From the concept of “Kaltlufttropfen” (cold air pool) to the cut-off low. The case of September 1971 in Spain as an example of their role in heavy rainfalls |
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Authors: | M-C Llasat F Martín A Barrera |
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Institution: | (1) Meteorological Hazards Analysis Team (GAMA), Department of Astronomy and Meteorology, University of Barcelona, Barcelona, Spain;(2) STAP, Instituto Nacional de Meteorología, Madrid, Spain |
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Abstract: | Summary Over the last 25 years the term “cold pool” has come to be used in many spheres as a synonym for floods. This has given rise
to a major confusion that has even moved into international scientific and technical circles. In this paper we analyse how
the concept of “cold air pool” has evolved from when it was defined at the beginning of the 20th century down to the present
day, in which the Spanish term “DANA” (similar to a cut-off low) has been introduced in order to avoid existing confusions.
In the course of the paper we take account of cold air pool genesis and their thermal and dynamic characteristics, and we
discuss the factors that have led to them being identified (erroneously) with heavy rainfall events. The study takes as its
basis a systematic analysis of all the cold air pools recorded in Europe, and particularly in the Iberian Peninsula, over
the period 1974–1983, as well as in studies of the floods recorded in eastern Spain since 1950 until nowadays. The discussion
done on the basis of this accurate analysis leads to the identification of a cold air pool as a type cut-off low (COL) and
justifying the use of this more generic term when a structure like this is present in a heavy rainfall event. For a better
illustration of the previous discussion and understanding of the role of COLs in intense rainfall events, we present the episode
of September 1971 in Catalonia, in which over 400 mm were recorded. The analysis was carried out with the MM5 initialised
with the ERA-40 re-analyses. The results show that the role of COLs in the heavy rainfall episodes recorded in Spain is mainly
dynamic, both in terms of the circulation they create at low levels and the potential vorticity anomaly generated. This circulation
draws in very warm, moist and potentially unstable air perpendicularly to the coast and the littoral mountain chains. The
factor of thermal instability, owing to the presence of cold air at medium and higher levels, shows itself to be more important
in zones where this warm moist advection at low levels is not as significant as in the Mediterranean zone. |
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