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A Brief History of Weather Forecasting

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Local weather details which really weren't understood-- were added ... The computers models become capable of simulating/forecasting local weather. ... – PowerPoint PPT presentation

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Title: A Brief History of Weather Forecasting


1
A Brief History of Weather Forecasting
2
The Stone Age
  • Prior to approximately 1955, forecasting was
    basically a subjective art, and not very
    skillful.
  • Observations were sparse, with only a few
    scattered ship reports over the oceans.
  • The technology of forecasting was basically
    subjective extrapolation of weather systems using
    the upper level flow (the jet stream).
  • Local weather detailswhich really werent
    understood-- were added subjectively.

3
Upper Level Chart
4
1955-1965 The Advent of Modern Forecasting
  • During this period, numerical weather
    predictionforecasting future weather with
    digital computers-- became the key tool in the
    meteorologists tool bag.
  • The launch of the first weather satellite (1960)
    gave meteorologists a view of the entire planet.
  • Weather radars were placed around the U.S.
    explicitly showing areas of precipitation.

5
Numerical Weather Prediction
  • The advent of digital computers in the late 1940s
    and early 1950s made possible the simulation of
    atmospheric evolution numerically.
  • The basic idea is if you understand the current
    state of the atmosphere, you can predict the
    future using the basic physical equations that
    describe the atmosphere.

6
Numerical Weather Prediction
  • One such equation is Newtons Second Law
  • F ma
  • Force mass x acceleration
  • Mass is the amount of matter
  • Acceleration is how velocity changes with time
  • Force is a push or pull on some object (e.g.,
    gravitational force, pressure forces, friction)

This equation is a time machine!
7
Numerical Weather Prediction
Using a wide range of weather observations we can
create a three-dimensional description of the
atmosphere… known as the initialization
8
Numerical Weather Prediction
  • This gives the distribution of mass and allows us
    to calculate the various forces.
  • Then… we can solve for the acceleration using
    Fma
  • But this gives us the future…. With the
    acceleration we can calculate the velocities in
    the future.
  • Similar idea with temperature and humidity.

9
Numerical Weather Prediction
  • These equations can be solved on a
    three-dimensional grid.
  • As computer speed increased, the number of grid
    points could be increased.
  • More (and thus) closer grid points means we can
    simulate (forecast) smaller and smaller scale
    features. We call this improved resolution.

10
A Steady Improvement
  • Faster computers and better understanding of the
    atmosphere, allowed a better representation of
    important physical processes in the models
  • More and more data became available for
    initialization
  • As a result there has been a steady increase in
    forecast skill from 1960 to now.

11
Forecast Skill Improvement
National Weather Service
Forecast Error
Better
Year
12
Satellite and Weather Radars Give Us a More
Comprehensive View of the Atmosphere
13
Camano Island Weather Radar
14
1995-2003 The computers models become capable of
simulating/forecasting local weather.
  • As the grid spacing decreased to 15 km and below…
    it became apparent that many of the local weather
    features could often be simulated and forecast by
    the models.

15
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19
The National Weather Service
Forecaster at the Seattle National Weather
Service Office
20
But even with all this improving technology, some
forecasts fail or are inadequate. Why?
21
Problems with the Models
  • Some forecasts fail due to inadequacies in model
    physics…. How the model handles precipitation,
    friction, and other processes.
  • Example too much precipitation on mountain
    slopes
  • Intensive work at the UW to address this problems.

22
Some forecasts fail due to poor initialization,
i.e., a poor starting description of the
atmosphere.
  • This is particularly a problem for the Pacific
    Northwest, because we are downstream of a
    relatively data poor region…the Pacific Ocean.

23
3 March 1999 Forecast a snowstorm … got a
windstorm instead
24
Eta Model Sea Level Pressure 12 UTC 2 March 99
Major Initialization Errors
25
Pacific Analysis At 4 PM 18 November 2003
Bad Observation
26
The problem of initialization should lessen as
new observation technologies come on line and
mature. New ways of using or assimilating the
data are also being developed.
27
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28
Seascan Unmanned Aircraft
29
Lack of Coastal Weather Information
  • There is a lack of detailed weather information
    immediately off the Northwest Coast.
  • Major issue… lack of a coastal weather radar.
  • The Northwest has the worst coastal weather radar
    coverage in the nation.
  • Often cant see the details of weather features
    before they make landfall. Seriously impacts
    short-term forecasts.

NWS Doppler Radar
30
Now
With Two New Radars
31
A More Fundamental Problem
  • In a real sense, the way we have been forecasting
    is essentially flawed.
  • The atmosphere is a chaotic system, in which
    small differences in the initialization…well
    within observational error… can have large
    impacts on the forecasts, particularly for longer
    forecasts.
  • Not unlike a pinball game….

32
A More Fundamental Problem
  • Thus, there is fundamental uncertainty in weather
    forecasts that can not be ignored.
  • Similarly, uncertainty in our model physics also
    produces uncertainty in the forecasts.
  • We should be using probabilities for all our
    forecasts or at least providing the range of
    possibilities.
  • There is an approach to handling this issue that
    is being explored by the forecasting
    community…ensemble forecasts.

33
Ensemble Prediction
  • Instead of making one forecast…make many…each
    with a slightly different initialization
  • Possible to do now with the vastly greater
    computation resources that are available.

34
Verification
The Thanksgiving Forecast 2001 42h forecast
(valid Thu 10AM)
SLP and winds
  • Reveals high uncertainty in storm track and
    intensity
  • Indicates low probability of Puget Sound wind
    event

1 cent
11 ngps
5 ngps
8 eta
2 eta
3 ukmo
12 cmcg
9 ukmo
6 cmcg
4 tcwb
13 avn
10 tcwb
7 avn
35
Ensemble Prediction
  • Can use ensembles to provide a new generation of
    products that give the probabilities that some
    weather feature will occur.
  • Can also predict forecast skill!
  • It appears that when forecasts are similar,
    forecast skill is higher.
  • When forecasts differ greatly, forecast skill is
    less.

36
Ensemble-Based Probabilistic Products
37
Forecast Dissemination The Achilles Heal
  • Although the technology of weather prediction is
    rapidly improving, our ability to communicate
    what we know to the public is inadequate.
  • Although the Internet and wireless communication
    providesfor the first timethe potential to
    distribute large amounts of weather information,
    we have not yet found an effective way to do so.
  • The amount of information is massive, how do we
    distill and filter it for a wide variety of
    users?
  • We are failing to communicate our degree of
    confidence in the forecasts.
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