The Two European Hydrological Cycles:

The Two European Hydrological Cycles:
The evidence from European Research Projects
Millán M. Millán, Dr.Ing.Ind., Ph.D.
Executive Director CEAM
(Mediterranean Centre for Environmental Studies)
Frankfurt GLOBAL BUSINESS week
Session II: Competing Demands for Scarce Water Resources in Agriculture,
Industry and Urban Agglomerations
Frankfurt 20-21 May, 2010

The first experimental data used for this work were obtained during the European
Commission Campaigns on Remote Sensing of Air Pollution in: LACQ (France, 1975),
TURBIGO (Po Valley, Italy, in 1979) and FOS-BERRE (Marseille, France, 1983).
Additional experimental and modelling results come from the European Commission
research projects: MECAPIP (MEso-meteorological Cycles of Air Pollution in the
Iberian Peninsula, 1988-1991), RECAPMA (REgional Cycles of Air Pollution in the
Western Mediterranean Area, 1990-1992), SECAP (South European Cycles of Air
Pollution, 1992-1995), T-TRAPEM (Transport and TRansformation of Air Pollutants on
East Mediterranean, 1992-1995), MEDCAPHOT-TRACE (The Mediterranean
Campaign of Photochemical Tracers-Transport and Chemical Evolution, 1993-1995),
BEMA I (Biogenic Emissions in the Mediterranean Area, Phase I/ 1994-1995),
VOTALP I (Vertical Ozone Transport in the Alps, 1996-1998), VOTALP II (Vertical
Ozone Transport in the Alps, Phase II, 1998-2000), BEMA II (Phase II/ 1996-1998),
MEDEFLU (Carbon and Water Fluxes of MEDiterranEan Forest and Impacts of Land
Use/Cover Changes, 1998-2000), RECAB (REgional Assessment and Modelling of the
CArbon Balance within Europe, 2000-2003), ADIOS (Atmospheric Deposition and
Impact of pollutants, key elements and nutrients on the Open Mediterranean Sea,
2001-2003), CARBOMONT (Effects of Land Use Changes on Sources, Sinks and
Fluxes of CARBOn in European MOuNTain Areas, 2001-2004), MICE (Modelling the
Impact of Extreme Events, 2001-2003), and FUMAPEX (Integrated Systems for
Forecasting Urban Meteorology, Air Pollution and Population Exposure, 2002-2005).
The new integrated project, CIRCE Climate Change Impacts in the Mediterranean
(2007- 2011)

� 21g(H 2 O)/kg(air)
2000 m �
� 20g(H 2 O)/kg(air)
2000 m �
60 �100 km
26º C
14g(H 2 O)/kg(air)
26º C
14g(H 2 O)/kg(air)

DAY 10:30

DAY 10:30 & NIGHT 22:30

DAY

DAY 10:30 & NIGHT 22:30

DAY

DAY 10:30 & NIGHT 22:30

Average surface pressure for Europe in July

4
3.5
3
2.5
2
1.5
1
0.5
01-Ene-00 31-Dic-00 31-Dic-01 31-Dic-02 31-Dic-03 30-Dic-04 30-Dic-05 30-Dic-06 30-Dic-07 29-Dic-08
4
3.5
3
2.5
2
1.5
1
Western Mediterranean Basin
Eastern Mediterranean Basin
0.5
01-Ene-00 31-Dic-00 31-Dic-01 31-Dic-02 31-Dic-03 30-Dic-04 30-Dic-05 30-Dic-06 30-Dic-07 29-Dic-08

4
3,5
3
2,5
2
1,5
1
WMB
EMB
Total Precipitable Water Column
1-ene 28-mar 23-jun 18-sep 14-dic

DAY 10:30 & NIGHT 22:30

Sea Surface Temperature SST (August 2003)

DAY 10:30 & NIGHT 22:30

Back trajectories (type V b ) that fed torrential rains in Germany and the
Czech Republic on 11-13 August 2002, from: Uwe Ulbrich et al. (2003)
Weather, 58, 434-443., and EC Project MICE (Modelling the Impact of Extreme
Events, 2001-2003). Note that one of the source regions is the Western
Mediterranean Basin, and compare with the average water vapour column
accumulated during August 2002, shown in the previous figure

UP TO 90% OF PRECIPITATION
IS OF ATLANTIC ORIGIN.
SURFACE EFFECTS ARE MINIMAL

UP TO 90% OF PRECIPITATION
IS OF ATLANTIC ORIGIN.
SURFACE EFFECTS ARE MINIMAL
MORE THAN 90% OF PRECIPITATION
IS WATER RECIRCULATED WITHIN
THE BASIN. SURFACE EVAPORATION IS
A KEY TRIGGER MECHANISM

Nature, vol 437/27, October 2005
Inventing an Icon: Hans Joachim Schellnhuber’s map of
global “tipping points” in climate change

British Isles & NW
Europe experience
floods in summer &
dry cold winters
Summer drought
followed by winter
floods in Southwestern
Europe &
Northern Africa
(+ ?) Perturbation
to the NAO (North
Atlantic Oscillation)
Perturbations (?)
to extra-tropical
depressions &
hurricanes, in
the Western
Atlantic South-
Eastern USA
Increased salty
outflow to
Atlantic
� LAND-USE &
SURFACE
CHANGES
Mud floods
Augmented
soil erosion
Torrential rains over
areas surrounding the
Mediterranean
increase in autumnwinter-spring
Condensation
nuclei
transported
across Atlantic
to Caribbean
Alter the MOISTURE�,
HEAT�, & pollutants�
added to seabreezes
Loss of summer storms.
Drought increases over
coastal mountains and
other inland areas
Changes the evaporation─precipitation
balance in the western basin. Salinity
increases in the Mediterranean Sea
Heterogeneous
reactions in
shallow clouds:
sulphatation
and nitrification
of Saharan dust
Combined greenhouse
heating additionally
increases the Sea
Surface Temperature by
the end of summer
African routes:
central and/or
Southern Atlas
(with vertical
recirculations)
* EVAPORATION
FROM SEA
+ Pollution Effects
(e.g., nucleation)
Cloud Condensation
Level rises above
coastal mountains
Non-precipitated H 2O vapour
& pollutants (O 3) follow the
return flows aloft, and pile
up in a system of layers, 4.5
to 5.5 km deep, over the sea
Aged airmass advected
to other regions, in part
(daily), or in toto (every
� 3 to 10 days) �
European
route-Vb tracks
Summer floods
in Central
Europe: 1997,
2002, 2005, ...

The Two European Hydrological Cycles:
The evidence from European Research Projects
by:
Millán M. Millán, Dr.Ing.Ind., Ph.D.
Executive Director CEAM, Valencia, Spain
Member of the External Advisory Group in "Global Change and Ecosystems" for the
European Commission's 6th Framework Programme.
Synthesis of the Chapter "Climate change and drought: The role of critical thresholds and
feedbacks", prepared by the author for the Report: Climate Change Impacts on the Water
Cycle, Resources and Quality, Research-Policy Interface, European Commission, 2007:
EUR 22422, Luxembourg: Office for Official Publications of the European Communities, 149
pp. (ISBN 92-79-03314-X)
Executive Summary
Probably the most common assumption regarding the hydrological cycle is that the water
resource is universally provided through precipitation by the large weather systems. As this
is not universally true in most parts of the world, the conception that the water resource is
there and all that is required is to manage it properly is perhaps the most widely extended
fallacy regarding the water cycle. In the northern hemisphere (Slide 2), this assumption
holds only in the Atlantic and Pacific Divides north of � 35º North Latitude. Moreover, it can
only be truly asserted for the western side of the continents, i.e., the European-Atlantic and
American-Pacific sides of the continents.
For any other Water Divide (Basin) in the world, other processes must be considererd; for
example, the available precipitation in the Central North American Basin, which is lodged
between the Pacific and North Atlantic Divides, is strongly modulated by conditions in the
Gulf of Mexico, i.e., by processes and conditions at the Regional to Subcontinental scales.
In the forested tropical areas (rainforests), after a masive inflow of water vapour at the
beginning of the wet season, the water is basically recirculated between the soil-forest and
the atmosphere, producing a daily afternoon-evening shower. Thus, rainforest means
exactly that; take the forest away and the rain goes along with it, for the area to become a
desert. The rainforest is probably the ultimate case where surface properties govern the
precipitation during the wet period.
This document presents our findings about the hydrological cycle in Southern Europe (the
Mediterranean Divide), and the path followed to reach these findings from information
gathered during various European Commission projects from 1974 to 1994. These had
alerted of a loss of summer storms around the Western Mediterranean Basin. This issue
was addressed in 1995 by re-analysing the meso-meteorological information obtained in nine
of the EC projects (Slide 3) and disaggregating the precipitation components for one of the
experimental areas used in the projects (Valencia, eastern Spain). The analysis shows that
there are three sources of rain in this region, viz: Atlantic Fronts, Summer Storms, and
Mediterranean Cyclogenesis, and that the last two are dominated by land-use-atmosphericoceanic
feedbacks.
Summer storms form (or used to form) in the afternoon, over the mountains surrounding
the basin at 60 to 80(+) km from the sea, in a coastal wind system that develops during
daytime and combines the seabreeze and the up-slope winds, henceforth called the
combined breeze. In this system the seabreeze develops at mid-morning and progresses
inland by incorporating, one after another, the up-slope wind cells formed earlier that day

(Slide 4). At the leading edge of the combined breeze the air is injected vertically upwards
into the return flows aloft, and these move seaward and sink over the sea, forming a
"closed" vertical circulation loop. Storms develop when the Convective Condensation Level
(CCL) of the incoming airmass is low enough for deep (moist) convection to be triggered at
the leading edge of the combined breeze. If storms do develop (Slide 5), the airmass
becomes mixed all the way up to the tropopause, and the closed-loop coastal wind system
becomes "OPEN".
The formation of storms, however, requires the addition of water (evaporated from the
surface) to counterbalance the heating that the airmass sustains as it moves inland along the
surface, i.e., to bring its CCL below the height of injection into the return flows. Otherwise, its
CCL will keep on rising, and a "first critical threshold" (or tipping point) may be crossed
when the CCL remains higher than the height of injection over the mountain tops. In
these conditions the coastal circulations will stay "CLOSED" (Slides 6 to 8) That is, storms
will not develop, and the return flows aloft will continue moving seaward during the whole
day, forming layers that contain the non-precipitated water vapour and the pollutants carried
by the combined breeze, and piling them up to 4500(+) m over the Western Mediterranean
Basin.
Moreover, if the closed-loop conditions develop in other coastal areas they can become
self-organised at the meso-� scale of the whole Western Mediterranean Basin. These
"closed loop" conditions can last from 3 to 9 consecutive days, and recur several times a
month, i.e., for a total of 12 to 24 days per month, from late spring to late summer (May-
September). During these periods, from � 1/4 to 1/2 of the depth of the layers accumulated
over the sea on the previous days is recirculated vertically over the basin each day. And,
because no storms develop, and there is no precipitation inland, these conditions can be
considered part of a first feedback loop that tips the local climate towards increased
drought inland (Slide 31).
Current numerical simulation models cannot capture all the details of the observed
processes; e.g., they cannot reproduce either the vertical recirculations or the layering over
the sea. Nevertheless, the outcome of these processes, i.e., the development of an
accumulation mode for water vapour (and pollutants) over the sea, has been validated with
water column data from NASA-MODIS since 2000. MODIS data show that there is an
intense and prolonged accumulation mode over the Western Basin and the Black sea in
summer, and two weaker accumulation modes over the Eastern Basin in spring and autumn
(Slides 9 to 17).
These vertical recirculation-accumulation modes make the weather and climate in the
Mediterranean Basin absolutely different from any other place in the world. Thus, in
contrast with regions dominated by advection, in the Western Mediterranean Basin, water
vapour and pollutants can accumulate in layers piled-up to 4500(+) m over the sea, for
extended periods during the summer. And, without requiring the high evaporation rates of
more tropical latitudes, the above-described mechanisms are able in just a few days to
generate a very large, deep and polluted airmass that increases both in moisture content and
in potential instability with each passing day.
At the end of each accumulation period (i.e., every few days) the moist and polluted
airmasses are vented out of the area and can contribute to summer floods in Central and
Eastern Europe (Slides 23-24). Moreover, during the vertical recirculations, the greenhouse
effect of the water vapour, photo-oxidants (ozone) and aerosols accumulated in layers over
the sea enhance the sea surface water heating during the summer (Slides 18 to 22). This
propitiates a second feedback loop (Slide 31) where Mediterranean Cyclogenesis fed by a
warm(er) sea contributes to an increase in torrential rains and floods in Mediterranean
coastal areas from autumn to spring (i.e., effects delayed by a few months with respect to the

closing of the first loop).
A "second critical threshold" can then be crossed in the second feedback loop during
intense Mediterranean cyclogenesis events, when torrential rains trigger mud flows over the
drier and vegetation-deprived mountain slopes already affected by the first feedback loop.
This can result in massive soil losses and a further reinforcement, and propagation of the
effects caused by the first feedback loop, etc. Available evidence not only indicates that
these processes and feedbacks have been operating in the Mediterranean for a long time,
but also suggests that fundamental changes, and long-term perturbations to the European
water-cycle (droughts and floods), are taking place right now.
This analysis further suggests that: up to 75% (or more) of the precipitation in southern
Europe, i.e., from Summer storms and Mediterranean Cyclogenesis, originates from water
that first evaporates and then precipitates, i.e., it is recirculated, within the major
Mediterranean drainage basin (i.e., all the areas south and east of the European Continental
Divide including the Mediterranean and Black seas, Slides 25 and 27). Moreover, on the
Mediterranean side of this divide, rain of Atlantic origin contributes less than � 20% of the
total (probably even less than that in Greece), in stark contrast with the Atlantic side of the
European Divide where � 80% or more of the precipitation is from water evaporated over the
Atlantic (Slides 28-29).
Finally, land-use appears to play a key role in the loss of summer storms, the shift to more
frequent vertical recirculation periods, and the amount of water recirculated within the basin.
These findings and the questions they raise are critical for European Union water policies in
Southern Europe because:
(a) water from precipitation tends to be treated as a given resource without duly
considering its origins,
(b) disaggregation of precipitation components is not normally performed in climatic
studies and,
(c) the feedback processes in the hydrological cycle, which govern the partitioning and
the recycling of the precipitation in each region, cannot be simulated in the Global
Models used to assess future water scenarios for Europe.
Thus, the decisions on the future of the water cycle, and particularly the timing of such
decisions, could be absolutely wrong and have catastrophic consequences for Southern
Europe. This work presents a summary of the evidence we use to support this statement
(Slide 31).