A statistical approach for defining speed thresholds for upper jet streaks (312)

Paula Sumrall, University of Missouri, Columbia, MO

Patrick Market, University of Missouri

 

Abstract:

 

How jet streaks are defined is often based upon long established contouring practices, whose origins often cannot be traced. For upper level jets, operational practice often emphasizes analysis of pressure surfaces (200 hPa, 250 hPa, and 300 hPa) over, say the level of maximum wind, as well as speeds in excess of 70 knots. A cursory survey of some more popular online sites supports the latter assertion: 300-hPa speeds are emphasized (often through shading) above 60 knots (COD), 70 knots (UNISYS) and 75 knots (SPC). A natural question that arises is: Could a more objective threshold be established for defining a jet streak? Current work focuses on a statistical evaluation of the 300-hPa level over the entire CONUS for one year of data (December 2012 to November 2013), selected arbitrarily. NARR fields are examined for 0000 UTC and 1200 UTC for each of the 365 days. The goal is to determine the validity of the ~70-kt isotach as a threshold to define jet streaks by comparing it to mean wind fields and their standard deviations. As of this writing, preliminary results for the winter 2012-2013 season (December 2012 to February 2013) reveal a mean wind speed over the entire CONUS of 61 knots with a standard deviation of 33 knots for the 3 month, twice daily sample (N=180), suggesting that winter season wind speeds in excess of ~90 knots. The year will be completed with normal distribution statistics at 300-hPa as well as the level of maximum wind speed.