Thunderstorm: a cumulonimbus cloud or collection of cumulonimbus clouds featuring vigorous updrafts, precipitation and lightning Thunderstorm: a cumulonimbus cloud or collection of cumulonimbus clouds featuring vigorous updrafts, precipitation and lightning ● Thunderstorms are responsible for most of what we refer to as “severe weather”, including high winds, lightning, tornadoes and hail - By official definition, a severe thunderstorm has at least one of the following: ● Large hail of ¾ inch diameter or greater ● Wind gusts of at least 50 knots ● A tornado ● Keep in mind that a thunderstorm is a form of cumulus cloud, which means it's basically a plume of warm, cloudy air (i.e., an updraft) in a conditionally unstable environment. The fuel for a thunderstorm is condensation in the cloud, which keeps the ascending cloudy air warmer than the surroundings. The Thunderstorm Cell: Labeling the Parts warm condensation evaporation cold ● The building block of a thunderstorm is the thunderstorm cell, consisting of a warm updraft and a relatively cold downdraft. ● The updraft is driven by condensation in the cloud, while the downdraft is driven by evaporation, as well as the drag produced by falling rain and hail. The Thunderstorm Cell: Labeling the Parts entrainment warm condensation evaporation cold The evaporation that helps drive the downdraft happens two ways: ● Rain falls into the unsaturated air below and evaporates ● Dry air is mixed with cloudy air across the edges of the cloud, through a process called entrainment The Thunderstorm Cell: Labeling the Parts entrainment warm condensation evaporation cold cold pool gust front ● As the downdraft reaches the ground, it spreads out and forms a region of relatively cold air called the cold pool. ● The leading edge of the cold pool is called the gust front, as the winds behind the gust front (on the cold side) are typically strong and gusty. The Thunderstorm Cell: Labeling the Parts warm air cold air gust front ● As the gust front spreads along the ground, it forces warm air up and over the front. ● Under the right conditions, this lifting can initiate new updrafts, which is an important process for maintaining and propagating the system. Photo of a gust front approaching outside the daycare. No walk today! Recipe for a Thunderstorm The driving force behind a thunderstorm is the latent heating in the updraft. So to get things going, we'll need: ● A liberal supply of warm, humid air ● A conditionally unstable environment, allowing the air to rise ● A lifting mechanism (the “trigger”) to raise the initially dry air to saturation And if it's a severe storm we're after, then we'll also need: ● A change in the background wind speed and direction with height (called wind shear) wind shear The Dependence on Shear ● The type of storm we get (and hence the intensity and duration) is strongly influenced by the wind profile in the environment (i.e., the environmental shear) In general, more wind shear tends to produce longer-lived and more intense storm systems! ● To a rough approximation, this means that severe thunderstorms and tornadoes often form under a branch of the jet stream Conditions for the April 15, 2011 Tornado Outbreak a change in wind speed and/or direction with height a trigger mechanism to provide lifting (cold front) a supply of warm humid air off the Gulf Thunderstorm Classification A given thunderstorm can be classified as: ● A single-cell (or ordinary-cell) thunderstorm, consisting of a single updraft/downdraft system ● A multicell thunderstorm, consisting of several thunderstorm cells evolving together as a unit ● A supercell storm, consisting of a single intense, rotating updraft and either one or two associated downdrafts (run away, run away) In addition, we also classify systems in which many individual storms are evolving together in the same general region....what are called mesoscale convective systems (MCS). But we'll come back to that later. Single-cell storms: ● A single-cell storm consists of a single updraft / downdraft pair ● Single-cells form in environments with weak wind shear ● The cell evolves through a series of stages, with the whole process taking roughly 30 to 60 minutes ● The motion of the storm follows the mean background wind Life cycle of a single-cell storm: Growth stage: air parcels Mature stage: evaporation Dissipating stage: the lifted to saturation by some leads to formation of a spreading cold pool cuts trigger process. Clouds downdraft. Cold pool the system off from warm build into towering forms and begins to spread air, killing the updraft and cumulus. along the ground. Most leading to dissipation. intense stage. A single-cell thunderstorm in the dissipating stage Multicell storms: ● A multicell storm consists of a small group of cells evolving together, but in different stages of development ● Multicells are found in environments with weak to moderate shear ● Individual cells grow and die in 30 to 60 minutes, but the collection as a whole can last much longer The view from Lowes in Bryan, earlier this spring Maintenance of a multicell storm: ● The key to multicell longevity is the spreading gust front ● As the gust front spreads it lifts warm air, initiating new updrafts ● The new updrafts form at the leading edge of the system, and the older cells shift to the back of the system and die mature cells move to the back of the new updrafts are system and eventually dissipate formed at the gust front a shelf cloud formed as air is lifted up and over the edge of the gust front a rotating roll cloud just behind the leading edge of the gust front Supercells: ● A supercell consists of a single intense, rotating updraft and either one or two associated downdrafts ● Supercells are typically found in high-shear environments ● The organization of a supercell is self-reinforcing, and a single supercell can last several hours rotating supercell wall cloud and associated tornado Supercells: ● Most of our intense tornadoes in the US are produced by supercell storms rotating supercell wall cloud and associated tornado Supercell Structure as Seen by Radar roughly N-S cross-section (A to B) radar echoes seen at various heights above roughly E-W the ground cross-section (C to D) B Bounded Weak Echo Region (BWER): C a hole in the radar signal where the updraft is so fast that precipitation has D hook A no time to grow echo updraft region front-flank downdraft rear-flank downdraft front-flank gust front rear-flank rotating gust front updraft Tornadic supercells usually have two downdrafts, each with an associated gust front. The single, rotating updraft is located where the two gust fronts meet. .