How do we predict weather and
climate?
Review of last lecture
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Well-defined pattern of heating, temperature, pressure
and winds around the globe.
Three-cell model. Mechanism for each cell
Two characteristics of zonal mean temperature structure
Two characteristics of zonal mean wind structure. Why
does westerly winds prevail in the extratropical
troposphere? What cause the jet streams?
Semipermanent pressure cells. Low pressure is
associated with clouds and precipitation. High pressure
is associated with warm surface temperature, drought,
and desert.
What drives the ocean surface currents? In the case of
Ekman spiral, what is the direction of surface current
relative to surface wind?
Review of last lecture
Tropical climate:
 Mean state: The two basic regions of SST? Which region has
stronger rainfall? What is the Walker circulation?
 El Nino and La Nina: Which region has warm SST anomaly
during El Nino? 4-year period.
 Land-sea contrasts: seasonal monsoon, diurnal sea and land
breeze
Extratropical climate:
 Mean state: westerly winds, polar vortex
 What is the primary way El Nino affect extratropics? (PNA)
 The oscillations associated with strengthening/weakening of polar
vortex: AO, AAO
Birth of global climate models
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1922 - Lewis Richardson’s
“forecast factory”: filled a vast
stadium with 64,000 people,
each armed with a mechanical
calculator. Failed!
1940s - von Neumann
assembled a group of
theoretical meteorologists at
Princeton to run the first
computerized weather forecast
on the ENIAC. The results were
encouraging.
1954, 1955 - Routine forecast:
The Swedish Institute of
Meteorology, the US JNWP.
Barotropic model.
World’s Major Climate Modeling Centers
The Global Climate System - Atmosphere,
ocean, biosphere, cryosphere, and geosphere
Framework of Climate System Model
Atmosphere
Land
Coupler
.
Ocean
Sea Ice
Atmosphere general circulation model
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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 (100-500 Km wide),
apply the basic equations within each grid and evaluate interactions with
neighboring points.
Atmosphere General Circulation Model:
Basic equations
(Conservation of monmentum)
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(Conservation of mass)
(Conservation of energy)
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This set of equations is
called the Navier-Stokes
equations for fluid flow,
which are at the heart of
the GCMs.
There are other
equations dealing with
the conservation of H2O,
CO2 and other chemical
species.
Regional model (Mesoscale model)
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Mesoscale: 1 Km- 1000 Km, 1 min - 1 day
Grid size: 1 Km - 10 km
Three characteristics:
Non-hydrostatic processes
Nested grid
Topography effects
Video: Constructing a climate model
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http://www.nassites.org/climatemodeling/page_3_1.php
The current status of weather and
climate predictions
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Weather prediction: Generally good within one
week, not good beyond 10 days. The skill
depends on season and location.
Tropical cyclone (hurricane, typhoon) prediction:
good in track, but bad in intensity
Climate prediction: Seasonal prediction
generally good within 6 months, but not good
beyond. Skill depends on season.
Climate projection: All models project global
warming in the 21st century, but with a 3-fold
difference in magnitude.
Why is it so difficult to understand
and predict weather and climate?
Problem I: Different parts of the world are
strongly connected to each other
(The “Teleconnection Problem”)
Global atmospheric flow
Example 1: Global impacts of El Nino
Example 2: Factors affecting U.S.
weather and climate
Arctic
N. Atlantic
Atlantic/
Sahel
MaddenJulian
Oscillation
El Nino
Amazon
One location is affecting every locations,
and in turn is affected by every locations
Problem II: Different components of the earth
system (atmosphere, land, ocean, ice, clouds, etc)
are strongly interacting with each other
(The “Feedback Problem”)
Example: Generation of hurricanes
Heat from the ocean
Problem III: The earth system composes of both very big
objects (such as the whole Pacific Ocean) and very small
objects (such as the cloud droplets), making it very difficult to
draw them on the same page
(The “Subgrid-Scale Problem”)
Summary
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4 components of the climate system model. 3 usages of
global climate models.
Atmosphere general circulation models: Grid size. Name
of the basic set of equations for atmosphere model.
Regional models: grid size. 3 characteristics.
The current status of weather and climate predictions: (1)
weather prediction good to 10 days, (2) tropical cyclone
prediction good in track but not in intensity, (3) climate
prediction good to two seasons, (4) climate change
projections have a 3-fold difference in magnitude.
The main reasons of the difficulties in weather and climate
predictions: (1) Teleconnection problem, (2) Feedback
problem, and (3) Subgrid-scale problem.