1. How do models work?
METR 2021: Spring 2009
Lab 10

2. How do we forecast the weather?
Use Numerical Weather Prediction (NWP) models
Fancy name for we let the computers crunch the numbers and we interpret the results
Models are run at least 4 times per day and forecast for anywhere between 12 to 384 hours into the future
But how does the model work?

3. History of NWP Lewis Fry Richardson first to attempt NWP
His attempt failed miserably in 1922
What were some limitations he faced?
Jule Charney 1950 first successful try
Equivalent barotropic model
Only 500-hPa heights forecasted
Today
Run models on desktop computers, forecast to 384 hours!

4. NWP basics
Each Model has a certain Domain or area over which it predicts the weather
Two types of models
Gridded models
Spectral models
Each model has some representation of the topography of Earth, some are better than others
Each grid point model has a certain resolution or spacing to grid points within the model
Spectral models increase resolution by keeping more parts of the wave series

5. Model Resolution
Vertical Resolution
Horizontal Resolution

6. NWP schematic

7. Forecasting: The Musical
Act One:
Sfc Observations (input: worldwide)
Radiosondes, Wind Profilers
Satellite, radar, aircraft, ship obs
Act Two:
Data Analysis and Assimilation
Error Checking (QC)
Gridding of Data (interpolation)
Smoothing (initialization)
Input into models (assimilation)

8. Forecasting: The Musical cont.
Act Three:
Numerical Modeling
Computer works to find solutions to the equations of the atmosphere
Act Four:
Forecast Dissemination (output: worldwide)
This is what meteorologists look at!!

9. Example of Model Output

10. Limitations to NWP Models
The Equations are inexact!!!
Must parameterize things
Errors in Initial State
The model is limited by quality of data going in
Inadequate Resolution
Not enough computer power to do it all
Most operational models cannot resolve T-Storms

11. Parameterizations Are we able to resolve boundary layer processes?
Can we know of all radiation scattering?
Is a typical convective cell 20 km wide?
Need to parameterize (make the model produce) phenomenon that are sub-grid scale

12. Parameterizations cont.
Many different parameterizations have been developed over the years
Each parameterization behaves differently
Convective Schemes
Kain–Fritsch
Betts–Miller
Boundary Layer
Which closure to use? (zero, 1, 1.5, or 2)

13. Inadequate Resolution
We keep getting smaller and smaller grid spacings…which helps, somewhat
What can be resolved for a given grid spacing?
20 km (2x = 40 km)
4 km (2x = 8 km)
Are there a lot of different phenomenon occurring that are smaller than 8 km?

14. Errors in Initial State
Error in data collection
Instrumentation errors
Gridding up observations
Smoothing of data
Too few observations for analysis
Features not resolved in observations data

15. Ensemble Forecasting
To account for errors in initial state, run model a number of times with slightly varying initial conditions (Lorenz – Chaos)
Used to asses confidence in models forecast ability
Also called spaghetti plots
Many different model runs for the same time
If it looks like spaghetti, low confidence
Less Variability = high confidence in forecast

16. Ensemble Plot

17. Research Models NWP are sometimes developed for research purposes
Usually higher resolution
Take a couple of days to a couple of weeks to run one model simulation that lasts 24 hours
Models to predict…
T-Storms
Tornadoes
Hurricanes