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NATS 101 Lecture 25 Weather Forecasting I

NATS 101 Lecture 25 Weather Forecasting I. Review: ET Cyclones Ingredients for Intensification. Strong Temperature Contrast Jet Stream Overhead S/W Trough to West UL Divergence over Surface Low If UL Divergence exceeds LL Inflow, Cyclone Deepens Similar Life Cycles. deepening. filling.

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NATS 101 Lecture 25 Weather Forecasting I

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  1. NATS 101Lecture 25Weather ForecastingI

  2. Review: ET CyclonesIngredients for Intensification • Strong Temperature Contrast • Jet Stream Overhead • S/W Trough to West • UL Divergence over Surface Low • If UL Divergence exceeds LL Inflow, Cyclone Deepens • Similar Life Cycles deepening filling Ahrens, Meteorology Today, 5th Ed.

  3. Reasons to Forecast Weather & Climate • Should I bring my umbrella to work today? • Should Miami be evacuated for a hurricane? • How much heating oil should a refinery process for the upcoming winter? • Will the average temperature change if CO2 levels double during the next 100 years? • How much to charge for flood insurance? • How much water will be available for agriculture & population in 30 years These questions require weather-climate forecasts for today, a few days, months, years, decades

  4. Forecasting Questions • How are weather forecasts made? • How accurate are current weather forecasts? • How accurate can weather forecasts be? We will emphasize mid-latitude forecasts out to 15 days where most progress has been made.

  5. Types of Forecasts Persistence -forecast the future atmospheric state to be the same as current state -Raining today, so forecast rain tomorrow -Useful for few hours to couple days

  6. Types of Forecasts Trend -add past change to current condition to obtain forecast for the future state -Useful for few hours to couple days

  7. Types of Forecasts Analog -find past state that is most similar to current state, then forecast same evolution -Difficulty is that no two states exactly alike -Useful for forecasts up to one or two days

  8. Types of Forecasts Climatology -forecast future state to be same as climatology or average of past weather for date -Forecast July 4th MAX for Tucson to be 100 F -Most accurate for long forecast projections, forecasts longer that 30 days

  9. Types of Forecasts Numerical Weather Prediction (NWP) -use mathematical models of physics principles to forecast future state from current conditions. Process involves three major phases • Analysis Phase (estimate present conditions) • Prediction Phase (computer modeling) • Post-Processing Phase (use of products) To justify NWP cost, it mustbeatforecasts of persistence, trend, analog and climatology

  10. Analysis Phase • Purpose: Estimate the current weather conditions to use to initialize the weather forecast • Implementation: Because observations are always incomplete, the Analysis is accomplished by combining observations and the most recent forecast

  11. Analysis Phase • Current weather conditions are observed around the global (surface data, radar, weather balloons, satellites, aircraft). • Millions of observations are transmitted via the Global Telecommunication System (GTS) to the various weather centers. • U.S. center is in D.C. and is namedNational Centers for Environmental Prediction (NCEP)

  12. Analysis Phase • The operational weather centers sort, archive, and quality control the observations. • Computers then analyze the data and draw maps to help us interpret weather patterns. Procedure is calledObjective Analysis. Final chart is referred to as anAnalysis. • Computer models at weather centers make global or national weather forecast maps

  13. Surface Data Sparse data over oceans and Southern Hemisphere Courtesy ECMWF

  14. Surface Buoy Reports Some buoy data over Southern Hemisphere Courtesy ECMWF

  15. Radiosonde Coverage Little data over oceans and Southern Hemisphere Courtesy ECMWF

  16. Aircraft Reports Little data over oceans and Southern Hemisphere Courtesy ECMWF

  17. Geostationary Polar Orbit Weather Satellites Satellite observations fill data void regions Geostationary Satellites High temporal sampling Low spatial resolution Polar Orbiting Satellites Low temporal sampling High spatial resolution Ahrens, Figs. 9.5 & 9.6

  18. Obs from Geostationary Satellites Courtesy ECMWF

  19. Temperature from Polar Satellites Courtesy ECMWF

  20. Atmospheric Models • Weather models are based on mathematical equations that retain the most important aspects of atmospheric behavior - Newton's 2nd Law (density, press, wind) - Conservation of mass (density, wind) - Conservation of energy (temp, wind) - Equation of state (density, press, temp) • Governing equations relate time changes of fields to spatial distributions of the fields e.g. warm to south + southerly winds  warming

  21. Prediction Phase • Analysis of the current atmospheric state (wind, temp, press, moisture) are used to start the model equations running forward in time • Equations are solved for a short time period (~5 minutes) over a large number (107 to 108) of discrete locations called grid points • Grid spacing is 2 km to 50 km horizontally and 100 m to 500 m vertically

  22. Model Grid Boxes 100-500 m 10-20 km

  23. 50 km “A Lot Happens Inside a Grid Box”(Tom Hamill, CDC/NOAA) Approximate Size of One Grid Box for NCEP Global Ensemble Model Note Variability in Elevation, Ground Cover, Land Use Rocky Mountains Denver Source: www.aaccessmaps.co

  24. 13 km Model Terrain Big mountain ranges, like the Sierra Nevada Range, are resolved. But isolated peaks, like the Catalinas, are not evident! 100 m contour

  25. Post-Processing Phase • Computer draws maps of projected state to help humans interpret weather forecast • Observations, analyses and forecasts are disseminated to private and public agencies, such as the local NWS Forecast Office and UA • Forecasters use the computer maps, along with knowledge of local weather phenomena and model performance to issue regional forecasts • News media broadcast these forecasts to public

  26. Suite of Official NWS Forecasts CPC Predictions Page

  27. Summary: Key Concepts Forecasts are needed by many users There are several types of forecasts Numerical Weather Prediction (NWP) Use computer models to forecast weather -Analysis Phase -Prediction Phase -Post-Processing Phase Humans modify computer forecasts

  28. Summary: Key Concepts National Center for Environment Prediction (NCEP) issues operational forecasts for El Nino tropical SST anomalies Seasonal outlooks 10 to 15 day weather forecasts 2 to 3 day fine scale forecasts

  29. Assignment for Next Lecture • Topic -Weather Forecasting Part II • Reading -Ahrens pg 249-254 • Problems -9.11, 9.15, 9.18

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