ATMO 336

Weather, Climate SocietyCyclones, Cyclogenesis

Weather Forecasting

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!

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

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

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

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

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

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

Gedzelman, p249

Vertical Motion

Mass Conservation leads to Upward motion beneath regions of divergence Downward motion beneath regions of convergence

Trough RidgeRidge

Sub-geostrophicSub-geostrophic

Super-geostrophicSuper-geostrophic

Convergence

Convergence Divergenc

Divergencee

DivergenceDivergence

ConvergenceConvergence

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.

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.

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.

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.

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.

upward motiondownward motion

Ahrens, Fig 6.21

Friction Induced Vertical Motion

DivergenceDivergence

Convergence

Convergence

DivergenceDivergence

ConvergenceConvergence

Surface Convergence and Divergence

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

5700 m5640 m

Simplistic Model for Homework

Cold

Warm

LLHH

WetDry

DryLLHH HH

SurfaceCyclone

SurfaceAnticyclone

SurfaceAnticyclone

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

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 SatellitesHigh temporal samplingLow spatial resolutionPolar Orbiting SatellitesLow temporal samplingHigh spatial resolution

Ahrens, Figs. 9.5 & 9.6

Courtesy ECMWF

T from (Mostly) GEO Satellitessw

eet

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 fieldswarmer 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)

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

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 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