How do we predict weather and How do we predict weather and climate?climate?

Review of last lectureReview of last lecture Well-defined pattern of heating, temperature, pressure Well-defined pattern of heating, temperature, pressure

and winds around the globe.and winds around the globe. Three-cell model. Mechanism for each cellThree-cell model. Mechanism for each cell Two characteristics of zonal mean temperature structureTwo characteristics of zonal mean temperature structure Two characteristics of zonal mean wind structure. Why Two characteristics of zonal mean wind structure. Why

does westerly winds prevail in the extratropical does westerly winds prevail in the extratropical troposphere? What cause the jet streams?troposphere? What cause the jet streams?

Semipermanent pressure cells. Low pressure is Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, is associated with warm surface temperature, drought, and desert.and desert.

What drives the ocean surface currents? In the case of What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current Ekman spiral, what is the direction of surface current relative to surface wind? relative to surface wind?

Review of last lectureReview of last lecture

Tropical climate:Tropical climate: Mean state: The two basic regions of SST? Which region has Mean state: The two basic regions of SST? Which region has

stronger rainfall? What is the Walker circulation?stronger rainfall? What is the Walker circulation? El Nino and La Nina: Which region has warm SST anomaly El Nino and La Nina: Which region has warm SST anomaly

during El Nino? 4-year period.during El Nino? 4-year period. Land-sea contrasts: seasonal monsoon, diurnal sea and land Land-sea contrasts: seasonal monsoon, diurnal sea and land

breeze breeze

Extratropical climate:Extratropical climate: Mean state: westerly winds, polar vortexMean state: westerly winds, polar vortex What is the primary way El Nino affect extratropics? (PNA)What is the primary way El Nino affect extratropics? (PNA) The oscillations associated with strengthening/weakening of polar The oscillations associated with strengthening/weakening of polar

vortex: AO, AAOvortex: AO, AAO

Birth of global climate modelsBirth of global climate models

1922 - Lewis Richardson1922 - Lewis Richardson’’s s ““forecast factoryforecast factory””: filled a vast : filled a vast stadium with 64,000 people, stadium with 64,000 people, each armed with a mechanical each armed with a mechanical calculator. Failed!calculator. Failed!

1940s - von Neumann 1940s - von Neumann assembled a group of assembled a group of theoretical meteorologists at theoretical meteorologists at Princeton to run the first Princeton to run the first computerized weather forecast computerized weather forecast on the ENIAC. The results were on the ENIAC. The results were encouraging.encouraging.

1954, 1955 - Routine forecast: 1954, 1955 - Routine forecast: The Swedish Institute of The Swedish Institute of Meteorology, the US JNWP. Meteorology, the US JNWP. Barotropic model.Barotropic model.

WorldWorld’’s Major Climate Modeling Centerss Major Climate Modeling Centers

The Global Climate System - Atmosphere, The Global Climate System - Atmosphere, ocean, biosphere, cryosphere, and geosphereocean, biosphere, cryosphere, and geosphere

Coupler .

Land Sea Ice

Atmosphere

Ocean

Framework of Climate System ModelFramework of Climate System Model

Atmosphere general circulation modelAtmosphere general circulation model

• General circulation models are systems of General circulation models are systems of differential equations based on the basic laws of physics, fluid motion, and chemistry.

• Scientists divide the planet into a 3-dimensional grid (Scientists divide the planet into a 3-dimensional grid (100-500 Km wide100-500 Km wide), apply the ), apply the basic equations within each grid and evaluate interactions with neighboring points. basic equations within each grid and evaluate interactions with neighboring points.

Atmosphere General Circulation Model: Atmosphere General Circulation Model:

Basic equations Basic equations This set of equations is This set of equations is

called the called the Navier-Stokes Navier-Stokes equationsequations for fluid flow, for fluid flow, which are at the heart of which are at the heart of the GCMs.the GCMs.

There are other There are other equations dealing with equations dealing with the conservation of Hthe conservation of H22O, O,

COCO22 and other chemical and other chemical

species.species.

(Conservation of monmentum)

(Conservation of mass)

(Conservation of energy)

Regional model (Mesoscale model)Regional model (Mesoscale model) Mesoscale: 1 Km- 1000 Km, 1 min - 1 dayMesoscale: 1 Km- 1000 Km, 1 min - 1 day Grid size: 1 Km - 10 kmGrid size: 1 Km - 10 km Three characteristics: Three characteristics:

1.1. Non-hydrostatic processesNon-hydrostatic processes

2.2. Nested gridNested grid

3.3. Topography effectsTopography effects

Video: Constructing a climate modelVideo: Constructing a climate model

http://www.nas-sites.org/climatemodeling/page_3_1.php

The current status of weather and The current status of weather and climate predictionsclimate predictions

Weather prediction: Generally good within one Weather prediction: Generally good within one week, not good beyond 10 days. The skill week, not good beyond 10 days. The skill depends on season and location.depends on season and location.

Tropical cyclone (hurricane, typhoon) prediction: Tropical cyclone (hurricane, typhoon) prediction: good in track, but bad in intensitygood in track, but bad in intensity

Climate prediction: Seasonal prediction Climate prediction: Seasonal prediction generally good within 6 months, but not good generally good within 6 months, but not good beyond. Skill depends on season.beyond. Skill depends on season.

Climate projection: All models project global Climate projection: All models project global warming in the 21st century, but with a 3-fold warming in the 21st century, but with a 3-fold difference in magnitude.difference in magnitude.

Why is it so difficult to understand Why is it so difficult to understand and predict weather and climate?and predict weather and climate?

Problem IProblem I: Different parts of the world are : Different parts of the world are strongly connected to each other strongly connected to each other (The “(The “Teleconnection ProblemTeleconnection Problem”)”)

Global atmospheric flow

Example 1: Global impacts of El NinoExample 1: Global impacts of El Nino

Example 2: Factors affecting U.S. Example 2: Factors affecting U.S. weather and climateweather and climate

El NinoMadden-Julian Oscillation

Amazon

ArcticN. Atlantic

Atlantic/ Sahel

One location is affecting every locations, One location is affecting every locations, and in turn is affected by every locationsand in turn is affected by every locations

Problem II: Different components of the earth Problem II: Different components of the earth system (atmosphere, land, ocean, ice, clouds, etc) system (atmosphere, land, ocean, ice, clouds, etc)

are strongly interacting with each other are strongly interacting with each other (The “(The “Feedback ProblemFeedback Problem”)”)

Example: Generation of hurricanesExample: Generation of hurricanes

Heat from the ocean

Problem IIIProblem III: The earth system composes of both very big : The earth system composes of both very big objects (such as the whole Pacific Ocean) and very small objects (such as the whole Pacific Ocean) and very small

objects (such as the cloud droplets), making it very difficult to objects (such as the cloud droplets), making it very difficult to draw them on the same page draw them on the same page

(The “(The “Subgrid-Scale ProblemSubgrid-Scale Problem”)”)

SummarySummary 4 components of the climate system model. 3 usages of 4 components of the climate system model. 3 usages of

global climate models. global climate models. Atmosphere general circulation models: Grid size. Name Atmosphere general circulation models: Grid size. Name

of the basic set of equations for atmosphere model. of the basic set of equations for atmosphere model. Regional models: grid size. 3 characteristics.Regional models: grid size. 3 characteristics. The current status of weather and climate predictionsThe current status of weather and climate predictions: (1) : (1)

weather prediction good to 10 days, (2) tropical cyclone weather prediction good to 10 days, (2) tropical cyclone prediction good in track but not in intensity, (3) climate prediction good in track but not in intensity, (3) climate prediction good to two seasons, (4) climate change prediction good to two seasons, (4) climate change projections have a 3-fold difference in magnitude.projections have a 3-fold difference in magnitude.

The main reasons of the difficulties in weather and climate The main reasons of the difficulties in weather and climate predictionspredictions: (1) Teleconnection problem, (2) Feedback : (1) Teleconnection problem, (2) Feedback problem, and (3) Subgrid-scale problem.problem, and (3) Subgrid-scale problem.