GLOBAL THUNDERSTORM ACTIVITY

摘要

Sprites, elves and other transient luminous events (TLEs) are known to exist only above thunderstorms. It is therefore important to know where these thunderstorms occur around the globe, and how their distribution varies temporally and spatially. The majority of thunderstorms on Earth occur within the tropical regions between ±30° latitude of the equator (～50% of the surface area of the globe). This is due to the maximum solar heating in the tropics, and the atmospheric general circulation patterns between the tropics and the sub-tropics (Hadley Circulation). Along the thermal equator, which migrates with the seasons, air masses from the northern and southern hemispheres converge along the intertropical convergence zone (ITCZ). This is the latitudinal position of the majority of the globe's rainfall and thunderstorm activity. However, in the tropics these thunderstorms are concentrated mainly over the continental regions (Americas, Africa and southeast Asia) with little thunderstorm activity observed over the oceans. The reason for the preference of thunderstorms to continental regions is likely related to the larger daily surface heating over land as compared with the oceans. In the extra-tropical regions thunderstorms form along the polar front, the boundary between warm moist air from the tropics, and cool dry air from polar regions. Recent satellite measurements of lightning indicate a mean global rate of ～45 flashes/second. In fair weather regions the integrated effect of global thunderstorms and other electrified clouds can be observed via the atmospheric global electric circuit. The global thunderstorms charge the Earth's surface negatively with a mean charge of 500,000 Coulombs, and a mean potential between the ionosphere (～80 km) and the Earth's surface of 250 kV. The diurnal variation of the atmospheric electric circuit (and global thunderstorms) has a maximum around 18 UT and a minimum around 03 UT known as the Carnegie Curve.