A7 Extreme Freezing Events

Topic

An extreme freezing event occurs when an atmospheric anomaly produces a massive accumulation of ice on the surface; this massive accumulation can take three distinct forms: freezing rain, which freezes upon contact with cold surfaces, coating them in a layer of solid ice; river ice jams, which block a river's channel and cause flash flooding upstream of the blockage; and unusual freezing of the topsoil, which damages ecosystems and the crops they support. An extreme freezing event—in any of these three forms—can cause immense damage to infrastructure, agriculture, and ecosystems in just a few hours.

Freezing rain forms from water droplets that remain liquid even at temperatures below zero—an unstable state that persists only until the droplets touch a surface; upon contact, the droplets freeze instantly, releasing the latent heat they had stored while in a liquid state. The rate at which freezing rain accumulates ice on a surface depends on three factors: the mass of liquid water striking the surface per unit of time, the temperature of the surface itself, and the atmospheric temperature profile above that surface—particularly when a layer of relatively warm air aloft overlies a layer of cold air near the ground.

River ice jams form when ice blocks carried by the river accumulate upstream of an obstacle; this accumulation of ice blocks generates hydrostatic pressure that can far exceed the pressure produced by an ordinary flood on the same river. A force-balance model within the river ice jam itself determines whether the accumulation of ice blocks remains stable—holding back the water—or suddenly gives way, triggering a catastrophic downstream release.

The behavior of an extreme freezing event—whether it manifests as freezing rain or a river ice jam—is highly erratic and depends sensitively on the initial conditions of the forming ice front; consequently, even minor differences in these initial conditions can, within just a few hours, result in the same type of massive damage to infrastructure, agriculture, and ecosystems mentioned at the beginning of this description.

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