Cut-off low systems

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 (Gimeno et al., 2007a). 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 require 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 y Price, 1989; Oltmans et al., 1992; Ancellet et al., 1994; Barsby y 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 et al., 1998, 1999) are now being substituted by much faster objective procedures (e.g. Nieto et al, 2005) allowing new directions of COLs related studies.

The climate change group of IDL has done some work on these systems namely helping to analyse the precipitation and cloudiness associated with COLs located over Iberia (Nieto et al., 2007a). Additionally we have looked in detail on how these systems are related with the occurrence of blocking systems over Europe (Nieto et al., 2007b) and with the decay of the Polar vortex (Gimeno et al., 2007b). The group has also contributed significantly with a new long-term climatology for the entire Northern Hemisphere using different Reanalyses datasets and methodological approaches (Nieto et al., 2008).

Figure 1. Diagram of the typical synoptic situation of a cut-off low showing the different stages of its life cycle using the geopotential field at 200 hPa.

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