NUMERICAL WEATHER PREDICTION K. Lagouvardos-V. Kotroni Institute of Environmental Research National...
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Transcript of NUMERICAL WEATHER PREDICTION K. Lagouvardos-V. Kotroni Institute of Environmental Research National...
NUMERICAL WEATHER PREDICTION
K. Lagouvardos-V. KotroniInstitute of Environmental Research
National Observatory of Athens
NUMERICAL WEATHER PREDICTION
K. Lagouvardos-V. KotroniInstitute of Environmental Research
National Observatory of Athens
WHAT IS THE NUMERICAL WEATHER PREDICTION?
WHAT IS THE NUMERICAL WEATHER PREDICTION?
A method to forecast weather conditions based on:
o Equations describing the flow within the atmosphere as well as many physical processes
o Translation into code executed on computerso Application on a specific geographic domain (grid)o Integration in time, based on initial and boundary
conditionso Provision of final products (forecasts of wind, temperature,
humidity, rain/snow, etc)
o1922: L. Richardson, forecast using human calculatorso1950: ΕNIAC forecast for three specific weather eventso1966: world coverage (USA)
o 1970: Foundation of the European Center for Medium-range Weather Forecasts (ECMWF)
1979 : first weather forecast issued by the ECMWFtoday: 10 days forecast, horizontal resolution~15 km
o 1990: first weather forecasts issued by universities, research centers and private companies.
HISTORYHISTORY
EQUATIONSEQUATIONS
Conservation of momentum (Newton’s law)3 equations for accelerations of wind (F = Ma)
Conservation of massequation for conservation of air (mass continuity)equation for conservation of water
Conservation of energy
equation for the first law of thermodynamics Relationship among p, V, and T
equation of state (ideal gas law)
NUMERICAL METHODSNUMERICAL METHODS
Write the governing equations in form of spatial and temporal derivatives and transform them into algebraic equations
Computers can solve these equations, usually using finite difference schemes (expansion in Taylor series) on a grid
All terms are defined and computed on a fixed grid.Example: 1-D advection equation ∂u / ∂t = -u (∂u / ∂x)
x x+1x-1
∆x ∆x
t
uu
t
u
x
uu
x
u
xx
x
xxx
tt
2
2
11
11
GRID
All atmospheric processes are described
within a grid
Need for a large number of calculations
3-D GRID3-D GRID
Physical processes-parameterizationsPhysical processes-parameterizations
Parameterizations are necessary for the representationof physical processes that are small in size or short in life, complex
or poorly known to be explicitly represented
Physical processes-parameterizationsPhysical processes-parameterizations
Parameterizations are necessary for the representationof physical processes that are small in size or short in life, complex
or poorly known to be explicitly represented
Global model (Source: USA)
Limited area- Region 1
Region 2
Weather forecasts at NOAWeather forecasts at NOA
Forecast errorsForecast errors
Limited knowledge of initial conditions
Limited knowledge of physical processes
(parameterizations!!!)
Result:
Forecast errors grow in time
Ensemble forecastingEnsemble forecasting
It is known that neither the models nor the initial conditions are perfect
Problem: deterministic forecasts have limited predictability
Possible solution: base the final forecast not only on the predictions of one model
(deterministic forecast) but on an ensemble of weather model outputs
ENSEMBLE FORECASTING
Based on perturbing the initial conditions (20-50 perturbed members) provided to
individual models, depending on a realistic spectrum of initial errors
OTHER APPLICATIONSOTHER APPLICATIONS
Model outputs can be used as an input to:
o Wave modeling and ocean circulationo Hydrological modeling for flood forecastingo Fire expansion modelso Air-quality models