Weather Forecasts: From Tragedy to Triumph Cliff Mass University of Washington
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Transcript of Weather Forecasts: From Tragedy to Triumph Cliff Mass University of Washington
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Weather Forecasts: From Tragedy to TriumphCliff Mass
University of Washington
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2012: Hurricane Sandy
125 dead, 60+ billion dollars damage
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Well predicted over a week ahead of time
ECMWF Forecast of Sea Level Pressure
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1938 Hurricane: Similar in Strength to Sandy
Nearly a thousand died
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Not forecast the day before
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1962 Columbus Day Storm
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Not forecast the day before either
Seattle Times
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Jan 1993: Inauguration Day Storm: Near Perfect Forecast
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Something Has Changed
Before 1990 the National Weather Service got virtually every major storm wrong, even the
day before.
After 1990, they gave good warnings for nearly all.
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Forecast Skill Improvement
ForecastError
Year
Better
National Weather Service
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Skill Improvements (ECMWF)
Major improvements, mainly due to satellite data and improved models
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The Revolution in Weather Prediction Technology
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The Key Technology of Modern Weather Forecasting is
Numerical Weather Prediction
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Numerical Weather Prediction
• The basic idea is that if you can determine the current state of the atmosphere (known as the initialization) , you can predict the future using the equations that describe the physics of the atmosphere.
• These equations can be solved on a three-dimensional grid.
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The “Primitive” Equations
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Numerical Weather Prediction• Numerical weather prediction is limited by the
available computer resources.
• As computer speed increases, the number of grid points can be increased.
• More (and thus) closer grid points means we can simulate (forecast) smaller scale features.
National WeatherService WeatherPrediction Computer
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NGM, 80 km,1995
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2007-2008
4-km MM5Real-time
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1.33 km resolution available on the UW web site
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But just as important has been the weather data revolution, with
satellites giving us three dimension data over the entire
planet
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Example: The Pacific Data Void No Longer Exists
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Cloud Track Winds
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Better than Star Trek!
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NOAA PolarOrbiter WeatherSatellite
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Satellite Sensors Provide Thousands of High Quality Vertical Soundings Daily over the Pacific
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Cosmic GPS Satellites Provide More Soundings!
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We are now starting to see frequent examples of forecast
skill past one week:
Hurricane Sandy is only one example
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Observed 180 hr (7.5 days)
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Forecast Skill Will Continue to Extend Further in Time…with
limits (about 2 weeks)
• More satellite assets will provide a far better description of the atmosphere.
• Better models and higher resolution
• Better data assimilation: how we use the observations to produce an initialization for our models.
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Increasing Resolution and Better Models Will Not Be Enough
The Next Major Revolution in Numerical Weather Prediction
Will Come Elsewhere
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The Transition from Deterministic to Probabilistic
Prediction
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A Fundamental Problem• The way we have been forecasting
has been 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….
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A Fundamental Problem
• Similarly, uncertainty in our model physics (e.g., clouds and precipitation processes) also produces uncertainty in forecasts.
• Thus, all forecasts have some uncertainty.
• The uncertainty generally increases in time.
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This is Ridiculous!
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Forecast Probabilistically
• 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
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Ensemble Prediction
• Instead of making one forecast…make many…each with a slightly different initialization or different model physics.
• Possible to do this now with the vastly greater computation resources that are available.
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Ensemble Prediction
•Can use ensembles to give the probabilities that some weather feature will occur.
• Ensemble mean is more accurate than any individual member.
•Can also predict forecast skill!•When forecasts are similar, forecast skill is generally higher.•When forecasts differ greatly, forecast skill is less.
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Prediction!
• The meteorological profession is rapidly gaining the ability to produce high-resolution probabilistic weather forecasts AND analyses.
• Probabilistic forecasts and analyses will be available for a wide range of weather parameters.
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The Nowcasting Revolution
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AMS Nowcasting Definition
A description of current weather and a short-term forecast varying from minutes to a few hours; typically shorter than most operational short-range forecasts.
American Meteorological Society’s Glossary of Weather and Climate
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During the past decade or so the geographical and temporal detail
the weather profession can provide has greatly increased.
• High resolution forecasting, NWS forecasts on a 2.5 km grid, radar data, satellite imagery, huge numbers of surface stations, and now probabilistic prediction!
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Example:The Pacific Northwest
Based on 72 different networks
3000-4000 observations per hour over WA and OR
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Traditional Approaches of Weather Information Dissemination Are Incapable of Delivering the Specificity and Detail
Meteorologists Can Provide
Typical TV weathercasters have only 2.5 minutes!
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Many of us worried about this problem in the 90’s but now the
solution is literally at hand
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Smartphones are Ideal for Weather Data Delivery!
• Lots of bandwidth
• They know where they are, so forecast information can be tailored to the user
• Substantial computational capacity.
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There are now thousands of weather apps for smartphones…and the best
are yet to come!
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Some Tragedies Continue
Little Improvement in Forecasting the Intensity of Hurricanes
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Tragedy: The U.S. is Now in Second or Third Place in Global
Weather Prediction
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The End