The Two European Hydrological Cycles:

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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
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