Numerical Weather Prediction

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Numerical Weather Prediction

• EAS 211
• Spring 2005
• 04/22/05

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NWP

• Computer models, numerical models attempt to
  predict the weather using 7 fundamental equations
  that govern how the basic parameters change in
  the atmosphere with time.
• The study of how we can physically and
  mathematically represent processes in the
  atmosphere is known as dynamics.

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Computer models cannot perfectly predict the
weather, obviously, but what makes them
inaccurate?

• Errors.
• Initial conditions
• Intrinsic to the model
• Errors perpetuate with time and the model becomes
  less and less accurate.
• Parameterizations.
• Methods for dealing with physical processes that
  are too small to include in the model.

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Why do we need parameterizations? How can a
process be too small?

• Planetary, synoptic, meso, and micro (or storm)
  scales
• The model equations used in the model are only
  used for one scale, so we eliminate all other
  scales.
• But often, the scales we eliminate are large
  contributors to the scale we are examining.
• Smaller scale motions not captured by the surface
  or upper air observing networks are still
  important to the larger scale.
• So we parameterize the effects of the smaller
  scale on the larger scale.

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What is parameterized?

• Convection
• MicrophysicsMoisture
• Turbulence
• Soil Moisture
• Radiation

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WE NEED BETTER OBS.

• Improvements in the density of our surface and
  upper air observing networks to better capture
  the small scale motions would lead to improvement
  in the model results.

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Current NWP models

• Run daily at the National Center for
  Environmental Prediction (NCEP)
• NGM
• ETA
• AVN
• ECMWF
• RUC
• MM5
• WRF
• GFS
• NAM

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NCEP Models

• NGMNested Grid Model
• 80 km
• Grid point model, meaning obs are converted to
  values at various points that are evenly spaced
  so the computer program can plug them into
  equations.
• Frozen model.
• Being phased out.

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NCEP models

• Eta
• Doesnt stand for anything.
• Grid point model using either a 22 or 12 km grid.
• Accounts for changes in terrain as a series of
  steps each of which corresponds to a level of the
  models vertical coordinate, eta. (Eta is
  analogous to pressure).
• NAM North American Mesoscale Model

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NCEP models

• AVN, MRF, GSM
• Global-spectral models
• Converts data into a large number of mathematical
  waves which the model uses for its calculations
  and then returns the waves in a manner that will
  produce a forecast map.
• These models go out farther in time than the
  regional models.
• AVN-160 hours

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NCEP Models

• ECMWF (European Center for Medium-Range Weather
  Forecasts) and Canadian
• Spectral models used for medium range forecasts
  (7-10 days)
• Considered by many to be the most advanced model
  in the world.

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NCEP Models

• MM5Mesoscale Meteorological Model Version 5.
  Penn. State and NCAR.
• WRFWeather Research and Forecasting Model (NCAR)
• MM5 is designed to run on the mesoscale.
• MM5 and WRF are primarily used for research
  purposed, but there are some available MM5 and
  WRF models running throughout the US.

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Ensemble Predictions

• Method uses a collection of two or more forecasts
  for the same time. These forecasts either use
  different initial conditions or are based on
  different forecasting procedures.
• The various forecasts represent possibilities of
  how the weather might change in the future.
• An estimate of the probabilities of various
  events as well as an average (consensus)
  forecast.
• This means if several forecasts all give the same
  result, this result is more likely to occur, we
  have more confidence in our forecast.

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Advice on NWP

• Models should not be used without first
  consulting the observed data and applying
  knowledge of atmospheric processes to determine a
  forecast.
• To use only model data is to succumb to what is
  called meteorological cancer
• Try to use your time wisely when creating a
  forecast rather than trying to determine which
  model to use. You will find that no model will
  predict as well as an experienced, knowledgeable
  human forecaster.

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Information on Models

• NCEP
• http//www.ncep.noaa.gov