Anomalous lightning activity over the Metropolitan Region of Sao Paulo due to urban effects

A significant enhancement in the number of negative cloud-to-ground (CC) lightning and a decrease in the percentage of positive CC flashes are observed over the city of Sao Paulo, similar to observations in other large urban areas. Strong evidence indicates that this anomalous behavior results from several mechanisms related to the urban effect. In this paper, we investigated the importance of the air pollution using CC lightning data provided by the Brazilian lightning detection network (BrasilDAT) for a 6-year period (1999-2004). Due to the large variations in the CC lightning activity in response to different meteorological processes, it is not an easy task to infer the contribution of air pollution to the enhancement in the lightning activity. In order to overcome such difficulty, two approaches were considered: (1) the weekly variation of the number of days with lightning in comparison to the mean concentration of particulate matter (PM(10)), as well as other thermodynamical parameters; (2) the variation of the number of CC flashes and the maximum storm flash rate per individual thunderstorm for different levels of pollution. The results of both analyses suggest that: first, the enhancement in the CC lightning activity during the week days over Sao Paulo metropolitan region is related to the MID concentration (pollution); second, the PM(10) concentration tends to increase the lifetime of the storms and, in consequence, the number of flashes per storm, and not the flash rate of the thunderstorm: and third, the effect of the pollution in the enhancement of the CC lightning activity is probably less significant compared to the effect of the urban heat island. (C) 2008 Elsevier B.V. All rights reserved.