ATMO 336 Weather, Climate Society Cyclones, Cyclogenesis Weather Forecasting
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Transcript of ATMO 336 Weather, Climate Society Cyclones, Cyclogenesis Weather Forecasting
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ATMO 336
Weather, Climate SocietyCyclones, Cyclogenesis
Weather Forecasting
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Recall: Uniform Circular Motion Requires Acceleration/Force
Initial Velocity
Final Velocity
Acceleration directed toward center of circleInitial
Velocity
Final Velocity
Circular Path
Circle Center
Centripetal (center seeking) acceleration is required for Centripetal (center seeking) acceleration is required for curved flow, i.e. to change the direction of the velocity vector! curved flow, i.e. to change the direction of the velocity vector!
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Flow Around Curved Contours
5700 m5640 m
Centripetal Acceleration is Required for Air Parcel to Curve
LL HHZero
Zero
Assume PGF constant size along entire channel
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Forces for Curved Flow
5700 m5640 m
Centripetal = PGF + CF
Centripetal << PGF or CF
Gradient Wind Balance
Wind
Wind
Geo
Win
dPGF
PGF
PGF
CF
CF
CF
Assume PGF constant size along entire channel
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Gradient Wind Balance: End Result
5700 m5640 m
Wind speeds are Slowest at trough Fastest at ridge
Slower than Geo Wind
Faster than Geo Wind
Geo
Win
d
Win
d Spe
ed
Incr
ease
sWind Speed
Decreases
Assume PGF constant size along entire channel
Therefore, wind speeds Increase downwind of trough Decrease downwind of ridge
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Gradient Wind Balance
Speeds and Areas: Increase downwind of trough Decrease downwind of ridge
Win
d Spe
ed
Incr
ease
sWind Speed
Decreases
Wind Speed
Decreases
5700 m5640 m
Area
Incr
ease
s
1
2
Assume PGF constant size along entire channel
Area
Decreases
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Divergence and Convergence
Parcel Shapes: Stretch Downwind of Trough so Area Increases Compress Downwind of Ridge so Area Decreases
Area I
ncrea
ses
Diver
gence
Area Decreases
Convergence
Assume PGF constant size along entire channel
Divergence: Horizontal Area Increases with Time Convergence: Horizontal Area Decreases with Time
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Divergence and Convergence
Diver
gence
Net M
ass L
oss
Convergence
Net Mass G
ain
Mass transport across channel
Large
Small
Assume PGF constant size along entire channel
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Gedzelman, p249
Vertical Motion
Mass Conservation leads to Upward motion beneath regions of divergence Downward motion beneath regions of convergence
Trough RidgeRidge
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Sub-geostrophicSub-geostrophic
Super-geostrophicSuper-geostrophic
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Convergence
Convergence Divergenc
Divergencee
DivergenceDivergence
ConvergenceConvergence
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Where Winds are Divergent?
Diver
gence
Diver
gence
Trough
Ridge
slower winds
faster windsRegions 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.
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What Increases Divergence?
Diver
gence
Diver
gence
Trough
Ridge
slower winds
faster winds
1) Stronger PGF because faster winds require larger centripetal accelerations. Divergence stronger along axis of jet stream.
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What Increases Divergence?
Div
erge
nce
Div
erge
nce
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.
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What Increases Divergence?
Div
erge
nce
Div
erge
nce
Trough
Ridge
slower winds
faster winds
3) Shorter wavelength because the sharper curvature requires larger centripetal accelerations. Divergence stronger downwind of shortwave troughs.
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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
tilt
Ahrens, Meteorology Today, 5th Ed.
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upward motiondownward motion
Ahrens, Fig 6.21
Friction Induced Vertical Motion
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DivergenceDivergence
Convergence
Convergence
DivergenceDivergence
ConvergenceConvergence
Surface Convergence and Divergence
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Summary: Curved Flow & Friction
• Curved FlowRequires Centripetal AccelerationDifference between PGF and Coriolis ForceSpeed Changes => Convergence-Divergence
• Frictional ForceCauses Winds to Turn toward Low Pressure Important in the lowest 1 km above the Surface Leads to Convergence-Divergence
• Curvature and FrictionProduce Cyclones and Vertical Motions
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5700 m5640 m
Simplistic Model for Homework
Cold
Warm
LLHH
WetDry
DryLLHH HH
SurfaceCyclone
SurfaceAnticyclone
SurfaceAnticyclone
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ATMO 336
Weather Forecasting
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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
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Forecasting Questions
• How are weather forecasts made today?• How accurate are current weather forecasts?• How accurate can weather forecasts be?
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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
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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)
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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
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Courtesy ECMWF
Sparse data over oceans and Southern Hemisphere
Surface Data
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Courtesy ECMWF
Some buoy data over Southern Hemisphere
Surface Buoy Reports
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Courtesy ECMWF
Little data over oceans and Southern Hemisphere
Radiosonde Coverage
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Aircraft Reports
Courtesy ECMWF
Little data over oceans and Southern Hemisphere
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Weather Satellites
Geostationary
Polar Orbit
Satellite observations fill data void regions
Geostationary SatellitesHigh temporal samplingLow spatial resolutionPolar Orbiting SatellitesLow temporal samplingHigh spatial resolution
Ahrens, Figs. 9.5 & 9.6
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Courtesy ECMWF
T from (Mostly) GEO Satellitessw
eet
spot
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T from Polar Satellites
Courtesy ECMWF
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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 fieldswarmer to south + southerly winds warming
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Atmospheric Models
Must contain representations of many of complex physical processes to produce a good forecast
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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
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Model Grid Boxes
Forecast average conditions within grid boxes shaped like brownies
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“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: www.aaccessmaps.co
Rocky Mountains
Denver50 km
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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.
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Take Home Points
Forecasts are needed by many usersThere are several types of forecastsNumerical Weather Prediction (NWP)
Use computer models to forecast weather-Analysis Phase-Prediction Phase-Post-Processing Phase
Humans modify computer forecasts