32nd Conference on Broadcast Meteorology/31st Conference on Radar Meteorology/Fifth Conference on Coastal Atmospheric and Oceanic Prediction and Processes

Monday, 11 August 2003
WSR-88D analysis of vortices embedded within a surface low pressure trough and subsequent convection initiation
Albert E. Pietrycha, NOAA/NWS, Amarillo, TX; and K. L. Manross
Poster PDF (800.5 kB)
On the afternoon of 2 July 2002 spatially isolated, deep moist convection initiated along a quasi-stationary surface trough of low pressure in north central Nebraska. Numerous vertically oriented, cyclonic vortices embedded along a surface trough were detected in the North Platte, NE, WSR-88D imagery 120 min prior to initiation. The vortices exhibited 3-6 km horizontal diameters, with a wavelength of ~12 km. The largest vortices persisted over 120 min as they propagated along the boundary at ~5 m s-1. Convection initiated southward along the boundary in the vicinity of the vortices and exhibited a similar spatial separation as the vortices. Within 30 min after convection developed along the boundary, two brief tornadoes were reported.

Previous research has documented the existence of vortices along boundaries. Studies of the Denver Convergence Vorticity Zone revealed rotational instabilities as vertically oriented vortices 4 km down to 200 m in diameter flowing along the boundary (e.g., Wilson et al. 1988; Szoke and Brady 1989; Wilczak and Christian 1990; Wilson et al. 1992; Pietrycha and Rasmussen 2001a). Wakimoto and Wilson (1989) demonstrated the genesis of non-supercellular tornadoes was due to the collocation of vortices embedded within a convergent surface boundary with a rapidly developing storm. Additionally, Pietrycha and Rasmussen (2001b) documented vortices along the Great Plains dryline. The vortices were resolved concurrent with a deceleration of dryline movement to nearly stationary, and while moisture differentials strengthened. The authors hypothesized that the vortices protect an ascending air parcel by inhibiting mixing thus allowing the parcel to reach its local lifted condensation level and level of free convection.

Based on the research and hypotheses documented above, in utilizing WSR-88D data there is circumstantial evidence the vortices on 2 July 2002 played a role toward convection initiation and the genesis of non-supercellular tornadoes. Data will be presented documenting a portion of the event. Additionally, the National Severe Storms Laboratory's Weather Decision Support System - Integrated Information (WDSS-II) will be utilized to examine if the vortices can be detected by any of its algorithms. The algorithms of specific interest will be the Azmuthal Shear and Azmuthal Convergence products employing a linear least squares derivative (Elmore 1994) of the radial velocity data, as well as the Rotation-Tracks composite. The results may prove beneficial toward providing the operational community with a new forecasting tool, and hence, a greater lead time for the detection of storm initiation.

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