thunderstorm Flashcards
what are associated with thunderstorms
- lightning and thunder
- gusty surface wind
- heavy rain/hail
thunderstorms are separated into which three storms
- ordinary storms
- multicell storms
- supercell storms
What meteorological conditions are necessary for the occurrence of a thunderstorm
- A conditionally unstable atmosphere
- Substantial boundary layer moisture
- A trigger to release the instability (e.g., low-level lifting) + parcel has to reach LFC
why is an absolutely unstable atmosphere unfavourable in the development of thunderstorm?
- bc the ELR is greater than the D/MALR, so convection is found from the start –> no CIN –> no accumulation of energy as energy is immediately released.
- no CIN –> no energy to form thunderstorm
discuss the energy of CIN in thunderstorm formation
larger CIN- more energy accumulation - stronger thunderstorm. however if CIN is bigger than the CAPE, thunderstorms do not form
why is it hard to form a thunderstorm
- atmosphere has to be conditionally unstable (stable at first unstable later) to prevent initial convection.
- however, without initial unstable condition parcels cannot rise. therefore other conditions are needed to rise the parcel
what are the factors that cause air parcel to rise without an unstable condition?
- Extra-tropical cyclone (low-pressure) convergence force
- Warm air rising along a frontal zone
- Large-scale uplift along mountain barriers
- Effect of small hills, sea-breeze front, localized convergence
- turbulent eddies
- unequal surface heating
what is the convective cloud growth when CAPE < CIN
cumulus
what is the convective cloud growth when CAPE > CIN
towering cumulus
what is the convective cloud growth when CAPE > CIN greatly
cumulonimbus
ordinary cell/single cell thunderstorm
- relatively small thunderstorms, rarely become severe
- isolated cumulonimbus cloud, short lived
- have a predicted life cycle
- single heavy rain
what are the conditions for developing ordinary cell thunderstorms
- very little vertical wind shear
- turbulence overturning
- localised surface wind convergence
- in warm humid air, far from fronts
what are the three stages of life cycle
- cumulus
- mature
- dissipating
discuss the cumulus stage
- Warm buoyant plume rising
- Inside cloud, transformation of water vapour to liquid or solid cloud particles releasing great amount of latent heat, the parcel is warmer than the surrounding hence continues to rise
- At edges of the cloud, entrainment happens, bringing drier air into the parcel. Evaporation happens to add water to the drier air – this cools the parcel down slowing the growth but this is necessary in order to moisten the air
- air moistens, RH increases, more condensation can take place –> more successive updrafts to sustain clouds to higher and higher levels –> towering cloud
- no rainfall or lightning at this stage
discuss the mature stage
- Growth of drops & ice continues until they become large and heavy –> updraft can no longer support – fall
- At the same time, entrainment happens. Raindrops evaporate, the air cools. The parcel is now heavy and cool– sinks
- Both these processes lead to development of a downdraft
- Most intense stage – heavy rain, thunder, lightning (occas. small hail)
- Maximum updraft and downdraft at middle levs– severe turbulence detrainment above that level
- Anvil starts to form at top as result of horizontal spread
- Raindrops are called supercooled drops above freezing level in the updraft, snow/ice are called snowflakes/pellets below frez. level in the downdraft
discuss the dissipating stage
- downdraft grows until it is greater than the updraft and totally cut it off –> no fuel supply for cloud development
- Without supply, droplets no longer form (no condensation), light precipitation
- The rain formed, dragging air down, cause downdraft. this upsets the warm air rising as they are at the same place. Self limiting itself.
- Rainfall stops; all that is left is the anvil
why does the rainfall declines in dissipating stage and how does this differ in different places/conditions
- Only about 20% of the water vapour condensed in the updraft actually reaches the ground as precipitation (rest evaporates in downdraft or left behind as cloud patches (then evaporates))
- In places with dry air, the raindrops evaporates before reaching the ground.
define multi-cell thunderstorm
Contain a number of cells, each at a different stage of development: once one cell subsides, another grows adjacent to the last cell
what promotes multi-cell thunderstorms
- Moderate-strong vertical wind shear: helps thunderstorm development and lifetime by separating the updraft from the downdraft
- The cool downdraft from an existing TS causes a “gust front” when it meets the surface. This may push up surrounding warm moist air(like cold front) –> uplifts, and trigger a new cell to develop, sustaining the multicell storm and its propagation
- high severity
windshear
- wind speed and direction change throughout the vertical profile.
- a type of turbulence
gust front
- the leadings edge of the cold downdraft of an existing TS.
- great temp drop, strong gust winds, turbulence (change in wind speed and air flow)
- aka mesohigh
- small scale front
- promotes the uplift of an adjacent TS, pushing up surrounding warm air
what clouds can gust front form
roll clouds. when the surface air is pushed up along the front.
squall lines
- Multicell thunderstorms organized in a line, favoured by the arrival of a cold front
- Frequent over midlatitudes
- much stronger than normal TS
- warm air forced up the cold front
Mesoscale convective systems
- Multicell thunderstorms organized in a large circular convective system
- Individual TSs work together to generate a long-lasting (> 6hrs) system moving slowly
Supercell thunderstorms
- Rotating updraft: rotation causes the storm to bring in more air, hence stronger.
- longer-lived (hours), dangerous
- low pressure formed by at centre of rotation, called a mesolow
- Rotation can lead to formation of a tornado
- No downdraft of air happen with the updraft of air at the same place, no limitation
formation of a horizontal shear/rotation
- strong and deep change in wind speed and direction with height.
- air is accumulated at both sides of the wind direction. air at the bottom rises to replace the air that are dragged on the other side.
- rotation is created.
how does a horizontal shear acquire vorticity about a vertical axis
- updraft of warm air
- more surface heating –> more updraft –> vertical motion with rotation
true or false:
mesocyclone induces more convergence and updraft
true
true or false
Precipitation falls ahead advected by strong wind
true
