MICROPHYSICS OF THE CATATUMBO LIGHTNING STRIKE

Abstract: The "Catatumbo Lightning" observed south of Lake Maracaibo (Venezuela) has unique characteristics in the world. Its phenomenology is discussed through the results of the expeditions carried out in the Ciénagas de "Juan Manuel" National Park. Additionally, microphysical magnitudes that allow modeling the phenomenon are estimated and the importance of the swamps as a causal agent of the atmospheric electrical activity observed in the region is discussed. Key words: Catatumbo lightning, atmospheric electricity.

1 .Introduction: Lightning and thunder are the most conspicuous manifestations of atmospheric electrical activity and occur very frequently in the region of the deltas of the Catatumbo, Zulia and Bravo rivers, south of Lake Maracaibo. Among such phenomena an inaudible lightning known as the "Lightning of Catatumbo" or "Lighthouse of Maracaibo" stands out for its persistent occurrence, its brilliance and luminosity. This phenomenon can be appreciated from hundreds of kilometers away, in the Andes, in the city of Maracaibo and from the Caribbean Sea, during almost the whole year (Centeno, 1968).

The occurrence of the "Lightning of Catatumbo" is very old, the first written mention dates from 1597, when Lope de Vega mentions it in the epic poem "La Drangontea". The naturalist Alexander von Humboldt describes the phenomenon as "electrical explosions that are like phosphorescent glows..." (Alvarado, 1956) and the geographer Agustin Codazzi (Codazzi, 1960) describes it "as a continuous lightning that seems to emerge in the swampy region of the Zulia River and its surroundings", south of Lake Maracaibo. It stands out for its continuous persistence in position and time, even in the period of drought, deriving from there the name of "Faro de Maracaibo" (Maracaibo Lighthouse).

Observations and Phenomenology.

The occurrence of the phenomenon covers an extensive region of about 300,000 hectares south-west of Lake Maracaibo, including the Bravo River, from its source to its mouth, the La Concepción River, part of the Catatumbo River, the Ciénagas de Juan Manuel de Aguas Claras and Juan Manuel de Aguas Negras, the lakes La Belleza, La Negra, La Estrella and other smaller lakes. Substantially it corresponds to a swamp ecosystem, swamp forests and mangrove swamps as well as an estuarine lacustrine delta system in the area of the mouth of the rivers in Lake Maracaibo. This large esplanade shares the same geological history that characterizes the southern part of Lake Maracaibo, forming a depression between the Perijá and Venezuelan Andes mountain ranges. The lagoons and flooded marshes continuously exhale methane from decomposing detritus and humus, with water depths varying between 2 and 9 meters.

The occurrence of the phenomenon covers an extensive region of about 300,000 hectares south-west of Lake Maracaibo, including the Bravo River, from its source to its mouth, the La Concepción River, part of the Catatumbo River, the Ciénagas de Juan Manuel de Aguas Claras and Juan Manuel de Aguas Negras, the lakes La Belleza, La Negra, La Estrella and other smaller lakes. Substantially it corresponds to a swamp ecosystem, swamp forests and mangrove swamps as well as an estuarine lacustrine delta system in the area of the mouth of the rivers in Lake Maracaibo. This large esplanade shares the same geological history that characterizes the southern part of Lake Maracaibo, forming a depression between the Perijá and Venezuelan Andes mountain ranges. The lagoons and flooded marshes continuously exhale methane from decomposing detritus and humus, with water depths varying between 2 and 9 meters.

The average annual temperature is 28º C, with a maximum of 30º -36º C in the town of Los Encontrados at 15 HLV, and the minimum between 23º and 25 ºC in the same place at 5 HLV. The winds in the region have two qualitatively different circulations. Below 1500 m a.s.l. there is a process of forced sliding due to the mountains of the Perijá and Andes ranges. At higher altitudes, above 3000 m asl, the direction of the winds and their average speed is characteristic for the rest of Venezuela (Gol, 1963).

Visibility of the phenomenon.

The hours of visibility of the phenomenon are variable, between 19 and 04 HLV, and seem to depend on the point of observation. At distances relatively close to the epicenters, in the interior of the marshes, the phenomenon begins to be observed with the disappearance of the zodiacal light, shortly after sunset. As the observer moves away from the epicenters, the relative height of the "Catatumbo Lightning" with respect to the horizon decreases, making its observation more difficult. Similarly, from high and distant observation regions, the visibility increases. As discharges are discharges inside cumulonimbus and stratocumulus clouds, an observer placed just at the epicenters, below the cloud layers where the discharges take place, does not appreciate the phenomenon.

Microphysical Model.

The extent of the permanently flooded marshes suggests that methane gas must play an important role in the microphysical processes that take place in the clouds of the region. Recent studies have pointed out the role of this molecule in certain climatological and oceanographic processes (Suess et al, 1999).

General.

As the methane molecule (CH4) is indissoluble in water, when it is generated in the marshes and lagoons it rises quickly because it is lighter than air, even above the water vapor clouds. This phenomenon increases in the hours following sunset, when the absence of solar irradiance prevents its photodissociation, which could explain why the lightning is only visible at night and never during daylight hours.

Methane generation by decomposition of detritus and humus in the marshes increases during the summer because the waters are shallower and the average temperature increases, facilitating the decomposition of organic material. This seems to explain why the "Catatumbo lightning" is more visible in dry periods than in winter.

Discussion.

According to the proposed model, methane would be the causal agent to understand the phenomenon known as the "Catatumbo Lightning". The concentration of this gas in the convective clouds over the region would cause the separation of electric charges inside the cloud cells, making possible the discharges (lightning) as well as the observed phosphorescence (lightning). This is in agreement with the current level of knowledge on electrical discharges in ionized gases and with the physicochemical properties of methane, as shown in the preceding section.

The concentration of methane in the troposphere varies locally up to even concentrations of 0.1 % ( Carman & Vincent, 1999). The fact that the discharges occur inside the clouds (cloud-cloud lightning) seems to rule out the existence of ionizing and geomagnetic agents in the substrate, and could also explain the non-detection of the phenomenon from meteorological satellites such as the "Optical transient Detector", designed to measure atmospheric electrical activity and storms. During the day the phenomenon would not occur because solar irradiance photodissociates methane continuously, preventing electrical self-polarization and excitation to metastable energy levels, in agreement with the observed phenomenology of the Catatumbo Lightning. After solar twilight or during a total eclipse as occurred in that region on 26-02-98, the electrical activity of the "Catatumbo Lightning" is manifested even before the temperature changes appreciably.

During the winter or after heavy rainfall over the region, the visibility of the phenomenon decreases or even disappears completely. This could be explained by the fact that intense and/or prolonged precipitation drags methane to the surface and decreases the relative concentration of the gas. Similarly, during the dry season (summer) evaporation and average temperature increase, allowing the volatility of the gas and its rapid ascent to the upper layers of the clouds where electrically self-polarized crystals would form.

CATATUMBO LIGHTNING OUR PHENOMENON