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Climate Seminar summary, Schemi e mappe concettuali di Climatologia

Politecnico di Milano (POLIMI)Climatologia
Prof. Renato Casagrandi

Systematic review of the seminar on climate by Dr. Silvio Gualdi

Tipologia: Schemi e mappe concettuali

2021/2022

Caricato il 31/05/2023

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Provide a basic understanding of the main features of the climate system and its variability to
illustrate the simple basic principles underpinning climate dynamics and climate predictions.
Lecture 1: The main basic features of the earth’s climate
✓ Basic characteristics of the earth’s climate
✓ Global energy balance
✓ Effects related to land-sea contrast and the seasonal cycle
✓ Main modes of climate variability (monsoon circulations, interannual variability (El
Niño) and decadal oscillations)
Lecture 2: Earth’s Climate system and its dynamic
• Dynamics of earth’s climate
• Simple radiative balance model
• Radiative-convective balance model
• Primitive equations of the atmosphere (shallow-water equations)
• Modes of climate variability
Lecture 3: Modelling the climate system
• Discretization and numerical solution to the primitive equations
• Development of climate models
• Deterministic chaos (Lorenz idealized system)
• Problem of predicting the evolution of the earth’s climate
• Predictability (1st and 2nd kind)
• Climate predictions and projections
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Provide a basic understanding of the main features of the climate system and its variability to illustrate the simple basic principles underpinning climate dynamics and climate predictions.

Lecture 1: The main basic features of the earth’s climate

✓ Basic characteristics of the earth’s climate

✓ Global energy balance

✓ Effects related to land-sea contrast and the seasonal cycle

✓ Main modes of climate variability (monsoon circulations, interannual variability (El

Niño) and decadal oscillations)

Lecture 2: Earth’s Climate system and its dynamic

  • Dynamics of earth’s climate
  • Simple radiative balance model
  • Radiative-convective balance model
  • Primitive equations of the atmosphere (shallow-water equations)
  • Modes of climate variability Lecture 3: Modelling the climate system
  • Discretization and numerical solution to the primitive equations
  • Development of climate models
  • Deterministic chaos (Lorenz idealized system)
  • Problem of predicting the evolution of the earth’s climate
  • Predictability (1st^ and 2nd^ kind)
  • Climate predictions and projections

• WEATHER

o Specific place and specific time o Atmospheres condition at a particular place over a short period of time

  • CLIMATE o Weather conditions over a place over a long period of time o Weather statistics over a region over time
  • Before 1950s: climate was a descriptive science, more geographical than physical. Koppen climate zones (tropical rainy, dry, warm temperate, cold temperate, polar, highlands)
  • Since 1950s o Improved observations lead to improved mathematical modelling o Climate is a statistical description of weather over a period of minimum 30 years (for robust statistics) o Modes of circulations Free atmospheric Time-scale 2 years Coupled ocean-atmosphere Time-scale weeks to decades Oceanic Time-scale: hundreds of years Cryosphere Time-scale: thousands of years
  • CLIMATE SYSTEM o (1975) A climate system is composed of the atmosphere, hydrosphere, cryosphere, land surface and biosphere o (1992) A climate system is the totality of the atmosphere, hydrosphere, biosphere and geosphere and their interactions weather is^ what you get climate whatyou expect
  • Climate is the balance between incoming solar radiations, terrestrial radiations and heat transport from low to high latitudes
  • The objective of oceanic currents and circulations is to transfer heat
  • Observed movements like cyclones, perturbations, etc., have as ultimate objective the heat transfer and transport
  • Convention for atmospheric circulation o Meridional: + towards north & - towards south o Zonal: + West to East & - East to West &
  • Warm air rises (tropics), as it looses energy it stops rising and moves towards higher latitudes, when it cools down it is dry and starts compressing downwards.
  • In tropics there’s convergence of low pressure, more precipitation
  • Meridional energy transport o Mean meridional transport MMC (Hadley Cell) ▪ Between equator and 15° MMC is very significant o Eddy motion (waves) ▪ Important for midlatitudes, like midlatitudes cyclones
  • How to measure change? Indicators. Sea Surface Temperature SST:
  • Pacific Ocean has a thermocline, warmer in west pacific. Rapidly cools down (in depth) in eastern pacific.

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  • Under perturbations positive feedbacks in the coupled oceanic-atmospheric circulation emerge.
  • Equatorial pacific oscillates between El Niño (warm phase) and La Niña (cold phase)
  • El Niño-ENSO (El Niño Southern Oscillation) o Warming of western pacific, weak winds o Reduced intensity of the winds leads to a convection shift towards east o Largest observed 1997- 98
  • La Niña o Cooling of eastern pacific, stronger winds o Largest observes 1988- 89
  • Indices of ENSO o SOI: Southern Oscillation Index. SLP @Tahiti – SLP @Darwin o SLP: Sea Level Pressure
  • Pacific decadal oscillation PDO o Long-lived El Niño like pattern of pacific climate variability o Spans 20 to 30 years, recently has shortened o Cool phase: lower than normal sea-surface temperature in equatorial eastern pacific. Last one 1947 to 1976, then 1999 to 2002, 2017 to 2013 o Warm phase: west Pacific Ocean cools and east warms. Last one 1977 to 1999, 2002- 2005 o Last PDO shift was in 2014, turned very warm (strongly positive)
  • AMO index: ten-year running mean of detrended Atlantic Sea surface temperature
  • Perturbations and interactions are constant at all time and spatial scales, we will always have variability.
  • Seasonal cycle, coming from orbit characteristics are fundamental and the main perturbation.

o TOA + Atmosphere= surface o Doing equilibrium knowing Te=255 K, Ts is 303 K (VERY HOT) o Planet is transparent for solar radiation but opaque to terrestrial radiation o This model can be extended to more and more layers, higher temperatures achieved each time. Will increase by number of layers considered. o What is wrong? Initial assumptions, real atmosphere is not opaque, it is not a real black body, it can’t absorb the entirety of the energy received

  • Leaky Greenhouse model o Emissivity (ε): ratio of energy radiated by an object and the energy radiated by a blackbody @same temperature. To an extent is the efficiency factor of the body compared to that ability of a blackbody. o Emissivity will depend on the composition of each layer. Important to consider water vapor, carbon dioxide, ozone, methane. o Radiative equilibrium: ▪ Stratospheric temp predicted is closed to observed but to cold in tropopause and to hot at the surface. ▪ Radiative equilibrium is unstable in the troposphere. ▪ What is wrong? It is missing convection! Convection is as important as radiation in transporting enthalpy vertically in the atmosphere and in the ocean, it controls distribution of water vapor and clouds (major constituents in radiative transfer) o Radiative-convective equilibrium: ▪ Consider buoyancy of air cells, weight (gravity) and pressure force. ▪ Better solution but results are still to hod for surface and too cold for tropopause. ▪ At this point no considerations of humidity have been done. o Moist convection/atmospheric humidity ▪ Usually confined to lowest km of the atmosphere ▪ Specific humidity (q) ratio of the mass of vapor in a certain volume to the total mass of air and vapor in the same volume. Will be higher in equator and decrease towards higher latitudes. Will decrease with altitude. ▪ Moist convection transports heat from surface, redistributes energy makes stratification stable. Energy is released in the middle of the troposphere by convective events (such as precipitation) the atmosphere responds with waaves of dissipating energy ▪ A hurricane of medium size can expend as much energy as 10,000 nuclear bombs ▪ Including moist convection finally leads to correct thermal distribution
  • The Governing Equations o 5 parameters define the state of the atmosphere and ocean at any time. Velocity in the 3 axis, pressure and temperature. And density can be inferred from pressure and temperature. o Equations with appropriate boundary conditions, they are sufficient to determine the evolution of the fluid.

o (1) Conservation of energy: thermodynamics heat equation, thermodynamic state in which the motion takes places o (1) Conservation of mass: continuity equation, change in the mass of a dry air parcel

  • Can be represented in pressure coordinates using hydrostatic balance o (3) Conservation of momentum: 3 equations motion for a fluid parcel in each of the orthogonal directions ▪ Forces on an elementary fluid parcel are considered: pressure gradient force, body force (weight) and friction. o Conservation of angular momentum: vorticity o This equations are accurate if applied to a fixed and inertial frame of reference which is NOT the case of our rotating planet. Coriolis forces need to be considered. o Coriolis force: tendency for a fluid parcel to turn. Right in northern hemisphere and left in south hemisphere. This force does no work o Simple model that allows to go from physical principals to equations to atmospheric descriptions
  • Shallow Water System solution o Developed by Matsuno in 1966 o Approximation where only 10 km of atmosphere are considered o Horizontal dynamics dominate over vertical dynamics o Idealized framework to better understand and investigate the dynamics of the tropical atmosphere o Wide spectrum of wave solutions
  • How can we model the climate system through mathematical relations? o Climate equations are very difficult but can be solved with numerical methods through computing power
  • Bjerkness. Sufficient conditions for the rational solution of forecasting problems are: 1. Sufcciently accurate knowledge of the state of the atmosphere at the initial time and 2- sufficient and accurate knowledge of the laws accorging a proceding state will develop from a previous one
  • Climate system simulations are based on solutions of the governing physical laws
  • Numerical solutions found throufh finite difference method, exploring solutions at different “locations” of the system and taking results as initial and boundary condition for next solution to be found
  • The first successful numerical weather forecast in march 1950 with computer ENIAC (electronic numerical integrator an computer
  • EMS Earth system models o Math equations representing physicial characteristics and process for each box o Equations are then discretized to finite difference. Finite volume, etc o Supercomputer solves equations and passes results to neighboring boxes, thus, calculating the next set of initial conditions o We can model dynamic components, we can’t model subgrid thus we parametrize. Those in a spatial time scale ofless than 1 km and 1 day
  • What can we do with the models: simulations, projections and predictions
  • Non-linear dynamics: non linear terms (gradients) can lead to big changes at a time t although at t0 very minimal changes of initial state
  • Deterministic chaos; perturbations can grow and small differences of initial conditions lead to large differences in the state of the system
  • Deterministic chaos was studied by Lorenz in a non-rotating reference frame o Mass conservation equation o Horizontal and vertical momentum, have very strong non linearities o Showed that deterministic predictability is an illusion
  • Predictability o First kind: initial conditions ▪ Atmospheres memory to initial conditions limited to 10 days ▪ Oceans memory to initial conditions from months to years o Second kind: comes from boundary conditions ▪ With boundaries we push a system to a preferred state
  • RCP: representative concentration pathway o 8.5 Buissiness as usual o 4.5 emissions decrease, concentrations remain

Old exam

  1. Which of the following oceans does not have any poleward energy transported directed to south? a. Atlantic b. Pacific c. Indian d. All
  2. Is there any surplus of absorbed energy at some latitudes on the planet? a. No, the absorption is uniform b. Yes, the tropics absorb more than the poles c. Yes, the poles absorb more than the tropics
  3. With the labelling “RCP 4.5”, climatologists refer to scenarios of: a. Very low emissions b. Moderatly low emeissions c. Moderatly high emissions d. Very high emissions
  4. How many of the primitive equations refer to the conservation of momentum? a. One b. Two c. Three
  5. The vertical profile of temperature of the atmosphere is a. Monotically increasing with altitude b. Monotonicaly decreasing with altitude c. Unimodal (increasing from soil to stratosphere then decreasing) d. Multimodal (with more than one local maximum)
  6. According to scenarios shown in the class, what would be the atmosphere temperature anomaly reached by the end of the century with the lowest emission scenario? a. 0.5° C b. 1°C c. 1.5°C d. 2°C
  7. How is the temporal regime of El Niño? a. Quite regular, with a period of 2 years b. Quite regular, with a period of 7 years c. Not so regular, but with a recognizable period of around 7 years d. Not so regular, but with a recognizable period od around 2 years
  8. The impressive log-log graph with spatiotemporal scales televent to climate science spans: a. 4 powers of ten b. 8 powers of ten c. 12 powers of ten d. 16 powers of ten