A study of coherent tropopause disturbances within the Northern Hemispheric circumpolar vortex

Coherent tropopause disturbances (CTDs) embedded within the Northern Hemispheric circumpolar vortex (CPV) are characterized by closed potential temperature and/or pressure contours on the dynamic tropopause. The interaction of CTDs with the polar-front jet stream (PFJ), as well as mergers with midlatitude disturbances, may result in intense cyclogenesis and associated high-impact weather. CTDs are of interest from a predictability perspective because the characteristic mesoscale structure of these disturbances may be inadequately sampled during their formation in arctic regions (initial analysis uncertainty) and because longwave radiative processes, which are hypothesized to play an important role in CTD formation, may be inadequately represented in global numerical weather prediction models (physics uncertainty).

The purpose of this presentation is to document the behavior of CTDs from climatological and case study perspectives. This research expands upon previous work by applying an objective tracking program to a medium-resolution dataset [NCAR-archived NCEP GFS final (FNL) analyses] during the extended cool season months of September through May for the period January 2000 through May 2004. Subjective tracking of CTDs for the 2002–2003 season (GFS initial analyses) is utilized to validate the objective tracking program. This subjective tracking is used in conjunction with real-time tracking utilizing a high-resolution dataset (GFS initial analyses) during the 2004–2005 winter season to document CTD behavior and to select case studies.

Distributions of CTD frequency, preferred genesis/lysis regions, and tracks will be presented. Frequency and genesis maxima are noted over Siberia and northern Canada. Lysis areas are distributed more uniformly. Tracks exhibit significant interannual variability depending on the areal coverage of the CPV. Findings indicate several categories of CTD behavior. Short-lived ( 3 days) disturbances tend to originate either in polar regions or along the cyclonic shear side of the PFJ. Invigoration of the PFJ by these CTDs is rare. Long-lived (1–5 weeks) disturbances tend to be of polar origin and may migrate to the southern periphery of the CPV with subsequent invigoration of the PFJ. An illustrative case study will be utilized to demonstrate CTD behavior and interactions with the PFJ and midlatitude weather systems, as well as limitations in model forecast skill for these interactions.