ATMO 336 Weather, Climate Society Cyclones, Cyclogenesis Weather Forecasting

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

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

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

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

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

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

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

Gedzelman, p249 Vertical Motion Mass Conservation leads to Upward motion beneath regions of divergence Downward motion beneath regions of convergence Trough Ridge

Sub-geostrophic Super-geostrophic

Convergence Divergenc e Divergence Convergence

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

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

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

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

Vertical Structure 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 tilt Ahrens, Meteorology Today, 5th Ed.

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

Divergence Convergence Divergence Convergence Surface Convergence and Divergence

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

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

ATMO 336 Weather Forecasting

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 CO 2 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

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

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 1.Analysis Phase (most expensive piece) 2.Prediction Phase (modeling, computing) 3.Post-Processing Phase (use of products) To justify NWP cost, it must beat no-brainer forecasts of persistence and climatology

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 named National Centers for Environmental Prediction (NCEP)

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 called Objective Analysis. Final chart is referred to as an Analysis. Computer models at weather centers make global or national weather forecast maps

Courtesy ECMWF Sparse data over oceans and Southern Hemisphere Surface Data

Courtesy ECMWF Some buoy data over Southern Hemisphere Surface Buoy Reports

Courtesy ECMWF Little data over oceans and Southern Hemisphere Radiosonde Coverage

Aircraft Reports Courtesy ECMWF Little data over oceans and Southern Hemisphere

Weather Satellites Geostationary Polar Orbit 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

Courtesy ECMWF T from (Mostly) GEO Satellites sweet spot

T from Polar Satellites Courtesy ECMWF

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

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

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 (10 8 ) of discrete locations called grid points Grid spacing is 5 km to 50 km horizontally and 100 m to 500 m vertically

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

“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 Source: Rocky Mountains Denver 50 km

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

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