ATMO 336 Weather, Climate Society Cyclones, Cyclogenesis Weather Forecasting

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ATMO 336Weather, Climate SocietyCyclones,
CyclogenesisWeather Forecasting

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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!
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Flow Around Curved Contours
Assume PGF constant size along entire channel
L
H
Zero
Centripetal Acceleration is Required for Air
Parcel to Curve
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Forces for Curved Flow
Assume PGF constant size along entire channel
PGF
Wind
PGF
Geo Wind
PGF
CF
CF
Wind
Centripetal PGF CF Centripetal ltlt PGF or
CF Gradient Wind Balance
CF
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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
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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
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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
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Divergence and Convergence
Assume PGF constant size along entire channel
Large
Mass transport across channel
Small
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Vertical Motion
Ridge
Ridge
Trough
Gedzelman, p249
Mass Conservation leads to
Upward motion beneath regions of divergence
Downward motion beneath regions of convergence
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Super-geostrophic
Sub-geostrophic
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Divergence
Divergence
Convergence
Convergence
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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
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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
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What Increases Divergence?
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?
faster winds
Ridge
Divergence
Trough
3) Shorter wavelength because the sharper
curvature requires larger centripetal
accelerations. Divergence stronger downwind of
shortwave troughs.
slower winds
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Vertical Structure
tilt
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.
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Friction Induced Vertical Motion
downward motion
upward motion
Ahrens, Fig 6.21
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Divergence
Convergence
Convergence
Divergence
Surface Convergence and Divergence
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Summary Curved Flow Friction

• Curved Flow
• Requires Centripetal Acceleration
• Difference between PGF and Coriolis Force
• Speed Changes gt 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

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Simplistic Model for Homework
5640 m
5700 m
H
Cold
L
Dry
Wet
Dry
Warm
L
Surface Anticyclone
Surface Anticyclone
Surface Cyclone
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ATMO 336Weather 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
• Analysis Phase (most expensive piece)
• Prediction Phase (modeling, computing)
• 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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Surface Data
Sparse data over oceans and Southern Hemisphere
Courtesy ECMWF
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Surface Buoy Reports
Some buoy data over Southern Hemisphere
Courtesy ECMWF
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Radiosonde Coverage
Little data over oceans and Southern Hemisphere
Courtesy ECMWF
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Aircraft Reports
Little data over oceans and Southern Hemisphere
Courtesy ECMWF
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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
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T from (Mostly) GEO Satellites
sweet spot
Courtesy ECMWF
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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 fields
• warmer 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)
Rocky Mountains

• Approximate Size of One Grid Box for NCEP Global
  Ensemble Model
• Note Variability in Elevation, Ground Cover, Land
  Use

Denver
Source www.aaccessmaps.co
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13 km Model Terrain
Big mountain ranges, like the Sierra Nevada, are
resolved. But isolated peaks, like the
Catalinas, are not evident.
100 m contour
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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