Analysis of Outflow Boundary Collisions in North-Central Alabama

Stephen J. Harrison Atmospheric Science Department, University of Alabama in Huntsville, Huntsville, Alabama

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John R. Mecikalski Atmospheric Science Department, University of Alabama in Huntsville, Huntsville, Alabama

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Kevin R. Knupp Atmospheric Science Department, University of Alabama in Huntsville, Huntsville, Alabama

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Abstract

Ninety-four outflow boundary (OB) collisions were documented in north-central Alabama over the summers of 2005–07 using the Advanced Radar for Meteorological and Operational Research (ARMOR) dual-polarimetric radar located at the Huntsville, Alabama, airport. These data were used to extend and verify previous research and to look for new correlations among the various factors that lead to convective initiation (CI) from OB collisions more frequently. For this study, CI is defined as the first occurrence of a ≥35-dBZ radar echo at an elevation angle of 0.8° and within 10 km of the point of collision, from a convective cloud. The radar reflectivity and angle of collision between both OBs along with time of day at which CI occurs most often were analyzed. Also, the presence of cumulus clouds along either/both OBs, or within the area of collision, was examined using Geostationary Operational Environmental Satellite-12 (GOES-12) visible imagery. A more detailed analysis of 23 of the 94 OBs that passed over the Mobile Integrated Profiling System instruments examines the relation among radar reflectivity, updraft magnitude, and water vapor enhancements. This analysis indicates that OB updraft magnitude is positively correlated with OB reflectivity factor. The main findings are that when OBs collide in a more head-on manner, when both colliding OBs have radar reflectivity values of 15 dBZ or greater, or when cumulus clouds preexist along at least one OB, CI is produced at a greater rate. These results, using a much larger dataset than had previously been used for colliding OBs, are subsequently compared with two existing studies.

Corresponding author address: John R. Mecikalski, Atmospheric Science Dept., University of Alabama in Huntsville, National Space Science and Technology Center, 320 Sparkman Dr., Huntsville, AL 35805-1912. Email: johnm@nsstc.uah.edu

Abstract

Ninety-four outflow boundary (OB) collisions were documented in north-central Alabama over the summers of 2005–07 using the Advanced Radar for Meteorological and Operational Research (ARMOR) dual-polarimetric radar located at the Huntsville, Alabama, airport. These data were used to extend and verify previous research and to look for new correlations among the various factors that lead to convective initiation (CI) from OB collisions more frequently. For this study, CI is defined as the first occurrence of a ≥35-dBZ radar echo at an elevation angle of 0.8° and within 10 km of the point of collision, from a convective cloud. The radar reflectivity and angle of collision between both OBs along with time of day at which CI occurs most often were analyzed. Also, the presence of cumulus clouds along either/both OBs, or within the area of collision, was examined using Geostationary Operational Environmental Satellite-12 (GOES-12) visible imagery. A more detailed analysis of 23 of the 94 OBs that passed over the Mobile Integrated Profiling System instruments examines the relation among radar reflectivity, updraft magnitude, and water vapor enhancements. This analysis indicates that OB updraft magnitude is positively correlated with OB reflectivity factor. The main findings are that when OBs collide in a more head-on manner, when both colliding OBs have radar reflectivity values of 15 dBZ or greater, or when cumulus clouds preexist along at least one OB, CI is produced at a greater rate. These results, using a much larger dataset than had previously been used for colliding OBs, are subsequently compared with two existing studies.

Corresponding author address: John R. Mecikalski, Atmospheric Science Dept., University of Alabama in Huntsville, National Space Science and Technology Center, 320 Sparkman Dr., Huntsville, AL 35805-1912. Email: johnm@nsstc.uah.edu

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