Pulse storm

A pulse storm is a single cell thunderstorm of substantial intensity which only produces severe weather for short periods of time. Such a storm weakens and then generates another short burst – hence "pulse". The term was coined by researchers at the National Severe Storms Laboratory in 1979 to describe a single storm cell briefly becoming severe within a cluster of multi-cellular thunderstorms, but has since been used to describe a variety of isolated and brief thunderstorms with both severe and non-severe characteristics.^[1]

Description

Single cell thunderstorms ordinarily form in environments with low wind shear and moderate instability, with the low wind shear contributing to a short average lifespan of less than an hour.^[2] When the instability, calculated by convective available potential energy (CAPE), is strong, the updraft will bring a larger amount of humid air very high above ground and generate a cumulonimbus cloud with high water and ice content.^[3] When the rain content, and even hail, falls from it, they can generate damaging winds brought about by downbursts. Rarely, a weak tornado develops in association with a pulse storm as the environment is only weakly sheared, or not at all.^[4]

Life cycle

Life cycle of a pulse storm.

One can distinguish three stages in the evolution of a pulse storm:^[3]

• Formation: the upward current of the cell intensifies and allows the condensation of water vapor from the rising air parcel. This forms a cumulus congestus, then a cumulonimbus when ice crystals form at its apex which spreads horizontally in contact with the tropopause.
• Maturity: downdrafts are emerging. This stage is accompanied by characteristic phenomena such as lightning and thunder, showers, and gust front.
• Dissipation: the cold pool descending from the cloud extends to the Earth's surface and helps to block the feed by pushing the updraft downstream. The outflow can then serve as a trigger for other single cell or even multi-cell thunderstorms.

See also

• Project NIMROD