7A.7 Synoptic and Mesoscale Regulation of Dryline Intensity and Associated Convection

Wednesday, 27 June 2007: 3:30 PM
Summit A (The Yarrow Resort Hotel and Conference Center)
David M. Schultz, University of Helsinki and Finnish Meteorological Institute, FI-00101, Helsinki, Finland; and C. C. Weiss

To investigate the role of synoptic-scale processes in regulating the strength of the dryline, a dataset is constructed of all drylines occurring within the West Texas Mesonet (WTM) during April, May, and June of 2004 and 2005. In addition, dewpoint and wind data were collected from stations on the western (Morton, MORT) and eastern (Paducah, PADU) periphery of the WTM domain(230 km across), generally oriented east–west across the typical location of the dryline in west Texas. Drylines were characterized by two variables: the difference in dewpoint between MORT and PADU (hereafter, dryline intensity) and the difference in the u component of the wind between MORT and PADU (hereafter, dryline confluence). A high degree of correlation existed between the two variables, consistent with a strong role for dryline confluence in determining dryline intensity. Some cases departing from the strong correlation between these variables represent synoptically quiescent drylines whose strength is likely dominated by boundary-layer mixing processes.

Composite synoptic analyses were constructed of the upper and lower quartiles of dryline intensity, termed STRONG and WEAK, respectively. STRONG drylines were associated with a short-wave trough in the upper-level westerlies approaching west Texas, an accompanying surface cyclone over eastern New Mexico, and southerly flow over the south-central United States. This synoptic environment was favorable for enhancing the dryline confluence responsible for strengthening the dryline. In contrast, WEAK drylines were associated with an upper-level long-wave ridge over Texas and New Mexico, broad surface cyclogenesis over the southwestern United States, and a weak lee trough—the dryline confluence favorable for dryline intensification was much weaker. A third composite termed NO BOUNDARY was composed of dates with no surface airstream boundary (e.g., front, dryline) in the WTM domain. The NO BOUNDARY composite featured an upper-level long-waveridge west of Texas and no surface cyclone or lee trough. The results of this study demonstrate the important role that synoptic-scale processes (e.g., surface lee troughs, upper-level short-wave troughs) play in regulating the strength of the dryline. Once such a favorable synoptic pattern occurs, mesoscale and boundary-layer processes can lead to further intensification of the dryline.

Subsequent sorting of the days by the type of convective weather along the dryline (no clouds, cumulus, cumulonimbus, severe weather, tornadoes) shows the effect of the synoptic and mesoscale environment on the development of convection. A logistic regression model is constructed to predict the occurrence of convection along the dryline, given the large-scale conditions. This model shows for the first time in the literature that dryline intensity is a significant factor for the severity of subsequent convection.

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