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Bipolar lightning is usually defined as a lightning flash where the current waveform exhibits a polarity reversal. There are very few reported cases of cloud-to-ground (CG) bipolar flashes using only one channel in the literature. Reports on this type of bipolar flashes are not common due to the fact that in order to confirm that currents of both polarities follow the same channel to the ground, one necessarily needs video records. This study presents five clear observations of single-channel bipolar CG flashes. High-speed video and electric field measurement observations are used and analyzed. Based on the video images obtained and based on previous observations of positive CG flashes with high-speed cameras, we suggest that positive leader branches which do not participate in the initial return stroke of a positive cloud-to-ground flash later generate recoil leaders whose negative ends, upon reaching the branch point, traverse the return stroke channel path to the ground resulting in a subsequent return stroke of opposite polarity. Key Points Existence of single-channel bipolar flashes Common characteristics of bipolar flashes How can positive and negative discharges use the same path to ground?

We investigated the seasonal patterns of Amazonian forest photosynthetic activity, and the effects thereon of variations in climate and land-use, by integrating data from a network of ground-based eddy flux towers in Brazil established as part of the ‘Large-Scale Biosphere Atmosphere Experiment in Amazonia’ project. We found that degree of water limitation, as indicated by the seasonality of the ratio of sensible to latent heat flux (Bowen ratio) predicts seasonal patterns of photosynthesis. In equatorial Amazonian forests (5° N-5° S), water limitation is absent, and photosynthetic fluxes (or gross ecosystem productivity, GEP) exhibit high or increasing levels of photosynthetic activity as the dry season progresses, likely a consequence of allocation to growth of new leaves. In contrast, forests along the southern flank of the Amazon, pastures converted from forest, and mixed forest-grass savanna, exhibit dry-season declines in GEP, consistent with increasing degrees of water limitation. Although previous work showed tropical ecosystem evapotranspiration (ET) is driven by incoming radiation, GEP observations reported here surprisingly show no or negative relationships with photosynthetically active radiation (PAR). Instead, GEP fluxes largely followed the phenology of canopy photosynthetic capacity (Pc), with only deviations from this primary pattern driven by variations in PAR. Estimates of leaf flush at three non-water limited equatorial forest sites peak in the dry season, in correlation with high dry season light levels. The higher photosynthetic capacity that follows persists into the wet season, driving high GEP that is out of phase with sunlight, explaining the negative observed relationship with sunlight. Overall, these patterns suggest that at sites where water is not limiting, light interacts with adaptive mechanisms to determine photosynthetic capacity indirectly through leaf flush and litterfall seasonality. These mechanisms are poorly represented in ecosystem models, and represent an important challenge to efforts to predict tropical forest responses to climatic variations.

Urbanization has an increasing contribution to anthropogenic cl imate forcing. The impact arises mainly from the Urban Heat Island (UHI) effect and aerosol anthropogenic emissions. An important but not completely understood consequence of this forcing is its effect on local lightning activity. Changes in lightning activity may result in a feedback on the climate system. In this article, it investigates changes in the lightning activity in the city of Manaus, located in the Amazon region of Brazil. It is found that, over the city, the lightning activity is larger than that in the regions around it and it has been increasing in the last four decades simultaneously with the increasing of its urban area. Our results suggest that such changes are caused by the UHI effect. The observations reported here are unique and relevant because Manaus is located in the central part of the Amazon rainforest and inside one of the three global lightning chimneys in the world.

This paper describes a comparative analysis of the thunderstorm incidence in Southeastern Brazil obtained from thunderstorm days observed at two different epochs (from 1910 to 1951 and from 1971 to 1984) and from lightning data provided by the Brazilian lightning location system RINDAT (from 1999 to 2006) and the Lightning Imaging Sensor (LIS) on board the TRMM satellite (from 1998 to 2010). The results are interpreted in terms of the main synoptic synoptic pattern associated with thunderstorm activity is the occurrence of frontal systems and their modulation by the South Atlantic Convergence Zone (SACZ) and topography. Evidence of urban effects is also found. The results are also discussed in the context of practical applications involving their use in the Brazilian lightning protection standards, suggesting that the present version of the Brazilian standards should be revised incorporating RINDAT and LIS data. Finally, the results are important to improve our knowledge about the limitations of the different techniques used to record the thunderstorm activity and support future climatic studies.

Lakes play a large role in global atmospheric and landscape carbon (C) processes, but their role may change as they become polluted with nutrients. Geographic regions rich in surface waters are also prone to agricultural and urban development and so may become increasingly eutrophic as the population rises. Here we develop C budgets of highly eutrophic lakes. These analyses show that lakes undergoing eutrophication can become atmospheric carbon dioxide (CO2) sinks because of the CO2 disequilibrium caused by extreme primary production. C budgets of such lakes show they absorb both landscape and atmospheric C, converting it into lake sediments and passing additional dissolved organic C (DOC) downstream. Eutrophication may cause a reversal in the role played by oligotrophic lakes by promoting atmospheric C sequestration as sediment and DOC. This means that as eutrophication increases from agriculture and urbanization, the expected large CO2 evasion to the atmosphere by natural lakes will decline substantially and inland C sequestration and enrichment of DOC in waters flowing to the sea will be augmented. Thus, we suggest that the global C role of eutrophication is worthy of future consideration because it represents an interface between 2 large, converging environmental problems, whose interaction may reverse the role of lakes in the global C cycle.

Este trabalho apresenta a análise das componentes do balanço de radiação à superfície sobre o Pantanal Sul Mato-grossense, a partir de medidas experimentais coletadas durante a estação seca, em setembro de 1999. Neste período, as componentes do balanço de radiação mostraram um ciclo diurno bem definido, associado à densidade de fluxo radiativo de onda curta de 850 Wm-2 ao meio dia. O albedo médio apresentou um comportamento quase especular, com valor mínimo de 0,16±0,02 para ângulos zenitais pequenos. Os valores de emissividade da superfície pantaneira, corrigida para temperatura radiativa de um corpo cinza, variaram entre 0,94 e 0,96. A partir das medidas diretas das densidades dos fluxos radiativos de ondas curtas e longas atmosféricas determinou-se os parâmetros ótimos empregados nas formulações propostas por Monteith e Unsworth (2008), Swinbank (1963) e Brutsaert (1975).

Changes in rainfall extremes and flooding are becoming more frequent in many countries, particularly in large cities where people and assets are concentrated. In the Metropolitan Area of the city of São Paulo (MASP) region, heavy or extreme precipitation events have important effects on society. Flash floods and landslides, associated with intense, but often brief, rainfall events, may be the most destructive of extreme events. Observations since the mid-1930s in the MASP region have shown significant increases in total and heavy rainfall and decreases in light rain. This was probably due to natural climate variability, but with some signals of the urbanization effect, especially during the last 40 yr. Here projections of future changes in rainfall extremes in the MASP region were derived from the Eta-CPTEC 40 km regional model nested in the HadCM3 global model, with 4 available realizations of the global model for the A1B emissions scenario to the end of the 21st century. Trends were assessed for significance using the nonparametric Mann-Kendall test. Projections, based on percentiles and on the number of days with rainfall above a certain limit, suggested: (a) an increase in total precipitation, (b) an increase in heavy precipitation and in the contribution to total precipitation from more intense rainfall events, and (c) the possibility of longer dry periods separating days with intense rain in the MASP region. The trends were stronger and more significant in the second half of the 21st century. We are aware that dynamical downscaling may not provide information at the weather station level and that climate modeling does not resolve all uncertainties. However, we believe that this exercise enables climate assessments that, in time, can be used for general public information.

The present study focuses on the impacts of extreme drought and flooding situations in Amazonia, using level/discharge data from some rivers in the Amazon region as indicators of impacts. The last 10 years have featured various once in a century droughts and floods in the Amazon basin, which have affected human and natural systems in the region. We assess a history of such hazards based on river data, and discuss some of the observed impacts in terms of vulnerability of human and natural systems, as well as some of adaptation strategies implemented by regional and local governments to cope with them. A critical perspective of mitigation of drought and flood policies in Amazonia suggests that they have been mostly ineffective in reducing vulnerability for the majority of the population, constituting, perhaps, examples of maladaptation via the undermining of resilience.

Phase transitions of atmospheric water play a ubiquitous role in the Earth’s climate system, but their direct impact on atmospheric dynamics has escaped wide attention. Here we examine and advance a theory as to how condensation influences atmospheric pressure through the mass removal of water from the gas phase with a simultaneous account of the latent heat release. Building from fundamental physical principles we show that condensation is associated with a decline in air pressure in the lower atmosphere. This decline occurs up to a certain height, which ranges from 3 to 4 km for surface temperatures from 10 to 30 degrees C. We then estimate the horizontal pressure differences associated with water vapor condensation and find that these are comparable in magnitude with the pressure differences driving observed circulation patterns. The water vapor delivered to the atmosphere via evaporation represents a store of potential energy available to accelerate air and thus drive winds. Our estimates suggest that the global mean power at which this potential energy is released by condensation is around one per cent of the global solar power – this is similar to the known stationary dissipative power of general atmospheric circulation. We conclude that condensation and evaporation merit attention as major, if previously overlooked, factors in driving atmospheric dynamics.