Climate models are tools used in climate research that range in complexity from simple zero-dimensional energy balance models to complex three-dimensional general circulation models. They work by solving equations that conserve mass, momentum, energy and other quantities in grid boxes. Climate models are evaluated by comparing their results to observations. They are used for applications such as detecting and attributing causes of climate change, making projections of future climate change, and studying past climates.

1. Climate Models
The Changing Climate
ATS 320
Lecture 14
November 2, 2015

2. Recap
Nathan Van Cleave
Miguel Velasco

3. This Lecture
3
Ryan Watts
Leo Williamson

4. Climate Models
Equations based on
conservation of
• mass
• momentum
• energy
• water
• carbon
• ...

5. Components
Included
• Atmosphere
• Ocean
• Sea Ice
• Land Surface (sometimes with
interactive vegetation)
• Ocean Biology and Chemistry (e.g.
Carbon, Nutrient, and Oxygen Cycles)
• (Ice Sheets, usually prescribed)
Models that include biogeochemistry are also often
called Earth System Models

6. Hierarchy of Climate
Models
• 0D Energy Balance Model (EBM)
• 1D EBM
• 1D Radiative-Convective Models
• Intermediate Complexity Models
(e.g. 2D EBMs)
• 3D General Circulation Models
(GCMs)
IncreasingComplexity

7. Energy Gain:
Absorbed solar
(shortwave) radiation
FSW=(1-a)S
Energy Loss:
Emitted terrestrial
(longwave) radiation
FLW
0D Energy Balance Model:
C∂T/t = FSW - FLW
T
Heat Capacity
Temperature
change with time

8. C∂T(φ)/t = FSW - FLW + Fin -
Fout
T(φ)
φ
Transport
Fout
Fin
1D Energy Balance Model
solves energy balance separately in
different latitude (φ) bands, includes
(meridional) heat transport between the
bands.
Only one single vertical layer is considered.
Earth receives less solar energy
per square meter at poles than
at the equator

9. C∂T(φ,λ)/t = FSW - FLW + Fin -
Fout
φ
Transport
2D Energy Balance Model
λ
solves energy balance separately in
each grid box on a latitude-longitude (φ,λ) grid
with transport between the boxes
T(φ,λ)
Only one single vertical layer is considered.

10. 1D Radiative Convective
Models
height z
Surface
shortwave
radiation
longwave
radiation
• Temperature
• Temperature
• Temperature
• Temperature
C∂T(z)/t = FSW,in - FSW,out + FLW,in - FLW,out +
Convection
• Temperature
Radiative fluxes are calculated similarly as in MOTRAN
Convection occurs if the stratification becomes unstable. In this case lapse rate is set to the moist adiabatic (6.5 K/km).

11. Three-Dimensional
General Circulation Models (GCMs)
Equations based on
conservation of
• mass
• momentum
• energy
• water
• carbon
• ...
Typically global climate models have
about 20-30 layers in the atmosphere
and about the same amount in the
ocean. The horizontal grid box size
varies from about 5 degrees (500 km)
to about 1 degree (100 km) or less.
Models with smaller grid box sizes
resolve more details (higher
resolution), but they are also more
computationally expensive to run.

12. Parameterizations
• Processes that cannot be resolved
(smaller than grid box size need to be
expressed in terms of resolved
quantities; often empirical formulas)
• Examples are clouds, convection
(atmosphere), and mixing (ocean)

13. Forcings
• Models are driven by boundary
conditions, e.g. incident shortwave
radiation at the top-of-the-atmosphere,
surface properties (albedo)
• Interior is solved without the use of
observations

14. http://www.image.ucar.edu/~nychka/Animations/BTSCAMT340.mp4
http://www.image.ucar.edu/~nychka/talks.html
http://vets.ucar.edu/vg/T341/
High-Resolution Climate Model
1024x512 grid points (30 km)
Water vapor

15. Model Evaluation
Models are tested by comparing results to observations.

16. IPCC 2013, Fig. 9.2
Surface (2 m) Air Temperature
30-40 different climate models from 10-20 different research institutions have participated in the 2013 IPCC. The multi model mean is the average of all
models. The right panel shows the differences between the models and observations.

17. Seasonality
December-January-February minus June-July-August
Models overestimate seasonality over land and underestimate it over the ocean.
IPCC 2013, Fig. 9.3

18. Precipitation
Models do less well in simulating precipitation.
Why?
Because precipitation is very intermittent and
depends strongly on the simulation of clouds
and convection, which are parameterized.
IPCC 2013, Fig. 9.4

19. Models simulate the observed temperature increase relatively well. Note the cooling associated with large volcanic
eruptions. Also note that the multi-model mean (thick red line) does not capture the observed (black thick lines) recent
global warming “hiatus”.
IPCC 2013, Fig. 9.8

20. Ocean Temperatures & Salinities
Black: Mean Values from Observations; Color: Multi Model Mean Bias (difference from observations)
IPCC 2013, Fig. 9.13

21. Black=Observations, Red=Multi Model Mean, Others: Individual Models

22. What are climate models used
for?
• Paleoclimate Studies
• Detection and Attribution
Is climate changing significantly and if so why?
• Projections
How may climate change in the future?

23. Detection and Attribution of Climate
Change
IPCC 2013 FAQ 10.1

24. Summary• Climate models are tools in climate research
• They range from simple (0D-EBMs) to complex
(3D-GCMs)
• They work by solving conservation equations
in boxes
• They are evaluated by comparison to
observations
• Applications are: detection and attribution,
projections, paleoclimate studies