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Stratospheric Research Group
FU Berlin

Alerting Criteria
for the STRATALERT and GEOALERT/STRATWARM messages

The magnitude of a stratospheric warming depends on the amount of temperature increase, the vertical extent, the areal extent, and the effect on the hemispheric (or even global) circulation. In spite of the relatively long period during which this stratospheric phenomenon has been studied, it is still not possible to define precise characteristics that will cover all winter stratospheric warming events. Therefore, the following (somewhat arbitrary) definitions are proposed:
  • A stratospheric warming is called MINOR if a significant temperature increase is observed (i.e. at least 25 degrees in a period of a week or less) at any stratospheric level in any area of the wintertime hemisphere, measured by radiosonde and/or indicated by satellite data. Warmings not meeting the criteria for this definition may be said to be local warmings;
  • A stratospheric warming can be said to be MAJOR if a 10-hPa or below the zonal mean temperature increases poleward from 60 degrees latitude and an associated circulation reversal is observed (i.e. net mean easterly winds poleward of 60 degrees latitude);
  • Another type of stratospheric warming not belonging to either of the above criteria is called the Canadian warming which originates through the pulsation of the Aleutian anticyclone with the reversal of the temperature gradient poleward from 60 degrees North.
These definitions lead to the following
alerting criteria:

A STRATALERT may be initiated when the temperature increases at least 25 degrees in a period of a week or less at any stratospheric level in any area.
A GEOALERT/STRATWARM alert may be initiated when the temperature increases at least 30 degrees in a week or less at 10-hPa or below, or at least 40 degrees above 10-hPa.
Both kinds of alerts will be discontinued when the circulation pattern returns to relatively undisturbed conditions.



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Last change: Montag, 01-Nov-2004 14:42:55 MET