Editorial: Special issue on cut-off low systems (COL)

Gimeno L., Trigo R.M. , Ribera P., Garcia J.A.
Meteorological and Atmospheric Physics. 96, 1-2, DOI 10.1007/s00703-006-0216-5

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A cut-off low (COL) corresponds to a closed low in the upper troposphere that has become completely detached (cut off) from the basic westerly current usually being advected equatorward of the mid-latitude westerlies. These systems are slow moving and often stay over the same region for several days, therefore capable of considerably affecting the weather conditions felt at the surface. Moreover, these features play a significant role in the tropospheric ozone balance through dissipation and mixing of stratospheric ozone. Because these structures requires information from the upper troposphere they were only described after the use of upper air soundings became available, i.e., after the 2nd World War (e.g., Douglas, 1947; Hsieh, 1949). Afterwards the interest on these events has decreased considerably, partially due to the inexistence of long and reliable climatologies.
In recent years there has been a significant increase on studies of polar and mid-latitude COLs (less of subtropical ones) focusing different perspectives, namely; (a) modelling of COL case studies and tropopause folds (e.g., Ebel et al, 1991; Langford et al, 1996), (b) impact of COLs on tropospheric ozone (e.g., Vaughan and Price, 1989; Oltmans et al, 1996; Ancellet et al, 1994; Barsby and Diab, 1995; Cuevas et al, 2000; Kentarchos et al, 2000; Baray et al, 2003), (c) mesoscale analysis of the life cycle of a COL (e.g., Ravetta and Ancellet, 2000; Gouget et al, 2000). Furthermore, COL climatologies based on (painstaking) visual inspection of thousands of charts (e.g., Price and Vaughan, 1992; Kentarchos and Davies, 1998, 1999) are now being substituted by much faster objective procedures (e.g., Nieto et al, 2005; Fuenzalida et al, 2005) allowing new directions of COLs related studies.
Because COLs are relatively frequent over and around the Mediterranean basin (i.e., southern Europe and northern Africa) it is natural that regional studies from this area are predominant in this special issue. Several studies describe the cloudiness associated with these systems while other focus on the precipitation patterns or analyze the evolution of the COL concept in recent years presenting characteristics about different events classified as COLs the time they occurred. Additionally, some of the studies deal with the identification of the mechanism responsible for the COL formation and lifecycle. Some papers relate inter-annual variability on COLs frequency and=or position with the different large-scale modes of atmospheric variability, like the NAO or the stratospheric polar vortex breakup. Intense precipitation over Manchuria (northern China) is related to the occurrence of COLs over that region in another paper. But COLs development is not exclusive of the Northern Hemsiphere, this special number includes two studies about the Southern Hemisphere, presenting a new climatology of COLs for the South America and also the complete lifecycle of a COL near Australia and some of its impacts.