How do models work?

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