Greece and Turkey.
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 215
using the experimental parameters measured during the EC pro-
jects. In the first place, Fig. 13 shows the profiles with the highest
and the lowest potential temperatures measured during the
RECAPMA project in the Balearic Basin. The values span the
312 K (39 °C) to 318 K (45 °C) range in the upper parts of the
flights, and 24 °C (297 K) to 26 °C (299 K) at the surface, and sug-
gest that the marine airmasses that enter the coastline gain from
13 K to 19 K by the time they become injected into the uppermost
return flows aloft.
During the EC projects, the climatic values (1966–1995) for the
water vapour mixing ratio and the temperature at the coast were
14 g/km and 26 °C (299 K), respectively. These yield a Lifting Con-
densation Level (LCL) at 780 m for the marine airmass at the
coastline (Iribarne and Godson, 1981). However, if the combined
breeze gains an average of 16 K along its land path (see Fig. 13),
the modified airmass needs to increase its water vapour mixing ra-
tio to P21 g/kg to keep its Convective (-orographic) Condensation
Level (CCL) below the approximate height of the coastal mountain
ranges, i.e., 2000 m altitude.
The one benefit of the closed vertical circulations is that the dis-
placed air volumes, per unit width along the coast, are reasonably
well defined, and this allows for tentative calculations. For exam-
ple, the average boundary layer depth along the path of the com-
bined breeze can be defined from Fig. 11 as 200 m. The average
run of the combined breeze at the coast (from Fig. 5) is 160 km
(Millán et al., 2000). Typical evaporation rates over irrigated areas
on the coastal plains are from 5 to 7 L/m
day, and over the moun-
tain maquia from 1 to 3 L/m
day. From these values, the water
vapour mixing ratio contributed by surface evaporation can be cal-
culated to be of the order of 5–6 g/kg under the best conditions
(Millán, 2008). This would yield a sum value 620 g/kg, which
would be just marginal to produce condensation at 2000 m, as
indicated in Fig. 14.
Heatings of 19 K were also observed (Fig. 13), which would re-
quire a mixing ratio of P25 g/kg to keep the CCL at 2000 m. That is,
an additional amount of 11 g/kg, which is nearly impossible to
reach under present evapotranspitration conditions. With the low-
er heating measured (13 K in Fig. 13), the airmass requires only an
additional 4 g/kg to reach condensation at 2000 m. It seems that
conditions in the past, with extensive forests and large coastal
marshes, limited the surface heating and provided enough mois-
ture (Fig. 14A) to generate precipitation almost every summer
day at the 1000 m altitude. In fact, the remains of extensive in-
land marshes, and the long drainage channels that desiccated
them, can still be seen in Barracas (Castellón, Spain) at the 980 m
altitude, and 60 km inland. That situation kept a large amount of
water recycling within the coastal system, i.e., including the evap-
oration, the precipitation, the surface water and the acquifers.
The fact that summer storms did not seem to falter as much
over Italy (Section 1) was brought to the author
´s attention by Dr.
S. Sandroni (of JRC-ISPRA) in 1979, during the EC Remote Sensing
Campaign in Turbigo, Italy (Sandroni and de Groot, 1980), and
the convergence of coastal airmasses towards the Apennines
(Cantú and Gandino, 1977) was mentioned while planning the
MECAPIP flights in 1987. In Italy the coastal circulations at either
side of the mountains begin as a ‘‘closed’’ loop during the morning,
Surface Layer Evolution (CAST-PUERTO)
Period: 1987/1995;
Nº Soundings: 818
Surface Layer Evolution (SICHAR)
0 100 200 300 400 500 600
0 100 200 300 400 500 600 0 100 200 300 400 500 600
Alitude (m)
Alitude (m) Alitude (m)
Period: 1987/1991;
Nº Soundings: 207
Surface Layer Evolution (VALBONA)
0 2 4 6 8 1012141618202224
0 2 4 6 8 1012141618202224 024 68
10 12 14 16 18 20 22 24
Period: 1989/1991;
Nº Soundings: 229
Fig. 11. Top left: One experimental deployment area used in nine EC projects in the Mediterranean; arrows mark the three tethered balloon sounding sites for the other
graphs. These show the hourly evolution of the mixed layer height (m), the experimental period (years), and the number of soundings used for the statistics at each site (see
216 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
until the two combined breezes converge over the Apennines in
the afternoon, or earlier if the added moisture is sufficient to reach
the CCL. This is different from the coastal mountains of Eastern
Spain, Southern France, and Northern Africa where the leading
edges of the combined breezes tend to encounter drier, more con-
tinental air on the inland side (Figs. 4, 6 and 8). More frequent
storms keep the surface wetter, and this, in turn, favours the devel-
opment of yet more storms on the following days (i.e., by generat-
ing more evaporation and less heating).
It seems interesting to explore a concept from chemical engi-
neering, i.e. the terms: ‘‘carrier’’, and reacting or ‘‘trigger’’ compo-
nents for the water vapour in the combined breeze. For example, if
it is assumed that the maximum water precipitated from a moist
airmass is 1/3 of the water vapour it contains. Then, the values in
Fig. 14 take a different meaning. That is, if the airmass requires a
mixing ratio of 21 g/kg to condense at 2000 m, the water vapour
at the beginning of the breeze, i.e., 14 g/kg, can be considered as
the ‘‘carrier component’’, which is necessary to obtain the right
percentages but is not sufficient for the condensation to occur.
The additional 7 g/kg contributed by evapo-transpiration along
its path becomes the ‘‘trigger component’’ required for condensa-
tion and precipitation to occur.
In this study area, the 7 g/kg value also happens to be the
upper limit (1/3) of the amount of water that could be recov-
ered by precipitation, i.e., from the 21 g/kg required for con-
densation. This yields another interesting concept, since it
implies that the precipitated water is the same amount (or less)
as the water evapo-transpirated along the path of the combined
breeze. This would further suggest that this amount of water is
simply recycled by this wind system under the right conditions.
Finally, this raises yet another question concerning the concept
of water resources. For example, whether the 120 L/m
per year
contributed by the storms could be considered the product of
15 L/m
per storm-event, times 8 storm-events/year. Are we see-
ing the same 15 L eight times? and, do we require the evapo-
transpiration from the surface to see that amount of water
those 8 times?
The most probable answer is yes, and if so, the concept of a free
(God-given) water resource, which only needs to be managed
properly, appears to be one of the great fallacies of conventional
wisdom. In this case, it seems that the storms must be ‘‘cultivated’’
Fig. 12. Comparison between temperature profiles over the places with triangles at right. Measured (solid black line, time in brackets), and simulated (small circles line, time
full hour). These are two extreme cases where the modelling results clearly underestimate the temperature structure measured by the instrumented aircraft.
0 10203040506070809010
Conc. O3 (ppb)
Height (m.)
10 15 20 25 30
Temperature (C)
318° K
315° K
312° K
26º C
299 K
303 K
Fig. 13. Range of temperature soundings measured over the Balearic Basin during
the EC’s RECAPMA project in July 1991. This overlay includes the profiles with the
highest and the lowest potential temperatures observed. The dashed 303 K
adiabatic profile (30 °C from 0 m to 2000 m) is included as a reference. Weak blue
lines are ozone concentrations.
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 217
Fig. 14. The summer shower/storm cycle on Western Mediterranean coasts. Note that the water evaporated at the coasts precipitates in the interior (see text) and, thus, that
the real water cycle includes the moisture in the soil, the marshes, the river flow and the acquifer that feeds the coastal marshes; i.e., they are all part of the same ‘‘water
Fig. 15. Postulated feedback loops between land-use perturbations in the Western Mediterranean basin and the climatic-hydrological system from the local through the
regional to the global scales. Beginning at the blocks Land-Use and Evaporation From Sea, the path of the water vapour is marked by dark blue arrows, the directly related
effects by black arrows, and the indirect effects by other colours. Critical thresholds are squared in red and indicate when the system tips to a different state. The various loops
are discussed in the text.
218 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
(Millán, 2012) in order to keep that amount of water recycling
within this system (Fig. 14). Similarly, in response to some of the
questions raised initially, one could also consider the amount of
water vapour that crosses the European Divide as a carrier compo-
nent that requires the additional water evaporated from within the
Mediterranean Catchment basin as the trigger component for the
rain events to occur. If this is so, how much of the water evapo-
rated within the basin is returned by the precipitation? This could
be another interesting topic for research and isotopic studies of
Finally, it seems that ‘‘closed’’ conditions in various parts of
the Mediterranean Basin may have begun with the desiccation
of coastal marches carried out by the Romans in Spain and
northern Africa (Bölle, 2003a,b) to avoid malaria. This was fol-
lowed by centuries of agricultural activities in the desiccated
areas, and deforestation and forest fires over the mountain
slopes. More recently, explosive urbanisation along the coasts
have sealed the soil with houses, asphalt, and cement, thus
increasing the heating of the airmass in the combined breezes.
All of these have contributed to the expansion of the areas with
closed vertical circulations. The dramatic increase in tropo-
spheric ozone in the late 1970s affecting crops in the Mediterra-
nean (Naveh et al., 1978; Lorenzini and Panattoni, 1986)
followed the intense industrialisation of the coasts (Refineries,
Power Plants) after the 1974 Middle East war.
The atmospheric circulations becoming closed in more areas,
through land-use changes, explains the accumulation of oxidants
and the ozone levels detected (Millán et al., 2000, 2002). Finally,
other significant changes in the rain runoff records detected at
the end of the 1970s, i.e., the ‘‘1980 runoff anomaly’’ (Cabezas,
2007), illustrated in Fig. 16, suggest that the atmospheric circula-
tions over the coasts of the Western Mediterranean Basin in sum-
mer had gone from being mostly open in the past (as they still are
over Central Italy) to being closed on the majority of the coasts
nowadays. This transition to a more closed system, i.e., to ‘‘the
recirculating cauldron’’ (Section 7), leads to the accumulation
mode for pollutants and water vapour, now detected by satellites,
which probably occurred at the end of the 1970s. As a result, the
1980 water runoff anomaly may just reflect the combined loss of
precipitation from the Atlantic frontal systems and from the sum-
mer storms, and that the focal areas for rains from Mediterranean
cyclogenesis got closer to the coast (Section 2). This should also be
studied further.
9. The accumulation mode and satellite water vapour data
As mentioned above, current numerical simulation (meso-
scale) models, even when using 2 km 2 km grids, do not capture
all the details of the observed processes; e.g., they cannot repro-
duce either the vertical recirculations or the layering over the
sea. Nevertheless, the outcome of these processes, i.e., the develop-
ment of an accumulation mode for water vapour (and pollutants)
over the sea, can be validated with water column data from the
NASA-MODIS satellite (Gao and Kaufman, 2003; King et al., 2003)
since the year 2000. The concept is shown in Fig. 6, and was intro-
duced in Section 5.
Fig. 17 shows the monthly averages of the NASA MODIS – Terra
measurements for July 2000 and 2005. The Day product derived
from the morning pass at 10:30 UTC emphasises the areas where
the satellite looks down the deep orographic-convection chimneys
developing at the fronts of the combined breezes, i.e., around the
edges of the basin (Figs. 6, 8 and 10). It also outlines the deep in-
land penetration of the seabreezes over the desert areas of Tunisia,
Libya and Egypt. The Day +Night product emphasises the areas
over which water vapour accumulation occurs at the end of the
diurnal vertical circulation cycle. That is, the water vapour that
did not precipitate over the coastal mountains in the afternoon fol-
lows the return flows aloft and fills in the Western Mediterranean
Basin (to 4500 + m) with layers of water vapour (and pollutants).
The same appears to occur over the Adriatic and Black seas.
This produces an ‘‘Accumulation Mode’’ in summer and, with-
out requiring the high evaporation rates of more tropical latitudes,
e.g., the Gulf of Mexico (Ulbrich et al., 2003), the mechanisms de-
scribed in the text are able, in just a few days, to generate a very
large, deep and polluted air mass that increases both in moisture
content and in potential instability with each passing day. Assum-
ing that the vertical circulation accumulation cycles last 3–5 days,
and that they recur several times a month, e.g., for an average of 21
recirculating days during July–August, it can be estimated that the
monthly averages obtained from the MODIS-Terra Day +Night
products (e.g., Figs. 17 and 19) are comparable to the column val-
ues of water vapour accumulated after approximately 3–4 days of
vertical recirculations.
The upper part of Fig. 18 shows two areas for which the daily
MODIS-Terra Day +Night product has been averaged for every
day of the year from 2000 to 2008. The results at the bottom part
of the Figure show that there is an intense and prolonged accumu-
Fig. 16. The 1980 water runoff anomaly as recorded by the Segura Watershed Authority (Confederación Hidrográfica del Segura). Provided by Prof. Sandra García
(Universidad Politécnica de Cartagena, UPCT).
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 219
lation mode over the Western Basin and the Black Sea in summer,
and two weaker accumulation modes over the Eastern Basin in
spring and autumn. This also brings out the point that meteorolog-
ical processes are different in the various Mediterranean sub-
10. What happens to the unprecipitated water vapour?
There are several significant consequences from the accumula-
tion mode. The first effect can occur every few days, when the
moist and polluted air masses are vented out of the area by an
upper atmospheric perturbation. Under certain conditions the
accumulated water vapour can contribute to intense V
tion events and summer floods in Central and Eastern Europe, as
illustrated in Fig. 19 (Ulbrich et al., 2003) and by modelling in
Gangoiti et al. (2011a,b). The response time from the accumulation
mode to the precipitation is of the order of days (to a week)
(Gangoiti et al., 2011a). Along with Fig. 3, they illustrate the inter-
connection between hydrological processes in Europe. That is, how
a local loss of summer storms around the Western Mediterranean,
caused by land use changes, leads to a local-to-regional vertical
recirculation-accumulation mode of water vapour over the sea,
and how the accumulated water vapour can be advected out of
the WMB and participate in major precipitation events, and floods,
in other parts of Europe.
The second effect occurs during the vertical recirculation-accu-
mulation periods. The greenhouse heating of the water vapour
(Figs. 17 and 18), the photo-oxidants (ozone) and the aerosols
accumulated in layers over the sea increases the Sea Surface Tem-
perature during summer. This propitiates a second feedback loop,
shown in Fig. 15, where Mediterranean Cyclogenesis fed by a war-
m(er) sea (Millán et al., 1995; Pastor et al., 2001) contributes to an
increase in torrential rains and floods in Mediterranean coastal
areas and islands, from autumn to spring. These effects have a de-
lay of a few months with respect to the closing of the first loop that
leads to the accumulation mode.
In this second feedback loop, another ‘‘second critical thresh-
old’’ or tipping point may then be crossed during intense Mediter-
ranean Cyclogenesis events, if torrential rains set off flash floods
and/or mud flows over the mountains already affected by the first
feedback loop, i.e., over drier and/or vegetation-deprived slopes.
This can increase erosion and/or produce massive soil losses which,
in turn, can further reinforce the first feedback loop, and the accu-
mulation mode. In this context, the Accumulation Mode could be
considered as ‘‘the memory of the system to land-use perturba-
tions’’, since it can propagate its effects (desertification) through
the increase in torrential rains to the same area, or (randomly, like
in a Russian roulette) to other parts of the Mediterranean basin.
For example, around the Mediterranean sea, deserts and desert-
like conditions are now found in proximity to a warm sea and,
thus, to a marine airmass with a high moisture content, e.g., the
coasts of Algeria, Tunisia, Libya, and Almeria in Southeastern Spain.
These regions were covered with vegetation in historical times,
e.g., during the Roman Empire (Bölle, 2003a). In Almeria, the oak
forests covering the mountains were cut down to fuel mines some
200 years ago (Fletcher, 1991; Charco, 2002), mud flows followed
in the early 1800s, leaving it the most desert-like area in Europe
(Grove and Rackham, 2001). Thus, the question is: did these areas
run the two feedback loops towards drought and desertification as
a consequence of desiccating the coastal marshes and removing
the forests?
The important point here is that the storms in the coastal sys-
tems could be recovered, i.e., to stabilise the local-to-regional com-
ponent of the hydrological cycle, but only if the second critical
threshold, i.e., the massive loss of soil, has not been crossed. This
Fig. 17. Monthly averages of the NASA MODIS-Terra water vapour data for July 2001 and 2005. The units are total precipitable cm.
220 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
would require judicious reforestation programs (Millán, 2012) to-
gether with appropriate management of the water cycle at the
same scale, e.g., to cultivate the storms, and could be regarded as
adaptation measures to climate change.
Fig. 15 shows a ‘‘third Atlantic-global feedback loop’’ with two
branches that could affect the North Atlantic Oscillation (NAO). The
oceanic component derives from losing the moisture accumulated
over the Western Mediterranean to feed summer floods in Central-
Eastern Europe. It would favour the output of saltier water to the
Atlantic, which has been considered an important tipping point
for world climate (Kemp, 2005). The other path involves perturba-
tions to the extra-tropical depressions and hurricanes in the Gulf of
Mexico caused by changing the characteristics of the Saharan dust
transported across the Atlantic (Prospero, 1996), and this will be
discussed now.
During the vertical recirculating periods, new Atlantic air enters
the WMB every day (mostly at night), mainly through the Carcas-
sone gap. Part of the vertically recirculated and aged airmass exits
to the Atlantic, either directly through Gibraltar and/or indirectly
through the Sicilian Channel (mostly during the day). In the latter
case, the airmasses can follow the Southern Atlas Corridor towards
the Canary Islands, and further to the Middle Atlantic, as illustrated
with satellite data in Fig. 20. This possibility was already postu-
lated in 1995 (Millán et al., 1997) and was considered by the EC
for his research programmes (EC, 2001). This transport mechanism,
and its continuity towards the Caribbean, has been simulated by
modelling (Gangoiti et al., 2006).
The airmasses that exit through the Sicilian Channel travel
along the southern Atlas corridor (Fig. 20) and provide a moist,
and very polluted, background airmass. Moreover, the upslope
winds on the south-facing slopes of the Atlas also generate vertical
recirculations in which the added moisture would favour the for-
mation of shallow clouds. NASA has detected dust layers of up to
7 km in height, topped by shallow clouds, over the southern Atlas
corridor as early in the year as March (Winker et al., 1996). These
clouds would provide the right environment for heterogeneous
reactions between the Saharan dust and the pollutants carried by
the Mediterranean airmass, i.e., for the sulphatation and nitrifica-
tion of the Saharan dust, which can be transported across the Cen-
tral Atlantic towards the Caribbean, and could explain available
data (Hamelin et al., 1989; Savoie et al., 1992, 2002).
It is also interesting that the southern Atlas corridor lies along
the headwaters of one of the endorheic basins shown in Fig. 2. This
appears to be another area where deforestation played a key role in
the loss of precipitation in the recent past. Fig. 20 shows large
amounts of water vapour following this path from the Mediterra-
nean to the Atlantic, e.g., up to 40 mm over Southern Algiers. The
questions are: did the massive deforestation of the Southern Atlas,
to fabricate railroad ties in the 1800s, have anything to do with
this? For example, by eliminating or diminishing the evaporative
component required to trigger storms, as in Almeria, and has it
gone through the second critical threshold of mud flows?
11. Comments concerning climate modelling and extreme
‘‘It is very likely that hot extremes, heat waves and heavy pre-
cipitation events will continue to become more frequent’’. This is
1-ene 28-mar 23-jun 18-sep 14-dic
Fig. 18. Top: Two areas in the Western (WMB) and Eastern Mediterranean Basin
(EMB) used to average MODIS-Terra data. Bottom: Ordinate, average of the daily
Day +Night product for the years 2000 to 2008. It shows the differences between
the accumulation modes for water vapour in the East and West Basins.
Fig. 19. Left: Monthly average of the MODIS water vapour column; Day +Night product for August 2002. The average is 3 cm (30 L/m
). Right: Back trajectories (type V
) that
fed rains in Germany and the Czech Republic on 11–13 August, 2002 (Ulbrich et al., 2003). This event took place over the European Continental Divide (Fig. 3).
M.M. Millán / Journal of Hydrology 518 (2014) 206–224 221
a conclusion common to all IPCC Assessment Reports. However,
‘‘although the ability of Atmosphere–Ocean General Circulation
Models (AOGCMs) to simulate extreme events, especially hot and
cold spells, has improved, the frequency and amount of precipita-
tion falling in intense events are underestimated’’ (IPCC, 2007).
Nevertheless, some of the observed anomalies and results of these
processes were indeed addressed both in the Fourth Assessment
Report AR4 (IPCC, 2007) and in the earlier Third Assessment Report
TAR (IPCC, 2001), as information resulting from EC research pro-
jects filtered into the system. This can be considered relevant as
AR4 Section Horizontal and Vertical Resolution states that
‘‘Changes around continental margins are very important for regio-
nal climate change’’.
The development of large meso-scale circulations around large-
enclosed (Mediterranean) or semi-enclosed (e.g., Sea of Japan) seas
could underscore two other aspects mentioned in AR4 when refer-
ring to Continental edge effects and lapse rates ( Water Va-
pour and Lapse Rate). This document states that ‘‘In the planetary
boundary layer, humidity is controlled by strong coupling with
the surface, and a broad-scale quasi-unchanged Relative Humidity
(RH) response is uncontroversial’’. The uncontroversial part of this
statement is questionable for any region outside flat terrains in
mid latitudes.
The reason: if the edges of the continents wrap around a large
interior sea like the Mediterranean, or a large semi-enclosed sea
like the Sea of Japan, Continental Edge Effects blend together to
create their own large meso-meteorological circulation and
dominate the weather patterns in these regions for months. Final-
ly, the Continental Edge Effect becomes particularly important if
the continental edges are backed by complex coastal terrains, that
is, coasts backed by high mountains up to 80(+) km from the sea.
The reason for this, as related above, is that the combined breezes
make the coastal boundary layers fold over themselves, creating a
multitude of ‘‘residual boundary layers’’ piled-up several km high
over the enclosed seas. This accumulation mode could explain
the observed temperature lapse rates (Figs. 7, 12 and 13) and the
water vapour profiles, i.e., the perturbed temperature and Relative
Humidity (RH), observed over some coastal areas, and over large
interior seas in sub-tropical latitudes.
12. Conclusions
To finalise, the following points should be considered regarding
extreme hydrometeorological events and possible climatic
(a) Drought and torrential rains in areas around enclosed seas in
the subtropical latitudes (e.g. Mediterranean, Sea of Japan,
South China Sea) are the result of a series of concatenated
meteorological processes involving water vapour accumula-
tion modes over the interior seas.
(b) These result from atmosphere–land–oceanic feedbacks and
the folding of the boundary layers over the seas, and, in par-
ticular over the WMB.
Fig. 20. Averages of the NASA MODIS-Terra measurements for September 2002 and 2005. The Day product shows large amounts of water vapour along the Southern Atlas
Corridor crossing from the Mediterranean to the mid Atlantic. This is now an endorheic basin (Fig. 2) where a series of ephemeral lakes remain witnesses to times of more
222 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
(c) The same processes can also lead to intense precipitation
events and summer floods in other parts of Europe (i.e.,
points along the European Continental Divide, or within
the Mediterranean Catchment side of the Divide).
(d) Moreover, through the intensification of the Atlantic-Medi-
terranean Salinity Valve at Gibraltar, the North Atlantic
Oscillation could be perturbed and affect precipitation
regimes on the Atlantic side of the European Continental
(e) Therefore, the local-to-regional perturbations initiated by
land-use changes at the local level may propagate their
effects to the Global Climate System.
(f) The basic atmosphere–land–ocean exchange governing
these processes, through BL parametrisations, is not (and
probably cannot be) included in current Atmosphere–Ocean
General Circulation Models (AOGCMs).
(g) Thus, the feedback processes in the hydrological cycle,
which govern the partitioning of precipitation and the recy-
cling of water vapour, as well as the development of extreme
hydro-meteorological events, cannot currently be simulated
in the AOGC Models used to assess extreme events in sub-
tropical latitudes.
This situation now presents the research community with very
important challenges to improve: first, meteorological forecasting
methods adapted to these processes, and second, Atmosphere–
Land–Oceanic parametrisations in Atmosphere–Ocean General Cir-
culation Models. It alerts that hasty decisions on the future of the
water cycle using current models could be wrong and have cata-
strophic consequences not only for Southern Europe, but also for
the rest of Europe.
This paper was supported by the Generalitat Valenciana, and
the projects CIRCE (EC), GRACCIE (Consolider-Ingenio 2010, SNRP)
and FEEDBACKS (Prometeo – Generalitat Valenciana).
The MODIS images used in this study were acquired with the
GES-DISC Interactive Online Visualization and Analysis Infrastruc-
ture (Giovanni) as part of NASA Goddard Earth Sciences (GES)
‘‘Data and Information Services Center (DISC)’’. The author would
like to acknowledge Prof. Lucio Alonso at UPV-EHU (Bilbao) for
preparing all the satellite water vapour averages shown, as well
as other colleagues who have cooperated with the preparation of
the Figures: R. Salvador (Figs. 4, 8–10 and 12), E. Mantilla (Figs. 5,
7, 8, 11 and 13), M.J. Estrela (Fig. 1), J.L. Palau (Fig. 18), I. Gómez-
Domenech, and G.Gangoiti (Fig. 12).
The author would like to dedicate this work to the memories of
Dr. Heinrich (Heinz) Ott (2004), for his encouragement and sup-
port at the beginning of this work, and to Dr. Anver Ghazi (
2005), who continued his support in difficult times.
Appendix A
The first experimental data used for this work were obtained
during the European Commission Campaigns on the Remote Sens-
ing of Air Pollution in: LACQ (France, 1975), TURBIGO (Po Valley,
Italy, 1979), and FOS-BERRE (Marseille, France, 1983). Additional
experimental and modelling results come from the European Com-
mission research projects: MECAPIP (1988–1991), RECAPMA
(1990–1992), SECAP (1992–1995), T-TRAPEM (1992–1995), MED-
CAPHOT-TRACE (1993–1995), BEMA I (1994–1995), VOTALP I
(1996–1998), BEMA II (Phase II/1996–1998), VOTALP II (1998–
2000), MEDEFLU (1998–2000), RECAB (2000–2003), ADIOS
(2001–2003), MICE (2001–2003), CARBOMONT (2001–2004),
FUMAPEX (2002–2005), and CIRCE (2007–2011).
The author participated directly in all the projects underlined
and, through his membership in the EC Project Cluster Evaluation
Groups, had first-hand access to the Progress Reports of the other
projects listed. The ten projects in bold print used the Valencia-
Castellón experimental area. The source of information and com-
ments from projects on Desertification was Dr. Heinrich Ott (EC).
The EC’s Joint Research Centre JRC-Ispra provided the funding for
the MECAPIP instrumented aircraft, the Commisariat à l’Energie
Atomique (CEA), Saclay, France, supported the RECAPMA flights
in the Western Basin, and the Israeli Air Force the SECAP flights
in the Eastern Mediterranean Basin. Complementary financing to
the EC funding was provided by the National Research Programs
in the countries of the participating teams. In the case of Fundación
CEAM, co-financing of these projects was provided by Spanish Na-
tional Research Program (SNRP, for the daily precipitation data),
the Generalitat Valenciana, and BANCAJA.
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224 M.M. Millán / Journal of Hydrology 518 (2014) 206–224
... For example, the dynamic factors (i.e., increasing surface roughness and urbanization expansion) have primarily driven the slowdown of summer SB speed (-0.033 m s −1 decade −1 ) in Shanghai (China) for 1994-2014 (Shen et al. 2019); and also in Colombo (Sri Lanka) for the same period, with a greater wind speed decline after 2010 (Shen et al. 2021b). On the opposite, the increase in air temperature has driven an annual and seasonal strengthening of the maximum SB speed in Adelaide (Australia) for 1955(Pazandeh-Masouleh et al. 2019, and recent studies evidence ongoing positive wind trends in Adelaide and Perth for 1994-2014(Shen et al. 2021b). In addition, solar radiation also determined winter variations on SB speed in Los Angeles (USA) for 1994-2014 (Shen et al. 2021a). ...
... It is also very interesting that a decrease in the SB speeds does not exclude a winter increase in its extremes, e.g., SB gusts and fronts (Laurila et al. 2021), possibly due to the direct or collateral effect of global warming on changes in atmospheric circulation and land uses (Miao et al. 2003). However, the weakening of summer SB speeds, and the declining of its activity may be explain reductions in summer inland precipitation with implications in amplifying droughts, land aridity and exacerbating wildfires (Millán et al. 2005;Millán 2014;Pastor et al. 2015;Pausas and Millán 2019). In specific locations, stronger but drier gusts may occur as response of the combined effect of air-temperature rise and soil depletion, urbanization and land degradation on reducing the moisture available for SB (Pausas and Millán 2019;Guion et al. 2021). ...
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Most studies on wind variability have deepened into the stilling vs. reversal phenomena at global to regional scales, while the long-term changes in local-scale winds such as sea-breezes (SB) represent a gap of knowledge in climate research. The state-of-the-art of the wind variability studies suggests a hypothetical reinforcement of SB at coastal stations. We first developed a robust automated method for the identification of SB days. Then, by using homogenized wind observations from 16 stations across Eastern Spain, we identified 9,349 episodes for analyzing the multidecadal variability and trends in SB speeds, gusts and occurrence for 1961–2019. The major finding is the opposite trends and decoupled variability of SB speeds and gusts: the SB speeds declined significantly in all seasons (except for winter), and the SB gusts strengthened at the annual scale and in autumn–winter, being most significant in autumn. Our results also show that the SB occurrence has increased across most of Eastern Spain, although presenting contrasting seasonal trends: positive in winter and negative in summer. We found that more frequent anticyclonic conditions, NAOI + and MOI + are positively linked to the increased winter occurrence; however, the causes behind the opposite trends in SB speeds and gusts remain unclear. The SB changes are complex to explain, involving both large-scale circulation and physical-local factors that challenge the understanding of the opposite trends. Further investigation is needed to assess whether these trends are a widespread phenomenon, while climate models could simulate the drivers behind these decoupled SB changes in a warmer climate.
... At a global scale, evapotranspiration from forests and vegetation represents the most important input for terrestrial precipitation (Sheil, 2018). Deforestation and changes in vegetation alter the global water cycle, modifying precipitation drivers elsewhere and increasing the frequency of extreme event occurrence across the globe (Millan, 2014). Recovery and protection of forested areas over a regional extent can regulate water flows and intensify water cooling (Ellison et al., 2017). ...
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Study region Watersheds of the Atlantic Coast of the United States (ACUS) spanning a latitudinal range of 30.8°N – 46.5°N Study focus This study assessed climate-induced changes (CMIP5-RCP 8.5) in projected streamflow and water temperature estimates from individual SWAT watershed models across the ACUS territory by mid and end of the century. Seasonal and spatial trends as well as relationships between hydroclimatologic variables were analyzed to identify opportunities for regional implementation of water management strategies. New hydrological insights for the region Changes in hydroclimatologic variables suggest spatial trends clearly differentiated by seasons. Northern and central watersheds are projected to experience the most dramatic changes in winter, summer, and spring streamflows (67 %, −25 %, and −24 %, respectively) and summer water temperature (6.2 °C), while southern watersheds presented the largest streamflow increase in fall (35 %), and water temperature changes greater than 3 °C for all seasons. These similarities and contrasts between ACUS watersheds’ hydrologic responses provide an opportunity for regional management of climate induced impacts on water resources. Mitigation strategies such as regional conservation of forests and wetlands can alleviate water scarcity and extreme flow events occurring across the ACUS under the assumed emissions scenario. Results suggest that changes in seasonal air temperature and water temperature may be linearly related in watersheds at lower elevations with no snow influence; while streamflow, precipitation, and air temperature changes have complex non-linear relationships.
... In general, comparative studies of the dynamics and transport of both dissolved and sediment loads in Serbia are represented in several papers [8,9,10,11]. In the last few decades, the extremes of meteorological-hydrological events in Europe have been particularly important for the occurrence of natural hazards [12]. Tošić and Unikašević (2014) showed that wet and dry periods become more frequent at the territory of Serbia after 1970 [13], pointing also to more frequent rainfall extremes and consequently torrential flood events [14,15]. ...
... Climate and human activities have altered water-cycle and runoff dynamics regional and globally [1][2][3]. Quantitively understanding the impact of climate change and human activities on runoff changes is a challenge, and there is always a debate about that [4,5]. Climate change can directly affect the hydrological cycle by changing precipitation or evapotranspiration [6,7]. ...
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Quantifying the impact of climate change and human activities on runoff changes is beneficial for developing sustainable water-management strategies within the local ecosystem. Machine-learning models were widely used in scientific research; yet, whether it is applicable for quantifying the contribution of climate change and human activities to runoff changes is not well understood. To provide a new pathway, we quantified the contribution of climate change and human activities to runoff changes using a machine-learning method (random forest model) in two semi-humid basins in this study. Results show that the random forest model provides good performances for runoff simulation; the contributions of climate change and human activities to runoff changes from 1982 to 2014 were found between 6-9% and 91-94% in the Zijinguan basin, and 31-44% and 56-69% in the Daomaguan basin, respectively. Furthermore, the model performances were also compared with those of well-known elasticity-based and double-mass curve methods, and the results of these models are approximate in the investigated basins, which implies that the random forest model has the potential for runoff simulation and for quantifying the impact of climate change and human activities on runoff changes. This study provides a new methodology for studying the impact of climate change and human activities on runoff changes, and the limited numbers of parameters make this methodology important for further applications to other basins elsewhere. Nevertheless, the physical interpretation should be made with caution and more comprehensive comparison work must be performed to assess the model's applicability.
... Fuente: Elaboración propia a partir de y Millán, 2014. Para que se formen las tormentas es necesario llegar al punto de condensación durante el ascenso de las masas de aire por las laderas. ...
Conference Paper
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Los sistemas de riego históricos de Sierra Nevada, de origen árabe, han sido adaptados al contexto socioecológico regional mediante la experimentación a lo largo de los siglos. Partimos de la hipótesis de que los agroecosistemas, manejados tradicionalmente, generan importantes servicios ecosistémicos. El objetivo principal de la investigación es profundizar en el conocimiento de estos sistemas e interpretar sus funciones en clave de Servicios Ecosistémicos y trade-offs. Los principales SE identificados son: el aumento generalizado de la humedad en las laderas, que lleva asociado un aumento de la biodiversidad, la regulación de los ciclos hidrológicos regionales y el aporte de agua para consumo urbano y agrícola. El principal conflicto está relacionado con el incumplimiento de los caudales ecológicos de los ríos impuestos por Plan Hidrológico 2015-2021 de la Demarcación Hidrográfica de las Cuencas Mediterráneas Andaluzas.
... Transpiration is such a process (Fig. 5b). This echoes the suggestion of Millán (2012Millán ( , 2014 made in the context of the Mediterranean region when he argued that the rainfall must be "cultivated" by maintaining a vigorously transpiring vegetation that can import moisture import from the ocean. ...
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The terrestrial water cycle links the soil and atmosphere moisture reservoirs through four fluxes: precipitation, evaporation, runoff, and atmospheric moisture convergence (net import of water vapor to balance runoff). Each of these processes is essential for human and ecosystem well-being. Predicting how the water cycle responds to changes in vegetation cover remains a challenge. Recently, changes in plant transpiration across the Amazon basin were shown to be associated exponentially with changes in rainfall, suggesting that even small declines in transpiration (e.g. from deforestation) would lead to much larger declines in rainfall. Here, constraining these findings by the law of mass conservation, we show that in a sufficiently wet atmosphere, forest transpiration can control atmospheric moisture convergence such that increased transpiration enhances atmospheric moisture import and resulting water yield. Conversely, in a sufficiently dry atmosphere increased transpiration reduces atmospheric moisture convergence and water yield. This previously unrecognized dichotomy explains the otherwise mixed observations of how water yield responds to re-greening, as we illustrate with examples from China's Loess Plateau. Our analysis indicates that any additional moisture recycling due to additional vegetation increases precipitation but decreases local water yield and steady-state runoff. Therefore, in the drier regions/periods and early stages of ecological restoration, the role of vegetation can be confined to moisture recycling, while once a wetter stage is achieved, additional vegetation enhances atmospheric moisture convergence. Evaluating the transition between regimes, and recognizing the potential of vegetation for enhancing moisture convergence, are crucial for characterizing the consequences of deforestation as well as for motivating and guiding ecological restoration.
... In spite of this, our results unveil a nonnegligible ozone formation from continental emissions taking place in the Iberian Peninsula and neighbouring countries, which contribute to increase tropospheric ozone downwind, that is, over the Mediterranean region. Bearing in mind the extensive knowledge about Mediterranean airpollutants dynamics, affected by recirculation processes over the basin (see Millán, 2014 and references therein), a common strategy to offset ozone precursors in Mediterranean countries should be adopted. Having in consideration that ozone pollution is one of the main air quality concerns in Southern European countries, the air quality benefit induced by the health crisis may persuade our politicians and policymakers to really battle against air pollution. ...
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In this work we investigate the variation in tropospheric ozone concentrations in south-western Europe in March and April 2020 in the context of COVID-19 disease, and to what extent the former situation has been recovered one year after the pandemic outbreak. To carry this study, data from 15 regional background sites in Spain, from 2010 onwards, are used. Historic (2010–2019) and most recent tropospheric ozone concentrations are compared. March and April 2020 ozone concentrations declined over 15% in most cases, rising to 23–28% at sites facing the Mediterranean. Most of the decay was related to the reduction of hemispheric background concentrations, but those sites downwind continental emissions from the Iberian Peninsula and neighbouring countries experienced an additional lessening. By exploring O3 concentrations one year after, March and April 2021, the general decline with respect to 2010–2019 persist but its magnitude was substantially lessened with respect to the strict lockdown period. This pandemic situation has unveiled that air pollution is not an endemic matter but it should be tackle with adequate actions. Ozone abatement plans for Mediterranean countries should need a pan-regional covenant in order to drop precursor emissions.
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Domingo Rasilla Presidente de la Asociación Española de Climatología (AEC) Tras el parón forzado por la COVID 19, la Asociación Española de Climatología retoma el curso habitual de actividades organizando el XII Congreso Internacional de la AEC, en colaboración con la Universidad de Santiago de Compostela y la Universidad de Vigo, bajo el lema “Retos del Cambio Climático: impactos, mitigación y adaptación”. En los años transcurridos desde nuestro último congreso, en Cartagena, el mundo ha experimentado una transformación, no sólo derivada de las consecuencias de la pandemia: una guerra en Europa, una crisis económica global etc… Quedan lejos los tiempos de la efervescencia mediática entorno al activismo climático, ya que el interés de la sociedad parece haber girado hacia otros temas. Sin embargo, las conclusiones globales aportadas por los diferentes grupos de trabajo del Sexto informe de evaluación del IPCC, al igual que fenómenos más domésticos como “Filomena”, “Gloria” o “Celia”, la ola de calor de junio pasado o las más de 20 000 ha quemadas en la Sierra de la Culebra, atestiguan las consecuencias de la crisis climática, y la necesidad de abordar medidas para su mitigación y soluciones de adaptación que suavicen el tránsito a este nuevo escenario. El XII Congreso Internacional de la AEC llega de la mano de un grupo entusiasta de profesores, los doctores Nieves Lorenzo, Alberto Martí y Dominic Royé, un equipo cuya formación multidisciplinar deja patente la necesidad de abordar el estudio del Clima desde una perspectiva transversal, una máxima que ha acompañado a la AEC desde el principio de sus actividades. A pesar de las numerosas dificultades que han jalonado su camino, nunca manifestaron deseo de abandonar la tarea encomendada. Por ello, en nombre de la Junta Directiva y de todos los socios de la AEC, quisiera agradecerles y felicitarles por su buen hacer y dedicación a lo largo de estos años. Extiendo este agradecimiento también a todos los organismos e instituciones que han apoyado este proyecto: AEMET, Xunta de Galicia, Meteogalicia, ABanca, Confederación Hidrográfica Miño‐Sil, Augas de Galicia, FIC, Meteored, Vexiza y Caixa Rural Galega. El XII Congreso Internacional coincide además con otro hito: este año la AEC cumple 25 años. Nacida como un foro de debate y de intercambio de información, la AEC ha protagonizado un espectacular avance de los estudios climáticos en nuestro país, materializado en una profunda renovación metodológica y temática. Esta historia de éxito recae, en gran medida, en una generación que, a la vez mentores y amigos, creó e impulsó la AEC, y que por razón de edad, está adquiriendo nuevas responsabilidades. Desde la Junta Directiva de la AEC queremos que estas líneas también sirvan de homenaje a esa generación; nuestro objetivo debe ser el prolongar y mejorar, si es posible, su legado para que la AEC perdure, al menos, otros 25 años. Por ello, la AEC debe adaptarse a un mundo en acelerada transformación, con el reto de ampliar sus actividades en ámbitos más allá del puramente académico, pues es evidente la necesidad de transmitir a la sociedad, de forma rigurosa pero también próxima, las consecuencias, cada vez más evidentes, de un cambio climático global.
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With increasing application of machine learning (ML) in hydrometeorology, we face the urge to demystify the black–box of ML because it usually does not provide physically interpretable information to users. Here, we demonstrate multiple post–hoc interpretation methods to evaluate feature effects for hydrometeorological prediction. These methods were integrated and applied to soil moisture (SM) as an example, and random forest was used to establish a predication model based on a FLUXNET site in Haibei, China. From different views of interpretability, feature importance, Shapley values, partial dependence plot, individual conditional expectation, accumulated local effect were used to investigate how features affect prediction. The result shows that comprehensive understanding can be achieved on relationship between predicted SM and affecting variables including lagged SM, precipitation, soil temperature. Thus, we advocated integrated interpretation tools to enhance practicability of ML for hydrologists and other physical scientists. A toolbox named as ExplainAI is provided.
This book provides an evaluation of the science and policy debates on climate change and offers a reframing of the challenges they pose, as understood by key international experts and players in the field. It also gives an important and original perspective on interpreting climate action and provides compelling evidence of the weakness of arguments that frame climate policy as a win-or-lose situation. At the same time, the book goes beyond providing yet another description of climate change trends and policy processes. Its goal is to make available, in a series of in-depth reflections and insights by key international figures representing science, business, finance and civil society, what is really needed to link knowledge to action. Different contributions convincingly show that it is time – and possible – to reframe the climate debate in a completely new light, perhaps as a system transformative attractor for new green growth, sustainable development, and technological innovation. Reframing the Problem of Climate Change reflects a deep belief that dealing with climate change does not have to be a zero sum game, with winners and losers. The contributors argue that our societies can learn to respond to the challenge it presents and avoid both human suffering and large scale destruction of ecosystems; and that this does not necessarily require economic sacrifice.
During June 1991 more than 30 scientific teams worked in Castilla-La Mancha, Spain, studying the energy and water transfer processes between soil, vegetation, and the atmosphere in semiarid conditions within the coordinated European research project EFEDA (European Field Experiment in Desertification-threatened Areas). Measurements were made from the microscale (e.g., measurements on single plants) up to a scale compatible with the grid size of global models. For this purpose three sites were selected 70 km apart and heavily instrumented at a scale in the order of 30 sq km. Aircraft missions, satellite data, and movable equipment were deployed to provide a bridge to the larger scale. This paper gives a description of the experimental design along with some of the preliminary results of this successful experiment.
This volume presents a collection of 110 research papers and posters related to the physio-chemical behaviour of atmospheric pollutants. The work is divided into three sessions: development of analytical methods to measure trace components of the atmosphere; atmospheric chemical and photochemical processes; and field measurements and their interpretation. Included are papers resulting from the Eurotrac project and the 1988 Polarstern cruise. Over 100 papers are abstracted in Physical Geography. -after Publisher
The Mediterranean basin is a unique feature of the Earth’s system for two reasons. Firstly, its geographical position is at the border between two climate regimes, and consequently climate and climate dependent living conditions in the area are very sensitive against even small shifts of this border. Secondly, due to the enclosed large sea, the bizarre coastal structure, and the mountain barriers, it has an extremely complex topography which gives rise to specific local climatic effects and climate gradients. Nowhere in the world is the Mediterranean climate as extended as at the border between Europe, Africa, and the Near East. It covers about 10 million km2, reaching from Portugal to south of the Caspian Sea with a core area that stretches over 46 degrees longitude. This is due to the long latitudinal extent of the Mediterranean basin with an area of about 6 million km2. Of this area, 2,496 million km2 is the surface of the sea.
During the period April 1989 through December 1990, O3 concentrations in the marine boundary layer at Barbados, West Indies, show a pronounced seasonal cycle. Daily averaged values in the winter and spring often fall in the range of 25-35 ppbv for periods of several days, and they seldom fall below 20 ppbv. In contrast, during the summer, values typically fall in the range of 10-20 ppbv. During the winter-spring period, there is a very strong negative correlation between O3 and a number of aerosol species, including NO3-. These anticorrelations appear to be driven by changing transport patterns over the North Atlantic as opposed to chemical reactions involving O3 and nitrogen species in the atmosphere. Analyses of isentropic trajectories clearly show that high O3 and low NO3- are associated with transport from higher latitudes and high altitudes. Conversely, high NO3- and relatively low O3 are associated with transport from Africa. Our study suggests that North America and the middle troposphere (and stratosphere) are not strong sources for NO3- over the tropical North Atlantic. The strong correlation of NO3- with Pb-210 and the weaker correlation with Saharan dust indicates that NO3- is derived principally from continental surface sources, probably in Europe and North Africa, but not from the Saharan soil material itself. During several extended periods, NO3- and Pb-210 were strongly correlated and their concentrations were high relative to nss SO4=; these factors, coupled with trajectories originating in Africa, suggest that African biomass burning was a significant source at these times. In contrast, biomass burning appears to be a minor source for O3 as measured at Barbados, perhaps accounting for an enhancement of about 5 ppbv at most during these periods.