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ATMO 336
Weather, Climate Society
Cyclones, Cyclogenesis
Weather Forecasting
Recall: Uniform Circular Motion
Requires Acceleration/Force
Circle
Center
Circular
Path
Initial
Velocity
Final
Velocity
Final
Velocity
Initial
Velocity
Acceleration
directed toward
center of circle
Centripetal (center seeking) acceleration is required for
curved flow, i.e. to change the direction of the velocity vector!
Flow Around Curved Contours
Assume PGF constant size
along entire channel
L
H
Centripetal
Acceleration
is Required
for Air Parcel
to Curve
Forces for Curved Flow
Assume PGF constant size
along entire channel
PGF
Wind
PGF
PGF
CF
CF
Wind
Centripetal = PGF + CF
CF
Centripetal << PGF or CF
Gradient Wind Balance
Gradient Wind Balance: End Result
Assume PGF constant size
along entire channel
Slower than
Geo Wind
Faster than
Geo Wind
Wind speeds are
Slowest at trough
Fastest at ridge
Therefore, wind speeds
Increase downwind of trough
Decrease downwind of ridge
Gradient Wind Balance
Assume PGF constant size
along entire channel
2
1
Speeds and Areas:
Increase downwind of trough
Decrease downwind of ridge
Divergence and Convergence
Assume PGF constant size
along entire channel
Divergence: Horizontal
Area Increases with Time
Convergence: Horizontal
Area Decreases with Time
Parcel Shapes:
Stretch Downwind of
Trough so Area Increases
Compress Downwind of
Ridge so Area Decreases
Divergence and Convergence
Assume PGF constant size
along entire channel
Large
Small
Mass transport
across channel
Vertical Motion
Ridge
Trough
Ridge
Gedzelman, p249
Mass Conservation leads to
Upward motion beneath regions of divergence
Downward motion beneath regions of convergence
Super-geostrophic
Sub-geostrophic
Divergence
Convergence
Where Winds are Divergent?
Regions downwind of 500 mb
troughs are favorable for surface
cyclones and upward motion.
faster winds
Ridge
Trough
slower winds
Cyclogenesis can only occur
where mass is being removed
from the column overhead.
Mass loss produces surface
pressure falls.
What Increases Divergence?
faster winds
Ridge
Trough
slower winds
1) Stronger PGF because faster
winds require larger centripetal
accelerations. Divergence
stronger along axis of jet stream.
What Increases Divergence?
faster winds
Ridge
Trough
slower winds
2) Bigger amplitude waves
because the sharper curvature
requires larger centripetal
accelerations. Divergence
stronger downwind of larger
amplitude troughs.
What Increases Divergence?
faster winds
Ridge
Trough
slower winds
3) Shorter wavelength because
the sharper curvature requires
larger centripetal accelerations.
Divergence stronger downwind
of shortwave troughs.
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.
Friction Induced Vertical Motion
downward motion
upward motion
Ahrens, Fig 6.21
Divergence
Convergence
Divergence
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
Simplistic Model for Homework
H
Cold
L
Surface
Anticyclone
L
H
Warm
Surface
Cyclone
H
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 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
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
Surface Data
Sparse data over oceans
and Southern Hemisphere
Courtesy ECMWF
Surface Buoy Reports
Some buoy data over
Southern Hemisphere
Courtesy ECMWF
Radiosonde Coverage
Little data over oceans and
Southern Hemisphere
Courtesy ECMWF
Aircraft Reports
Little data over oceans and
Southern Hemisphere
Courtesy ECMWF
Weather Satellites
Geostationary
Polar Orbit
Ahrens, Figs. 9.5 & 9.6
Satellite observations
fill data void regions
Geostationary Satellites
High temporal sampling
Low spatial resolution
Polar Orbiting Satellites
Low temporal sampling
High spatial resolution
sweet spot
T from (Mostly) GEO Satellites
Courtesy ECMWF
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 (108) 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)
Rocky Mountains
Approximate Size of One
Grid Box for NCEP
Global Ensemble
Model
Note Variability in
Elevation, Ground
Cover, Land Use
50 km
Denver
Source: www.aaccessmaps.co
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
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
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