Extreme weather and climate

what is an extreme event?

• UNUSUAL

• IMPACTFUL

We need the sun!

• without sun earth’s temp would stabilise around -240 degrees in a few million years

• surface of ocean would freeze, plants would die

We need the atmosphere!

• without it temp would be -18 degrees

• but no protection against earth’s incoming radiation

• really cold nights and hot days, heat energy is not trapped

• no sound!

• no oceans

• no blue sky, no red sunsets

we need earth rotation!

or really hot, humid equator and freezing poles (steady decrease)

fun fact equator rotates fastest

History of the climate and future projections

energy budget

• what comes in: the sun (we receive 0.000000045% of the energy emitted by the sun! → still a lot)

• annual average solar irradiance (i.e. solar constant) = 1361 Wm-2!

• +- 3% variations throughout year because of elliptical orbit

(average) total solar irradiance → maximum amount of energy delivered to earth from the sun BUT

• but only half of the earth is lit by the sun at once

• and most of the sun’s rays hit earth on an angle

• so we divide 1361 Wm-2 by 4

  • taken from (the area of a disc/the surface area of a sphere)

    

• global average is closer to 340 W/m-2

cycles of incoming energy

can lead to small changes in climate that have a flow-on feedback effect

seasonal cycle

our weather and ocean systems do a lot of work moving the energy from the northern to southern hemisphere as needed and vice-versa!

solar activity cycle

11 years

• solar flares → hot

• sunspots → magnetic interactions cool different spots of the surface of the sun by about 1800 degrees

  • from 6000 to 4200

  • increases global temp by like 0.1 degree (hypothesis), so doesn’t really have any effect

• we’re currently coming off a solar maximum, high frequency of aurora events

energy budget graphic

planetary albedo

• clouds and aerosols (dust, dirt, ash, pollen, water vapour in volcanic ash, sea salt (big CCN btw!!), soot, etc.)

• 2023 was hottest year on record, even though a La Nina event

  • but shipping industry changed regulations to make fuel a lot cleaner → fewer aerosols, less sunlight reflected out into space

• global temperature and volcanoes → make things distinctly cooler

aerosols’ effects

• directly increase albedo caused by dust (pollution)

• indirectly increase the number of CCN making cloud drops smaller for the same amount of water, which increases albedo (but is tricky to calculate)

albedo values for different materials

positive feedback loop

• UNSTABLE system

  • like a ball on a hill → if you push it, things will change and change will amplify

  • e.g. ice-albedo effect

  • 

negative feedback loop

• → STABLE system

  • if you push it things will return to normal

greenhouse duh

things like CFCs and CO2 absorb the longwave radiation re-emmitted by the earth, and break up O3 which protect atmosphere

summary of weather

geostationary

satellites orbit earth once per day so seems like they’re staying still

but doesn’t work over the poles lol

PGF

• proportional to the number of isobars per unit distance (gradient)

  • change in pressure over a particular distance

density

kg/m3 and can be thought of like acceleration → how much would air like to accelerate due to differences in pressure

old mate with rotation

fluid dynamics principles

• conservation of mass

  • amount of air, water, etc.

• conservation of energy

  • temperature, kinetic

• conservation of momentum

  • wind

fluid includes atmosphere

atmospheric structure

• tro

  

  posphere → 80% of the atmosphere’s mass, whole land surface, all water vapour

  • tropopause → separation line between troposphere and stratosphere

    • higher in equatorial regions and lower at the poles

    • due to the convection and surface heating in the tropics thickening the layer where there is air turning

    • temperature generally decreases with height but very near the surface this isn’t always the case, often due to overnight surface cooling.

      • inversion → increasing temperature with height

• stratosphere → 20% of the atmosphere’s mass

  • much drier than the troposphere → not much water vapour

  • WARMER → ozone absorbs shortwave radiation from the sun

    • as the temperature rises with height this layer is very stable → not a lot of vertical motion, doesn’t move down

    • stratospheric ozone = good!

    • in tropical regions, convection can pass through the tropopause (marked by the anvil in cumulonimbus clouds) and into the stratosphere → how moisture gets there

      or volcanic eruptions

    • nacreous clouds form in the stratosphere in polar regions

      • due to chemical interactions and waves generated in the lower atmosphere which propogate upwards

• mesosphere

  • where meteors burn up

  • much lower ozone concentrations in this layer compared to the stratosphere → temperatures decline with height again

• thermosphere

  • temperatures rise with height as ions and electrons are excited by solar radiation and there is greater absorption at higher levels

  • temperature starts to become academic as there are fewer particles (but they are high energy

  • above about 100km there is no effective mixing. this can be used as a definition of where space begins.

• ionosphere → parts of the thermosphere and mesosphere

  • in this layer atoms are stripped of their electrons so they become charged

  • this is the consequence of absorption of energy due to cosmic rays and solar winds → aurora borealis and aurora australis

  • green: excited oxygen between 150 and 200km

  • red: excited oxygen above 250km

  • blue/purple: excited nitrogen below 100km

Composition of the earth’s atmosphere

• 78% nitrogen

• 21% oxygen

• ~0.9% argon

• trace amounts of water vapour (up to 1%), CO2, and other gasses, methane, ozone, nitrous oxide

• aerosols → pollutants and other particulates

• molecular weight of atmosphere is pretty close to nitrogen ~ 28.97 (average of all the elements found approximately)

air pressure

• consider air as an ideal mixture of gas and an ideal gas

  • molecules don’t really interact

pressure

hydrostatic equation

pg

pressure gradient

hydrostatic

high pressure and low pressure acting on the air are equal and therefore the air is not being pulled up or down

at a constant temperature, we can

• relate pressure changes to changes in height in the atmosphere (exponentially related to height)

  • can use this to build an altimeter!

combining hydrostatic equation and ideal gas law

force balance

means vertical motion is constant → again, hydroSTATIC

pg

= pressure gradient

reasons for thermally-driven overturning circulation!

if pressures are fixed in the top and bottom of the atmosphere, the only thing that will determine height of column of air is temperature

(basically, high- and low-pressure systems self-explanatory but just backing it up with some maths)

• halfway point is still 500 for both!

• now there is a difference in pressure for both columns of air at the same height!!

air will move from high to low

• air moves out of the tropics

  • therefore tropics will have less air → lower pressure

  • so now there is a DIFFERENT pressure difference at the bottom → air will move back!!!!!

CORIOLIS EFFECT!

• coriolis force compensates for f=ma not accounting for rotation

• coriolis force:

  • is greatest at the poles (local vertical aligned with earth’s rotation)

  • is zero at the equator (local vertical not aligned with rotation)

  • acts perpendicular to wind direction (right in NH, left in SH) → towards equator, essentially

coriolis force: the maths

understanding coriolis

old mate cells!!

why wind moves paralell to isobars

• very good approximation to the actual wind in the middle troposphere (about 5000m or higher)

  • very good when no friction, and when isobars are straight.

  • but:

    • only good for large-scale approximation

    • not great when isobars are curved (they usually are)

      • apparent centrifugal forces must be considered too!

    • it’s not great at the earth’s surface

      • friction impact (think of last sem)

        • winds at surface point slightly from high→low pressure

geostrophic balance

looking at cells/circulation globally

summary trust

you can measure rainfall with satellites!!

• water/precipitation reflects microwaves back to satellites

• microwaves make water move around a bit which shoots radiowaves back

what will climate change mean for atmospheric circulation/precipitation?

future areas with biggest increases in temperature!

• arctic is warming more rapidly than other places → very reflective things (ice and snow) are being removed, so heating is accelerating faster than other places

• land tends to warm more rapidly

insolation

= solar constant (radiation coming from sun)

• amount of incoming solar radiation perpendicular from the sun

• determined by level of radiation from sun and our distance from sun

• ~1361 W/m^2

  

when something has temperature, it

emits radiation (not just a particular radiation, a whole spectrum of light)

blackbody curve

can perfectly absorb all the energy that lands on it and perfectly emit all the energy possible

notice! brightest part at top of the curve is YELLOW

if an object is only 3000K, most intense light is a lower wavelength = infrared (flattest curve, smallest area underneath)

Wien’s displacement law

Stefan-Boltzmann law

solar radiation absorbed is only on the sunny side!

radiative balance

solar input of energy on sunny side BUT emission loss of energy in all directions (!!)

so for balance, incoming = outgoing

so if we know the albedo, we can figure out the radium equilibrium temperature because we know the solar constant! (basically just combining two above equations)

surface temp and radiation

the boltzmann feedback requires the emission temperature to be near equilibrium. surface temperature adjusts to match

strength of the greenhouse effect

the difference between the radiative equilibrium temperature and the surface temperature

balance differs by 1W/m^2 ****and that is what is causing global warming

precise values diagram → do we need to know this?

how does absorption work?

• H20 example → will rotate faster and vibrate if hit by incoming infrared photons

  • symmetric/asymmetric stretch, bend, and can vibrate in different waves → means different wavelengths absorbed by a single molecule

    • speed/frequencies very specific and dependent on the shape and weight of the molecule

• CO2 example

  • earth emits a lot of frequency at its preferred wavelength

  • molecule bends

• BASICALLY: (different molecules need a photon of specific wavelength)

atmospheric window region

• wavelengths that don’t interact, that are re-emitted by earth straight back out into space

  • rest (356/396 watts) radiated from surface is absorbed by atmosphere (mostly water)

if a planet has an atmosphere, it has a greenhouse effect!

water vapour positive feedback!

understanding how feedbacks enhance a small change → for every 1 degree of warming we get from CO2 in the atmosphere, that effect is accelerated by the water cycle by another 2 degrees!!

tropical cyclone structure and dynamics

• aka tropical storms, hurricanes, typhoons

• tropical cyclones → warm-cored, cyclonically rotating atmospheric vortices driven by air-sea enthalpy fluxes, and are mostly in hydrostatic and gradient wind balance (except near the eyewall and within the boundary layer)

hydrostatic balance

• → vertical acceleration isn’t very strong

  • except in boundary layer bc of imbalance of forces due to friction

technical definition

• warm-cored, non-frontal low-pressure system of synoptic scale developing over warm waters

  • having organised convection

  • a wind speed of at least 34 knots or 63 km/h

  • extending more than halfway around near the centre

  • persisting for at least 6 hours

  • before that = tropical low

• mature and intense tropical cyclones exhibit a high degree of symmetry with well defined eyes

tropical cyclone structure

two important conserved variables

specific moist entropy -> ‘up,’

absolute angular momentum -> ‘in,’ and, ‘out,’

• don’t think you need to know the maths that well?

  • R=radius, v= velocity (bigger in centre decreases moving out), f=Coriolis (doesn’t change unless latitude changes

  • MEANS as the air parcels spiral towards the centre and radius decreases, V (velocity) has to increase for M to remain constant

more cyclone structure/movement

Tropical wind profiles

• Most intense winds occur in the innermost ~100km surrounding the eye

• Wide range of observed radius of maximum winds (RMW) -> 10-110km

• Wind speed decays rapidly just beyond the RMW, but decreases more gradually at larger radii

Tropical cyclone structure and energetics

• Converting into

  heat energy

  mechanical energy

Carnot cycle (heat engine) -> stages of conversion

isothermal expansion

• Surface winds increasing increases the heat as you move towards the centre

• Temperature stays constant, moisture is acquired through evaporation of water as winds are increasing

• air accelerating as it moves towards the centre of the cyclone increases the flux of vapour from the ocean -> latent heat, not sensible heat

Adiabatic expansion as air rises

• High entropy air is conserved but temperature decreases

• g. from 29 to -70 degrees Celsius

Isothermal compression

Heat radiatively lost from top out into space

Adiabatic compression

• Air slowly sinks down to the surface

• (but in actuality, not idealised, a lot of air is mixed as it sinks rather than conserved)

Precipitation structure of a mature TC

• Concentrated in:

  • ‘Eye wall

  • Spiral bands

WMO tropical cyclone programme

• ‘bhola cyclone,’ hit Bangladesh in 1970

  • 300000 deaths due to storm surge

  • Triggered a coordinated response for the mitigation of cyclone disasters, resulting in the WMO TCP

Spatial distribution of tropical cyclone genesis

Large-scale conditions for tropical cyclone genesis

• Source of low-level (previous disturbance)

  vorticity

• Coriolis parameter needs to be sufficiently high, can’t be on equator

• Low deep-layer vertical shear (not much criss crossy wind)

• Sea surface temperature excess above 26 degrees to a depth of 60m

  • Threshold actually depends on global climate and will likely increase with global warming! -> shifts in a relative sense to the climate

• Vertical gradient of theta between sfc and 500mb (huh)

• Middle tropospheric relative humidity (big one!)

Annual tropical cyclone frequency -> a bit of a mystery

-> always around 80 per year with a standard deviation of only 7

Could help set the earth’s energy balance

Seasonality by region

• Jan-march is peak but nov-april in aus

• Peak in September-oct in northern hemisphere

• Basically, summer!

Tc-related mortality

Usually actually flooding/storm surge!

Tropical cyclone jasper (2023)

• Huge rainfall and flooding in cairns

• Rainfall of over 2m in a week

• Most rainfall-intensive on record

TCs are responsible for how much rain?

30-50% of annual rainfall maxima, especially in WA northern desert!

Need to resolve intense rainfall near the centre of tropical cyclones -> at the moment we don’t have that that much, around 100km

climate change and tropical cyclones

• They’ll likely decrease with global warming

  • But , so we are left with models as our primary tool

    no existing theory for tropical cyclone frequency

  • And if we go with high-resolution models decrease is actually less

With 2 degrees warming:

• But is expected to increase like 15% and even up to 30%!!

  rain rate

• Wind speed will increase about 5%

• TC intensity will increase about 5%

• TCs will intensify more quickly

  • Rapid intensification -> Increase in maximum winds of at least 30kt in a 24h period

thickness

• the distance between the 1000 and 500hPa pressure levels

• proportional to the mean temperature over this layer

  • i.e. higher thickness = warmer air

• used to identify cold and warm air masses

equation

fronts and thickness

• located at maximum gradients and kinks in

  • air pressure

  • thickness

• identified by

  • sharp temperature changes over short distances

  • shifts in wind direction

  • rapid changes in air pressure

  • changes in air moisture content

  • clouds and precipitation patterns

• seem to run perpendicular to isobars and parallel with thickness lines!

the thermal wind

• the vector difference due to the change in geostrophic wind with height

  • represents the change in geostrophic wind speed and direction with height

  • oriented parallel to isotherms (thickness lines!)

  • helps identify advection of cold and warm air masses

• can tell us if the average temperature is increasing or decreasing

  • 

    indicating whether advection of air into a region is cold or warm

cold air advection:

• colder airmass moves to replace warmer air

  • *geostrophic wind vectors turn clockwise with height → veering

  • thermal wind vector aligned parallel to the mean isotherms

  • warm air to the left of thermal wind vector, cold air to the right

warm air advection

• the airmass within a layer is warming, as a warmer airmass moves in to replace colder air

  • *geostrophic wind vectors turn anticlockwise with height → backing

  • thermal wind vector aligned parallel to the mean isotherms

  • warm air to the left of thermal wind vector, cold air to the right

practice using hypsometric equation!

Synoptic

Implies both time and space scales

• Time: 1-7 days

• Space:

  • Horizontal: about 1000-3000km across

  • Vertical: up to 12-16km (depth of the troposphere)

• weeklong timescale,

  • highs, mid-latitude and tropical lows

  • thunderstorms or tornadoes or climate change

    not

First synoptic chart

1816 -> Heinrich Brandes (using observations sent by mail)

First operationally useful chart

• 1849 -> Smithsonian institution (telegraph observations)

• Australia’s first chart in 1877

fronts in north

north Atlantic/Europe tends to have a lot of occluded fronts and warm fronts and complicated things happening

Pressure contours are in hPa (1 Pa = 1 Nm/-2), usually spaced at 4 hPa

but hey! remember prac 1? (south?)

Vertical motion

• Descending in highs

• Rising in lows

• Condensation/clouds: rising air more likely to lead to rainfall

  • as air cools and condenses

  • Air warms when it descends

dew point

when condensation occurs

The temperature to which air must be cooled (at constant pressure and water vapour content) for saturation to occur