Transcript INTRODUCTION - University of Crete

NUMERICAL WEATHER
PREDICTION
K. Lagouvardos-V. Kotroni
Institute of Environmental Research
National Observatory of Athens
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 computers
o Application on a specific geographic domain (grid)
o Integration in time, based on initial and boundary
conditions
o Provision of final products (forecasts of wind, temperature,
humidity, rain/snow, etc)
HISTORY
o1922: L. Richardson, forecast using human calculators
o1950: ΕNIAC forecast for three specific weather events
o1966: world coverage (USA)
o1970: Foundation of the European Center for Medium-range
Weather Forecasts (ECMWF)
1979 : first weather forecast issued by the ECMWF
today: 10 days forecast, horizontal resolution~15 km
o1990: first weather forecasts issued by universities, research
centers and private companies.
EQUATIONS
Conservation of momentum (Newton’s law)
3 equations for accelerations of wind (F = Ma)
Conservation of mass
equation 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 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-1
∆x
x
u x 1  u x 1
x 
2x
x
x
u

u
u
t 1
t 1

x
t
2t
u
x
x+1
3-D GRID
GRID
All atmospheric
processes are described
within a grid
Need for a large number
of calculations
Physical processes-parameterizations
Parameterizations are necessary for the representation
of physical processes that are small in size or short in life, complex
or poorly known to be explicitly represented
Physical processes-parameterizations
Parameterizations are necessary for the representation
of physical processes that are small in size or short in life, complex
or poorly known to be explicitly represented
What happens inside the clouds
WITHIN THE GRID
Constructing the initial conditions
Initial conditions: surface obs
Initial conditions: buoys
Initial conditions: soundings
Initial conditions: airplanes
Initial conditions: satellites
Weather forecasts at NOA
Global model
(Source: USA)
Limited area- Region 1
Region 2
Limited area model: area 1
Limited area model: area 2
Final forecast product
Final forecast product
Forecast errors
Limited knowledge of
initial conditions
Limited knowledge of
physical processes
(parameterizations!!!)
Result:
Forecast errors
grow in time
Ensemble 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
Ensemble forecasting
OTHER APPLICATIONS
 Model outputs can be used as an input to:
o
o
o
o
Wave modeling and ocean circulation
Hydrological modeling for flood forecasting
Fire expansion models
Air-quality models
ATMOSPHERIC RESEARCH
THEORY
OBSERVATIONS
MODELS