Climatology of Convection in Florida: Implications for Mercury Deposition

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Sunday, 17 January 2010
Exhibit Hall B2 (GWCC)
Eleanor Ferguson, University of Michigan, Ann Arbor, MI; and F. J. Marsik, D. J. Posselt, and G. J. Keeler

Elevated levels of mercury found in wet deposition measured across the State of Florida have generated debate over the potential sources of mercury responsible for these observations. Previous studies have variously attributed these elevated levels of mercury to local anthropogenic sources (Dvonch et al. 2005) and the scavenging of a global pool of mercury from the upper troposphere by tall convective cells (Guentzel et al. 2001, Holmes et al. 2008). One may hypothesize that if mercury scavenging by tall convective cells does play a critical role in the elevated mercury wet deposition observed in the State of Florida, then precipitation cell structure should show some correlation to mercury wet deposition levels.

The first stage of this project developed a detailed climatological profile of precipitation cells impacting Broward County, Florida during the period of June 1, 1995 June 30, 1996. For this period, NEXRAD Doppler radar data from Miami, Florida were visually examined and storm events affecting Broward County were classified by storm morphology. The seven morphologies included in the classification system were non-convective storms, isolated cells, clusters of cells, linear with no stratiform rain, linear with trailing stratiform rain, linear with leading stratiform rain, and bow echoes. Other variables recorded include event duration, direction, maximum echo top, average cell lifetime, occurrence of hail and whether the precipitation hit the site at Davie, FL. Events affecting Davie, FL in each 24 hour period (8 am to 8 am) were combined and assigned the dominant direction and classification for that period. This allowed for comparison with mercury data collected corresponding to the same time period at Davie, FL by Dvonch et al. (2005).

This preliminary study found no correlation between storm type, dominant direction, or echo top height and the concentration or bulk wet deposition of mercury at the Davie, FL site during this time period. This may suggest that either local sources dominated the inputs into wet deposition regardless of the storm category/characteristics or that the precipitation cells associated with all of the storm categories considered reached sufficiently high to potentially scavenge elevated mercury aloft. In an effort to better understand these results, a second precipitation cell climatological analysis was performed for the period of June 1 June 30 2009, a period during which daily mercury wet deposition measurements were conducted for a number of urban and remote sites across the State of Florida. If the scavenging of elevated mercury by deep convection does play a key role in the wet deposition for this region, one might hypothesize that such a signal will be more apparent for the remote sites. Results from both analyses will be presented.