21. Computer Models

Question 1

To study the ocean, we use theory, observations, and:

Answer 1

numerical models

Question 2

_______ can observe from ocean processes almost everywhere every few days. But they can only observe some processes, and only near the sfc

_____ can measure more variables, and deeper into the water, but measurements are sparse

___ ____ provide the only useful, global view of ocean currents

Answer 2

satellites

ships

numerical models

Question 3

What is a model?

(according to Entry 4 and Entry 12 of the Webster Dictionary!)

Answer 3

Entry 4= a usually miniature representation of something

Entry 12= a system of postulates, data, and inferences presented as a mathematical description of an entity or state of affairs

Question 4

any kind of representation of the real world=

Answer 4

a model

Question 5

Numerical model=

Answer 5

a set of equations and parameters that describe the time and spatial evolution of physical processes that occur within the earth system (oceans here)

Question 6

Most models are what type of equation?

Answer 6

differential equations

Question 7

Give the 3 main elements of a model

Answer 7

1. governing equations
2. initial conditions (temp, salinity, & velocity fields)
3. Boundary conditions (atmosphere above, basin geometry, bottom topography)

Question 8

Describe discrete vs continuous models

Answer 8

Continuous: dt and dx are very very small (graph is a solid, smooth line)

Discrete: chunky (bar graph)

Analogy: continuous= water running from a tap as a steady stream, while discrete= random drips, no steady stream

Question 9

What are 3 air-sea interactions that a 3D ocean model would consider?

Answer 9

water vapor
heat
momentum (winds)

Question 10

Ocean processes need to be parameterized for 2 main reasons:

Answer 10

• we don’t want to spend the computational resources to directly treat/ resolve them as they’re too small and complex
• we don’t completely understand the process

Question 11

In low resolution models, what is the main issue?

Answer 11

meso-scale eddies

Question 12

In high resolution models, what are 2 main issues?

Answer 12

• internal wave mixing
• flow-topography interactions

Question 13

List 3-5 ocean processes that are commonly parameterized

Answer 13

• mesoscale eddy effects
• coastal processes
• ice-ocean mixing
• friction
• surface mixed layer processes

Question 14

Calibration=

Answer 14

adjustment of independent variable (parameters) in order to match observations

Question 15

Describe what a parameterization is

Answer 15

= a model within a model

Things the models do not resolve, we have to provide them those things

eg. coefficient of diffusion is a constant given to the model

Question 16

Give 4 ‘requirements’ that a valid model should meet

Answer 16

1. contain no coding errors
2. have sound metrics
3. give the same results on different computers
4. agree with analytic solutions (where available)
   - This is tricky for oceans: analytical solutions do not exist. & comparing with observed data is not ideal

Question 17

Give up to 4 problems with comparing models with observed data for ‘validation’

Answer 17

• limited data coverage
• time variability
• spatial scale of model vs data (hard to collect observed data)
• many unresolved processes

Question 18

Give up to 6 uses of models

Answer 18

• hypothesis testing
• sensitivity analysis
• what-if scenarios
• estimation of hard-to-measure quantities
• extrapolation
• forecast

Question 19

Ocean general circulation models are based on a series of coupled partial differential equations. The full set involves __ (how many).

Answer 19

7

Question 20

Parameterizations and results of ocean general circulation models depend on what?

Answer 20

grid resolution

Question 21

What are the main 8 processes considered in climate models?

Answer 21

1. atmosphere
2. land sfc
3. ocean & sea ice
4. aerosols
5. carbon cycle
6. dynamic vegetation
7. atmospheric chemistry
8. land ice

Question 22

T/F

The complexity of climate models has increased a lot over the last few decades

Answer 22

true

Question 23

Use FAR, SAR, TAR, and AR4 as examples of improvements in model resolution

Answer 23

FAR (1990)= 500km resolution, not good

SAR (1996)= 250km resolution, better

TAR (2001)= 180km resolution: much better

AR4 (2007)= 110km resolution (can clearly see topography)