ATMO 336 Weather, Climate Society Cyclones, Cyclogenesis Weather Forecasting

1. ATMO 336Weather, Climate SocietyCyclones, CyclogenesisWeather Forecasting

2. Recall: Uniform Circular Motion Requires Acceleration/Force Circle Center Final Velocity Acceleration directed toward center of circle Circular Path Final Velocity Initial Velocity Initial Velocity Centripetal (center seeking) acceleration is required for curved flow, i.e. to change the direction of the velocity vector!

3. 5640 m 5700 m Flow Around Curved Contours Assume PGF constant size along entire channel L H Zero Centripetal Acceleration is Required for Air Parcel to Curve

4. 5640 m 5700 m Forces for Curved Flow Assume PGF constant size along entire channel PGF Wind PGF Geo Wind PGF CF CF Wind Centripetal= PGF +CF Centripetal<< PGF orCF Gradient Wind Balance CF

5. Wind Speed Increases 5640 m 5700 m Wind Speed Decreases Gradient Wind Balance: End Result Assume PGF constant size along entire channel Faster than Geo Wind Geo Wind Wind speeds are Slowest at trough Fastest at ridge Slower than Geo Wind Therefore, wind speeds Increase downwind of trough Decrease downwind of ridge

6. Wind Speed Increases Area Increases Wind Speed Decreases Wind Speed Decreases Area Decreases Gradient Wind Balance Assume PGF constant size along entire channel 5640 m 2 5700 m 1 Speeds and Areas: Increase downwind of trough Decrease downwind of ridge

7. Area Increases Divergence Area Decreases Convergence Divergence and Convergence Assume PGF constant size along entire channel Parcel Shapes: Stretch Downwind of Trough so Area Increases Compress Downwind of Ridge so Area Decreases Divergence: Horizontal Area Increases with Time Convergence: Horizontal Area Decreases with Time

8. Divergence Net Mass Loss Convergence Net Mass Gain Divergence and Convergence Assume PGF constant size along entire channel Large Mass transport across channel Small

9. Vertical Motion Ridge Ridge Trough Gedzelman, p249 Mass Conservation leads toUpward motion beneath regions of divergence Downward motion beneath regions of convergence

10. Super-geostrophic Sub-geostrophic

11. Divergence Divergence Convergence Convergence

12. Where Winds are Divergent? Regions downwind of 500 mb troughs are favorable for surface cyclones and upward motion. faster winds Ridge Divergence Trough Cyclogenesis can only occur where mass is being removed from the column overhead. Mass loss produces surface pressure falls. slower winds

13. What Increases Divergence? faster winds Ridge Divergence Trough 1) Stronger PGF because faster winds require larger centripetal accelerations.Divergence stronger along axis of jet stream. slower winds

14. faster winds Ridge Divergence Trough slower winds What Increases Divergence? 2) Bigger amplitude waves because the sharper curvature requires larger centripetal accelerations.Divergence stronger downwind of larger amplitude troughs.

15. What Increases Divergence? faster winds Ridge Divergence Trough 3) Shorter wavelength because the sharper curvature requires larger centripetal accelerations.Divergence stronger downwind of shortwave troughs. slower winds

16. tilt Vertical Structure tilt Fundamental Fact: Cyclone deepens only if divergence in column exceeds convergence! This condition can occur if the system tilts toward the west with height Westward tilt aligns upper-level (UL) divergence over the surface low and … Results in low deepening Ahrens, Meteorology Today, 5th Ed.

17. Friction Induced Vertical Motion downward motion upward motion Ahrens, Fig 6.21

18. Divergence Convergence Convergence Divergence Surface Convergence and Divergence

19. Summary: Curved Flow & Friction • Curved Flow Requires Centripetal Acceleration Difference between PGF and Coriolis Force Speed Changes => Convergence-Divergence • Frictional Force Causes Winds to Turn toward Low Pressure Important in the lowest 1 km above the Surface Leads to Convergence-Divergence • Curvature and Friction Produce Cyclones and Vertical Motions

20. H H Simplistic Model for Homework 5640 m 5700 m H Cold L Dry Wet Dry Warm L Surface Anticyclone Surface Anticyclone Surface Cyclone

21. ATMO 336Weather Forecasting

22. Reasons to Forecast Weather • 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? These questions require weather-climate forecasts for today, a few days, months, years, decades

23. Forecasting Questions • How are weather forecasts made today? • How accurate are current weather forecasts? • How accurate can weather forecasts be?

24. 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 (most expensive piece) • Prediction Phase (modeling, computing) • Post-Processing Phase (use of products) To justify NWP cost, it mustbeatno-brainer forecasts of persistence and climatology

25. 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)

26. 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

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

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

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

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

31. 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

32. T from (Mostly) GEO Satellites sweet spot Courtesy ECMWF

33. T from Polar Satellites Courtesy ECMWF

34. 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 warmer to south + southerly winds  warming

35. Atmospheric Models Must contain representations of many of complex physical processes to produce a good forecast

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

37. Model Grid Boxes Forecast average conditions within grid boxes shaped like brownies

38. 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

39. 13 km Model Terrain Big mountain ranges, like the Sierra Nevada, are resolved. But isolated peaks, like the Catalina’s, are not evident. 100 m contour

40. Take Home Points 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