EOSC 114 - Storms

Cloud-to-Ground Lightning

• When one end of the bidirectional leader network connects to the ground/object on the ground

• Lightning from the anvil (positive strike) travels a longer distance

• To make spark - must have 3 billion volts/km

• Can be positive or negative

  • Negative strikes: more numerous and come from cloud base, shifts towards ground

  • Positive strikes: 

    • Less frequent

    • Come from anvil 

    • are much stronger

    • Primary cause of natural wildfires

    • 10-25% of Canadian CG lightning is positive

    • Come up from ground 

How does lightning form?

• Thunderstorms can separate + and - in and around them

  • Creates powerful electric fields

• Formed between two oppositely charged regions

• Leaders branch out to oppositely charged polarities to attempt to neutralize the separation

Cloud Flash

• Most lightning flashes occur within a storm and are called this

• Intercloud lightning

Return Stroke

The bright flash of light produced when the positive charge from the ground returns to the negative charge in the cloud after the leader has established a path

Multistroke CG Flash

• After a - CG return stroke, other return strokes follow

• Causes other return channels to flicker

• Due to orientation of charged regions in the storm

Positive CG

• The peak charge can be 10x more powerful than the avg CG

• Most powerful and super hot

• Only account for 5-10% of worldwide flashes

• Strange and smooth channel to the ground

  • Smooth channel lightning

• Visible flashes often show just the channe;

  • Branchless lightning (the branches are hidden in the clouds)

Negative CGs

• Lots of branches 

• “Bolt from the blue”

Massive Complexes

• Sometimes storms grow upscale into massive complexes

• Horizontally layered charge regions, serve as the electrical conduit

• Travels horizontally

  • Almost always positive in polarity

Upward lightning

Travels upwards from tall objects from the ground into the storm. Almost always have positive upward leaders (AKA CG lightning)

Intracloud Lightning

A type of lightning that occurs entirely within a single thundercloud, or sometimes between two different clouds, without striking the ground. This is also referred to as a Cloud Flash.

____ has the highest density of lightning worldwide.

Africa

____ is “lightning alley” in North America.

Florida

How far away is lightning?

• Sound travels more slowly than light

• Count the number of seconds between when you see the lightning and hear the thunder

• Divide this number by 3 to estimate the range in km to the lightning

  • I.e. 9 sec difference = 3km

  • 15 sec difference = 5km

Lightning safety

• Monitor the weather conditions

• 30/30 rule: if 30 sec or less between when you see the lightning flash and hear thunder, move indoors and stay there until 30 minutes after last lightning or thunder

• Safe places:

  • Fully enclosed vehicle with the windows up

  • Substantial permanent building, but do not use hard-wired telephones

• If you are stuck outdoors, avoid:

  • Small structures (huts, rain shelters)

  • Nearby metallic objects (pole, fence)

  • Trees, water, open fields, hill tops, etc.

• If caught in the open, do the lightning safety crouch

  • Feet together, hands over ears

• If people nearby are struck by lightning, try reviving with CPR

Thunderstorm

• Thick clouds with lightning and thunder 

• Looks like a mushroom

• Strong updrafts and downdrafts (turbulent)

• If the updraft is strong, then a dome of clouds overshoot above the anvil (mushroom top)

• Anvils can be 100s km in diameter

• Main updraft (the stem of the mushroom) is 15km diameter

• Storm energy is from temp and humidity

Thunderstorm Cells

• Make up thunderstorms

• Evolve in 15-45 mins

• Most thunderstorms have 2 or more cells (each in diff. stages

  • Called multicell thunderstorms

  • Last longer than any individual cell

Squall Line

• A line of thunderstorms

  • “linear”-ish

• Often form along a cold front

Thunderstorm Cell Life-Cycle

1. Cumulus stage: updraft, no rain, no anvil

2. Mature stage: up and down drafts, heavy rain, anvil

3. Dissipating stage: downdraft, lgith rain, fuzzy anvil

Multi-cell Thunderstorms

• Each cell only lasts 15-45 minutes, but the cluster, made up of multiple cells at various stages in their lifecycle, can last several hours

Storm Energy

• Solar energy

• Surface heat budget

• Daily cycle

Solar energy

• Absorbed at 3 diff. heights:

  • Top (thermosphere): absorption of non-visible light

  • Middle (stratopause): absorption of ultraviolet by ‘good’ ozone

  • Bottom (earth surface): line shines through lower atmosphere with little direct heating of air but heats the ground instead

  • Then the warm ground heats air in troposphere (the bottom 11km and powers storms

Surface Heat Budget

• Some solar energy reflects back into space from clouds and the ground

• Some is absorbed by the ground (ground gets warmer)

• The warm ground affects the air:

  • Sensible heat (warms the air): air temp. increases

  • latent heat (evaporates water from lakes, vegetation, etc.): air humidity increases

• Both temperature and humidity are important because they are fuel for storms

Daily cycle

• Solar heating during the day = input (like charging a battery)

• Infrared radiation (IR) cooling = day and night = loss (like discharge)

• Greatest accumulation of heat near sunset everyday (at the end of each charging cycle)

• Late afternoon and early evening = most likely time of day for thunderstorm formation 

How do we observe and monitor storms?

Remote sensing (i.e. radars and satellites)

• A remote sensor is an instrument that is remote from (outside of) the storm, but can still measure the storm

• They can see visible images of thunderstorms by being able to see the top of the anvil cloud

___ shows clouds during the daytime only, while ___ can show clouds both day and night.

Visible images, IR images

• IR = infrared

  • High altitude clouds are highlighted in red, because these are often the tops of dangerous thunderclouds

Radar

Sees the precipitation inside the storm; namely, in the up& downdraft stem of the mushroom cloud.

dBZ

• Radar echo intensity (in decibals)

• A disaster intensity scale for radar-echo strength.

• An indicator of rainfall-rate.

Supercell Thunderstorm

Very large, rotating single-cell thunderstorm. These cause the most violent tornadoes, large hail, frequent lightning, heavy rain and strong winds. They typically have striations (layers) around the mesocyclone.

Mesocyclone

A rotating thunderstorm.

Supercell Types

1. Low precipitation (LP) supercell: can produce lots of hail

2. Classic supercell: rainy downdraft and rain-free updraft

3. High precipitation (HP) supercell: updraft mostly surrounded by rain

• Some are a hybrid and contain features of 2+ more types

Why shouldn’t you go into fast flowing water, even in a car?

It takes just 30cm of flowing water to carry off a small car. More than 50% of the deaths from flooding each year in the US happen in vehicles.

Downbursts & Gust Fronts of air

• Downdraft speeds of 20-90km/h

• horizontal wind speeds near ground of up to 250km/h

• Microbursts are small diameter (~1km) downbursts

Downbursts

Cold, dense air sinking

Downbursts - How does it happen?

Thunderstorms can create dense air where rain falls due to evaporative cooling of ‘virga’ (precipitation that evaporates before it reaches the ground).

Downbursts - Risks

Invisible, hazard to aircraft

Gust front

Leading edge of cold, horizontal straight-line winds

Gust front - how does it happen

Downburst air hits ground and spreads outward in straight lines

Haboob 

• If on dry ground

• A severe dust storm or sandstorm

• Visible because in dry, arid regions, the strong winds of the gust front blast into the dry soil and lift the vast quantities of dust and sand high into the air

• Can reduce visibility

Arc Cloud

• If moist air 

• A long, low lying, often ominous looking cloud formation that takes the shape of an arc or wedge (shelf clouds that are wedge-shapes and roll clouds that are tube shaped)

• Visible because in areas with sufficient moisture in the lower-level air, the warm and moist air ahead of the gust front is forcibly lifted up and over the incoming cold air wedge

  • As the air rises and cools, its water vapour condenses, forming a dramatic line of cloud along the entire leading edge of the cold outflow

Gustnado

• A small short lived, ground-based vortex

• Tiny weak tornado

• Visible because the intense wind shear and turbulence where the cool outflow meets the warm surface air can sometimes create a temporary rotating column of air

• Rotation usually only extends a few hundred ft above ground and is made visible by the debris it stirs up from the surface

• NOT A TRUE TORNADO because it is not connected to the rotation within the mesocyclone

Gust Front - RIsks

• Can blow down large trees and destroy weak structures (mobile homes; out-buildings)

• Hazard to aircraft during take off and landing

How to stay safe during downbursts and Gust Fronts

Avoid weak buildings and trees that could fall.

Storm Organization

Storms have special organization and capability to:

• Draw in humid air

• Condense the moisture in this air

• Release its heat into the storm, increasing the strength of the storm

• Results in precipitation and violent winds

Humidity

The amount of water vapour in the air. There are many ways to quantify humidity:

• Mixing ratio

• Saturation - an equilibrium between evaporation and condensation

• Storms strengthen when latent heat turns into sensible heat

Mixing Ratio

The amount of water vapour divided by the amount of all other gases

Saturation

• Vapour to liquid = condensation

• Liquid to vapour = evaporation 

• The mixing ratio tends to approach an equilibrium where condensation matches evaporation.

• Value is the maximum humidity air can hold

  • Important in controlling atmospheric humidity

  • Warmer air can hold more water vapour at equilibrium than colder air

  • Air that contains the max. amount of water vapour (cloudy/foggy) = ___

  • Air holding less (not cloud) = ____

Advection

Movement of air by the wind. Water vapour can be ___ into a thunderstorm by the wind.

Adiabatic Cooling

When a thermal of unsaturated air rises ___ (with no heat transfer to the surrounding environment), the air cools roughly 10 degrees celsius per km of rise because of pressure differences

Storms strengthen when latent heat turns into sensible heat

• If saturation humidity value becomes smaller than the actual humidity then condensation occurs

• This condensation does three things:

  • Releases the stored latent heat back into the sensible heat to make storms warmer (makes storms even stronger)

  • Reduces the humidity down to the equilibrium (saturation) value

  • Produces or increases liquid cloud drops, which can grow to become rain drops

• The release of latent heat increases the strength of the storm

Recognizing Tornadoes

• Most tornadoes are made visible by cloud-water droplets (funnel cloud)

• Or dust and debris from the ground (debris cloud

• Some are invisible

Are tornadoes attached to thunderstorms?

• Yes

• Only a small percentage (20-30%) of supercell thunderstorms produce tornadoes

• In N. America, most thunderstorms move from SW towards the NE

• Supercell storms:

  • Strongest

  • Strong tornadoes are most likely associated with supercell storms

Best thunderstorms viewing is?

• Off to the side of the storm path

• Preferred side is to the SE of the storm

Recognizing supercell rotation

• Striations around the mesocyclone

• A rotating wall cloud

• Strong tornadoes are most likely associated with supercell storms

Wall cloud

• An isolated lowering of cloud base…

  • beneath rising cumulus towers

  • On the SW flank of the storm

  • Outside of (SW of) the precipitation region

• Tornadoes come from rotating ___ in supercells

Enhanced Fujita Scale

• Used in n. America

• Determined by amount of damage to buildings

• EF0 = very weak tornado (may break a few windows)

• EF5 = very strong tornado (totally destroy whole buildings)

Tornado Safety

• Usually short lived and only last minutes

• Typical damage path

  • Narrow (usually width of. house, up to size of city block)

  • Damage paths often 1-10 km long

• They can occasionally last for hours, have damage paths 1km or more wide, or have paths of a 100 km long

Safest Places to be During a Tornado

Indoors:

• get below ground, in a basement or storm cellar

• Get out of mobile homes (worst place to be in during a tornado)

Outdoors on foot:

• Get into a ditch/hole

• Place body below line of fire of fast moving debris

In car:

• Drive away from tornado on best convenient road

• Preferably to the right/left of translation direction of tornado

• Do NOT hide under highway bridge/overpass

Tornado Risk

• US

• Middle East

• East Asia

• Japan

• Australia

• South Africa

• More

Tornado Outbreaks

• Often associated with squall lines

• Squall lines often form along cold fronts

• As the cold front moves toward the southeast, the thunderstorms along the front move toward the northeast

Tornado Watch

• 6-12 hr forecast

• A broad region within which tornadoes are favourable or likely later in the day

• you can continue your normal activities

• you should monitor emergency announcements on news or weather radio

Tornado Warning

• Tornados actually detected now by:

  • Doppler radar with tornado vortex signature (TVS)

  • Human spotter or other government official sees a tornado

• Tells you where the tornado is, where it is moving, warning for specific towns or counties within the expected path, and tornado sirens activated in those towns

Storm Energy: From Heat to Motion

• Air motions = wind

• Can cause damage directly and blow in more humid air (storm fuel) 

  • AKA Moisture advection

  • Positive feedback = longer storms

  • Thus storms become organized

Force

• m x a

• Forces create winds

• If you push an object harder (more force) then it accelerates faster in the direction you push it

Acceleration

• change in velocity during time interval

• measured in m/s²

• a = vnew-vold/ change in time

Air Parcel

• Hypothetical blob of air about the size of a city block

• Air parcel movement is wind (horizontal/vertical)

• vnew = vold + (f/m) change in time → forecast equation for wind

• Tells us how winds increase/decrease or change direction depending on the forces that act on air parcel

Buoyancy Force

• vertical

• Causes up and downdrafts

Pressure-gradient (PGF)

• Horizontal or vertical

• Horizontal PGF → horizontal winds

Why does warm air updraft and cold air downdraft?

Temperature affects the density of air and density affects buoyancy.

Buoyancy

• Air parcel depends on the difference between parcel temp and temp of surrounding air

• Warm air is less dense (upward force)

• Colder air is more dense (downward force)

• Causes hot air balloons to rise

• Drives thunderstorms

  • Condensation in thunderstorms releases latent heat

  • It warms the thunderstorm air, making it buoyant and causing the air to rise

  • Drives violent updrafts in thunderstorms

Pressure

• Drives winds

• Pressure difference across a distance = pressure gradient 

  • pressure pushing on one side of air parcel vs. pressure pushing on other side

• Hurricanes are strengthened by pressure- gradient forces

  • Latent heating from condensation in the centre of the hurricane causes the centre to be warmer

  • The warm air expands, creating low pressure near the surface in the centre

  • low pressure at the bottom = pressure gradient

  • Gives the spiral inflow into the bottom of a hurricane

  • Inflow advects in more fuel making the hurricane stronger

How does temperature alter pressure to drive horizontal winds?

• Horizontal changes in temp 

• Horizontal changes in pressure that increase with height

• Pressure gradient increasing at higher altitudes 

• Drives faster winds at higher altitudes

• This type of pressure-gradient force drives the violent wind in hurricanes

Continuity Concept

• Air molecules tend to spread themselves smoothly and evenly

• Don’t leave gaps

• They don’t get bunched together

• Air is spread relatively evenly

• Leads to circulations

  • Air molecules are smoothly and evenly distributed in space

  • Buoyant air parcel rises, leaves hole where it used to be → a partial vacuum

    • Has lower pressure than surrounding air

  • Surrounding air sucked in to fill hole → maintain

  • Air above the rising parcel is compressed and has higher pressure = expands laterally

  • Net result: initial vertical motion due to buoyancy generates horizontal motion in surrounding air = circulation

Circulation

• Develop smoothly and continuously

• try to maintain continuity as air parcels start to move 

• Can be driven by buoyancy in the vertical or by horizontal pressure gradients

  • Linked to the effect of continuity

Atmospheric Rivers

• Pressure gradients drive this

  • When the air hits mountain ranges and is forced to rise, the air cools at the adiabatic lapse rate (10 degrees celsius/km) causing water vapour to condense and rain to form

• Can carry 2x< the volume of the Amazon River

• About 400-800km wide, 3km deep, and 100s of km long

• Originate in the tropics

Abbotsford Flooding 2021

• Heavy, prolonged precipitation from a series of atmospheric rivers

• led to severe and disastrous flooding in the Sumas Prairie 

• Resulted in landslides, impacting some major roads in BC

Thunderstorm Hazards

• Lightning

• Downpours of rain and flooding

• Downbursts of air and gust fronts

• tornado

• hail

hurricane Hazards

• Contain thunderstorms

• storm surge/ coastal flooding

• High waves

• Coastal erosion

Hail Safety

• bring/wear safety glasses in case hail breaks the windows in car

• If possible, turn away from storm and drive away

• Stay under a roof, inside a car, under a farm tractor, etc. to protect yourself from falling hail 

Size of Hurricanes and Thunderstorms

• Height = 15km for hurricanes and thunderstorms

• Diameter = 150-300km for hurricanes, compared to 15km for thunderstormes

Hurricane/Typhoon Structure

• Tropical cyclones, surface winds turning counterclockwise

• Eye = center of hurricane (relatively clear and calm and low pressure at sea level)

• Hurricanes are made of thunderstorms

  • Eye wall = ring of thunderstorms around the eye

  • Spiral bands = bands of thunderstorms extending out from the eye wall

Tropical Cyclones

• Organized to create their own fuel 

• Longer lasting thunderstorms 

  • Ambient atmosphere happens to have right amount of wind shear that:

    • continually blows fresh fuel into storm

    • or blows thunderstorm to new regions having boundary level fuel

    • Supercell that can last for hours

Why can hurricanes and their thunderstorms last for weeks?

They manipulate the environment that create new fuel of warm humid air from heat stored in the ocean.

Fuel Creation Method

• low pressure in eye sucks in boundary-layer air (air in bottom 1km of the atmosphere

• As air gets closer to eye it moves faster

• These faster winds create larger ocean waves

• Evaporation from ocean surface is enhanced with spray from waves

• Adds significant amounts of moisture into boundary-layer air

• So when this boundary layer air reaches base of eye wall it is warm and exceptionally humid

• Warm sea surface is needed

  • Self fueling method works only if ocean surface temperatures are > 26 degrees celsius and warm surface waters are 60m deep

Hurricane Season

Most hurricanes in late summer and in early fall when waters are warmest with maximum extent and depth 

• Official N. Atlantic hurricane season is June through November

• Most N. Atlantic hurricanes occur August through October

Hurricane Formation and Movement

Hurricanes form in the tropics but not at the equator

Hurricane Existence

• Hurricanes can persist only if central pressure in the eye remains low and the hurricane remains over the warm ocean

• Hurricanes weak and die if they cannot generate sufficient fuel of warm humid air and larger scale weather systems can also interfere with hurricanes

Hurricane Paths

• Steered by the general circulation in the atmosphere

• Westerlies, Bermuda High, Trade winds

Why can hurricanes last for weeks in spite of all the air molecules continually being blown into the core?

Heavy rain condensation and precipitation from the thunderstorms in the eye wall cause the hurricane core to become very warm relative to its surroundings.

How do eye-wall thunderstorms warm the core?

The warm core creates high pressure at the core top and low pressure at the core bottom, strengthening the cyclone. Result is a vertical circulation:

• Strong updrafts in the eye-wall tstorms

• outflow at hurricane top away from core

• Weak downward motion in the eye and outside the hurricane

• inflow at hurricane bottom towards core

• Cycles back into base of eye-wall tstorms as a complete circulation

Hurricane Prediction and Safety

• Hurricane predictions are inaccurate

• Therefore forecast maps of hurricane danger usually include probabilities

• there is uncertainty until the hurricane gets close to shore

• To stay safe, don’t buy or build houses on or near the beach in SE USA and plan in advance for evacuation 

Canadian Hurricane Centre

• Part of the Meteorological Service of Canada (ECCC)

• Responsible for providing info and warnings on tropical cyclones in Canada

• Located in Dartmouth, NS