M3 Convection and Convective Systems

Topic

Atmospheric convection is the mechanism by which warm, moist air rises in intense vertical currents, releasing latent heat during the ascent; this heat fuels the growth of cumulus clouds and—if growth continues—cumulonimbus clouds. It is precisely this release of latent heat that keeps the rising air warmer than the surrounding air, causing it to accelerate upward rather than come to a halt.

Atmospheric convection does not always occur on the same scale: it ranges from individual cumulus clouds—measuring one to five kilometers across and lasting only half an hour to an hour—to mesoscale convective systems, which span over a hundred kilometers and can remain active for more than six hours. The ultimate size and duration of a convection event depend on two environmental conditions, and this dependency explains why the scale of convection varies so significantly from one event to another.

These two environmental conditions are available convective energy and wind shear; together, they determine the form that atmospheric convection takes. When available convective energy is high but wind shear is low, the result is isolated, short-lived single-cell convection; conversely, when wind shear is also high, that same available energy organizes into much longer-lasting structures, such as supercells or mesoscale convective systems. Thus, wind shear is the factor that causes available energy to evolve into an organized storm rather than dissipating within an isolated cell. The fundamental unit of all this atmospheric convection is the cumulonimbus cloud; within it, the updraft—the rising current that sustains the cloud—feeds on the air rich in available convective energy found in the lower troposphere, reaching speeds of between five and fifty meters per second. Accompanying this updraft is the downdraft, generated as falling rain evaporates within the cloud and cools the surrounding air; this cooling makes the air denser than its surroundings, causing it to descend forcefully toward the ground.

In supercells, this same updraft performs an additional function: it tilts the horizontal vorticity already present in the ambient air toward the vertical. It is this tilting—known as the Rotunno-Klemp mechanism—that causes the updraft itself to begin rotating, giving rise to the mesocyclone that characterizes the supercell.

Organization into a mesoscale convective system, by contrast, does not depend on updraft rotation but rather on wind shear strong enough to spatially separate the updraft from the downdraft. This separation allows both currents to coexist without interfering with one another, enabling the system to remain active for many hours. When these convective systems must be represented in low-resolution numerical models—which cannot resolve them explicitly—adjustment schemes are employed. These schemes eliminate available convective energy over a period of one to two hours, effectively replacing the actual process of atmospheric convection with a statistical approximation of its effect.

The resulting behavior varies depending on the form the atmospheric convection takes: an ordinary cumulus cloud forms, matures, and dissipates in a mere half-hour to an hour; A multicell storm is sustained by successive cells that renew themselves every fifteen to twenty minutes; a supercell maintains persistent rotation throughout the storm's duration; and a mesoscale convective system can propagate overnight, driven by its own cold air pool—the dense, cold air accumulated near the ground by downdrafts—causing the storm to travel far from its point of origin. Thus, from the simplest individual cumulus cloud to the most extensive mesoscale convective system, this entire hierarchy of atmospheric convection is governed by the same balance between the available convective energy driving it and the wind shear determining its organization.

ID:4985