Effects of large urban areas over precipitation and lightning - a review and future perspectives

The urban effect on the lightning activity remains a controversial topic, beginning with works in 90's and then followed by several studies in US, Europe and Brazil. Most of them speculated on a combination of two factors to explain their findings: the increase of cloud condensation nuclei (CCN) due to anthropogenic emission of particulate matter (PM10) and SO2, and the enhancement of the convergence due to the heat island effect. All of them discuss a particular topic related to CCN interactions with cloud microphysics and the urban heat island (UHI) effect. The increase of temperature in urban areas is believed to be caused by UHI effect and it is related to many physical differences between urban and rural areas, among them absorption of sunlight, increased heat storage of artificial surfaces, obstruction of re-radiation by buildings, absence of plant transpiration, and differences in air circulation. The UHI magnitude, defined as the difference between urban and rural temperatures is influenced by the Bowen ratio of contrasting surfaces, synoptic weather conditions, urban morphology, timing of temperature observations, and categorization of “rural” areas adjacent to the city. The influence of aerosols on cloud microphysics, rainfall and cloud electrification has been comprehensively studied and the effect of aerosol concentration on cloud electrification has been discussed by a number of authors. According to first works in Brazil, the urban effect on lightning activity can be a combination of a thermodynamic effect (the enhancement of the sensible heat flux due to the UHI) and a microphysical effect (due to a high concentration of anthropogenic aerosol in the atmosphere). More recent studies suggested that the large urban area of São Paulo city leads to a negative anomaly of the intra-cloud activity and a decrease in the percentage of positive lightning. They proposed that the anthropogenic particulate could appreciably affect the charge separation in the urban thundercloud by increasing the negative lightning activity. Thus, lightning can be impacted by changes in aerosol loading, showing changes in either polarity or amount of lightning as the aerosol loading changes. One of those impacts is the weekly cycle (or weekend effect). The probability to detect a weekly cycle in a climatic data is very low. However, such cycles exist within urban centers and surrounding areas and their causes are attributed to anthropogenic actions. In Brazil, it was found a weekly cycle of the lightning activity over São Paulo city. The same was found for the sulfur dioxide (SO2) and particulate matter (PM10). Other evidences of the weekly cycle were also found in India and Israel. A major problem with experimental verification of the dominant physical controls on thunderstorm electrification and lightning over urban areas is the need for simultaneous control on thermodynamic and aerosol variables. As discussed, for many observed phenomena, both thermodynamic and aerosol explanations are possible, and even plausible. The GoAmazon project provides a new and improved opportunity to attempt disentangling aerosol effects from thermodynamic ones. The challenging problem of lightning enhancement over urban areas has great relevance to Manaus, the focal point for GOAmazon, as a large city surrounded by river water and rainforest. The naturally controlled experiment of the plume is expected to provide a most productive path to understanding Manaus's influence on lightning activity as shown by the attached LIS lightning density map.