Unit 5 Section 1 Thunderstorms Meteorology Unit 5 Section 1 Thunderstorms

Thunderstorms Thunderstorms are convective storms Form with rising warm air Need unstable environmental conditions As long as rising air parcel is warmer than the air surrounding it, it will rise Warm air is less dense than cold air

Thunderstorms Each air parcel is called a “cell” Involve wind shear – the change in wind speed and/or direction over a distance

Unstable Environmental Conditions Unequal heating at the surface Effect of terrain Converging surface winds = contrasting temperatures and humidities Diverging upper-level winds + converging surface winds + rising air Usually many factors working together Can form in warm or cold air temperatures

Severe Thunderstorms To be considered severe, a storm must have at least one of the following: Large hail >0.75 inches in diameter Surface wind gusts of 58mph or more Produces a tornado

Simple Thunderstorm Many names: “Scattered thunderstorms” “Pop up storms” “Ordinary cell thunderstorms” “Air-mass thunderstorms”

Simple Storm Rarely become severe Less than a kilometer wide Predictable life cycle, complete in less than an hour Form in regions with limited wind shear – wind speed and direction do NOT abruptly change

Simple Storm Life Cycle 1. Cumulus stage 2. Mature stage 3. Dissipating stage

Cumulus Stage Growth stage Warm air rises, cools, and condenses into a cumulus cloud or cluster of clouds Vertical Growth – Warm, humid air at the top of the cloud dissipates, which adds more moisture into the air, which cools and condenses, which adds more cloud to the top Appears as a dome or tower Can form a Towering Cumulus cloud in a matter of minutes Not enough time for precipitation to form, no thunder and lightning

Cumulus Stage

Mature Stage Most intense stage As the cloud builds above freezing level, cloud particles get larger and heavier and begin to fall Drier air is drawn into the cloud, causing some raindrops to evaporate, which cools the air Air is colder and heavier, forms a downdraft Downdraft gets stronger if falling precipitation pulls air with it

Mature Stage Mature storms have the original convective updrafts and downdrafts occurring at the same time Top of cloud, containing ice crystals, reaches stable air and flattens out into an anvil shape Severe turbulence in the middle of the cloud from updrafts and downdrafts Lightning and thunder are present Heavy rain and occasionally small hail

Too much talking. Not enough pictures.

Mature Stage

Mature Stage Feel a downrush of cold air on the surface before precipitation When downdraft of cold air reaches the surface, it spreads out horizontally Gust front – surface boundary that separates the advancing cold air from the warm surface air High wind shear, wind is rapidly changing speed and direction Forces warm air back up into storm, which adds to updraft

Gust Front

Dissipating Stage Usually 15-30 minutes after start of mature stage Updrafts weaken as gust front moves away Downdrafts dominate, destroy humid updrafts = no fuel for storm Light precipitation Cloud disperses from ground up, the cirrus anvil is often the last thing to be seen

Life Cycle

Complex Thunderstorm “Multi-cell thunderstorm” “Supercell thunderstorm”

Multi-cell Thunderstorm Contain a number of cells, can be in different stages of development Form in regions with moderate to strong vertical wind shear

Multi-cell Thunderstorm Wind on one side of the storm is stronger, causing it to tilt When tilted, downdrafts will not cut off updrafts = storm doesn’t lose fuel Storm will survive longer Can become intense and produce severe weather

Multi-cell Thunderstorm Air at the gust front may form a shelf cloud that is attached to the base of the storm Roll clouds may form just behind the gust front

Microbursts Beneath an intense thunderstorm, downdraft may be very localized, forming a microburst Can produce intense winds up to 170mph in a small area Heavy damage to trees and structures Can capsize boats and crash planes

Microbursts

Mesoscale Convective Complexes (MCCs) Large, circular convective weather system made of individual multi-cell thunderstorms Can be 1,000 larger than a simple thunderstorm Can often cover an entire state Move slowly and can exist for 12 hours or more

MCCs

MCCs Pros – provide significant portion of seasonal rainfall over grain belt of USA Cons – can produce severe weather, like hail, high winds, flash floods, and tornadoes

Supercell Thunderstorms Supercell- A large, long-lasting thunderstorm with a single, violently rotating updraft Form in regions with strong vertical wind shear Tilted so storm doesn’t lose fuel May exist for hours

Supercell

Supercell Thunderstorms Wind shear may be strong enough to create rotation Can produce updrafts of 100mph or more, damaging surface winds, and tornadoes Strong updrafts suspend hailstones for long periods of time, allowing them to grow Updraft can be so strong that precipitation can’t fall through it, creating a rain-free base

Supercell

Done.