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

Coherent tropopause disturbances (CTDs), which are tropopause-based mesoscale vortices, are found within the Northern Hemispheric (NH) circumpolar vortex (CPV). CTDs are characterized by closed potential temperature and/or pressure contours on maps of the dynamic tropopause (DT). The interaction of CTDs with the polar-front jet stream (PFJ), as well as mergers with midlatitude disturbances, may result in intense cyclogenesis and high-impact weather associated with surges of arctic air into middle latitudes. This evolution is particularly likely to occur when the CTD is strong (i.e., relatively high central pressure on the DT) and thus relatively close to the surface, and when the Rossby penetration depth is large due to low static stability beneath the disturbance (e.g., as is the case for a CTD moving over the Gulf Stream). CTDs also 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, contributing to initial analysis 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 higher-resolution dataset [NCAR-archived NCEP GFS final (FNL) analyses] than that used previously (GFS initial analyses) for the period from January 2000 through May 2004, allowing better definition of the mesoscale structure of CTDs. Subjective tracking of CTDs for the NH extended cool-season months of September 2002 through May 2003 utilizing GFS initial analyses is conducted to validate the objective tracking program. This subjective tracking is used in conjunction with real-time tracking utilizing GFS initial analyses during the 2004–2005 winter season to document CTD behavior and to select case studies.

Distributions and annual time series of CTD frequency, preferred genesis/lysis regions, and tracks will be discussed. Characteristics of CTDs (e.g., central pressure, radius) vs. track longevity, as well as variation of these characteristics during the CTD life cycle, also will be considered. 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 appear to be of the most interest from the standpoint of high-impact midlatitude weather events. These disturbances tend to be of polar origin and may migrate to the southern periphery of the CPV and subsequently invigorate the PFJ. Frequency and genesis maxima for the long-lived disturbances 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. An illustrative case study (the 22–24 January 2005 northeastern U.S. snowstorm) will be utilized to demonstrate CTD behavior, as well as interactions with the PFJ and midlatitude weather systems. This case study will be conducted using GFS initial analyses, allowing critical mesoscale features to be resolved adequately. Results will be posted at: www.atmos.albany.edu/student/kravitz/index.html as they become available.