Tuesday, 22 January 2008
Cell mergers and their impact on cloud-to-ground lightning over the Houston area
Exhibit Hall B (Ernest N. Morial Convention Center)
Hypotheses advanced in studies such as METROMEX in the early 1970's and other subsequent analyses suggest that convective intensity may be impacted by urban forcing of enhanced merger activity downstream of urban corridors. The suggestion here is that cities may exert an indirect anthropogenic “forcing” of parameters related to convection and associated phenomena such as lightning and precipitation. This paper investigates the urban merger-hypothesis by examining the role of convective cell mergers on the existence and persistence of the Houston lightning “anomaly”. Using 8 summer seasons of peak columnar radar reflectivity, cloud-to-ground (CG) lightning data and the TITAN cell-tracking algorithm a two-dimensional cell merger climatology is created for a domain covering portions of eastern Texas and Louisiana. Results from the tracking and analysis of over 3.8 million “composite” cells are presented and quantify the general importance of cell-mergers on the upscale growth of convection. Not unexpectedly, we find that merger-driven enhancements in convection induce a positive response (on the order of 46%) in ground-flash densities throughout the domain, with areas of enhanced ground flash densities typically being co-located with areas of enhanced merger activity. However, while mergers over the Houston area (relative to elsewhere in the domain) do result in more vigorous convective cells that produce more lightning with larger flash densities, we find that CG lightning contributions due to mergers are distributed similarly throughout the domain. Hence while we demonstrate that cell mergers do greatly impact the production of lightning, the urban cell merger hypothesis does not explain, exclusively, the presence of a local lightning maximum near and downstream of Houston.