Ensemble weather forecasting

Ensemble Weather Forecasting

What is Ensemble Weather Forecasting?

  • An ensemble weather forecast is a collection of forecasts that illustrate the range of possible future weather conditions.

  • Multiple simulations are executed, each with slight variations in initial conditions and weather models.

  • These variations embody the inherent uncertainty in initial conditions and approximate models, leading to diverse potential weather outcomes.

Importance of Measuring Uncertainty in Forecasts

  • The uncertainty associated with forecasts implies that various scenarios can occur, which should be reflected in the forecast.

  • Single 'deterministic' forecasts may mislead users since they don't provide information on uncertainty.

  • For instance, in agriculture, farmers require knowledge of possible conditions for crop protection, highlighting the need for understanding the range of forecasts at various times.

Advantages of Ensemble Prediction

  • Ensemble predictions generate multiple possible outcomes, indicating the likelihood of different scenarios in the coming days and how far into the future forecasts remain useful.

  • A smaller range of predicted outcomes results in a forecast being termed 'sharper'.

  • Reliable ensemble forecasts ensure that if a forecast claims a 70% chance of temperatures exceeding a certain threshold, this should occur in 70% of all such forecasts made.

Is Forecast Uncertainty Due to Lack of Knowledge?

  • Partially; deficiencies in our understanding significantly elevate the forecast uncertainty.

  • There is ongoing research to enhance knowledge regarding initial conditions and atmospheric processes that models must replicate.

The Chaotic Nature of the Atmosphere

Sensitivity to Initial Conditions

  • The atmosphere behaves as a chaotic system, meaning it is highly sensitive to initial conditions.

  • A minor change in input can lead to substantial discrepancies in the output forecast.

  • This contrasts with non-chaotic systems, where small initial discrepancies result in minor output variations.

  • Consequently, it is crucial to examine how sensitive the atmosphere is to initial conditions during weather forecasting, achieved through ensemble forecasting which evaluates a spread of possible outcomes.

History of Ensemble Forecasting at ECMWF

Introduction of Ensemble Forecasting

  • The European Centre for Medium-Range Weather Forecasts (ECMWF) commenced operational weather predictions in 1979, initially lacking uncertainty estimates.

  • Research efforts aimed at exploring ensemble approaches began in the 1980s, culminating in the first ensemble forecasts in 1992 after a decade of development.

Producing Ensemble Forecasts at ECMWF

Methodology of Ensemble Forecasting

  • ECMWF generates a total of 51 forecasts:

    • Control Forecast: Utilizes the best available data and unwavering models.

    • Perturbed Forecasts: The remaining 50 forecasts include slight perturbations of initial conditions and modeling.

  • Together, these forecasts construct the ensemble, which depicts the range of possible outcomes and their likelihoods.

Future of Ensemble Forecasting

  • ECMWF articulated its 10-year strategy titled "The Strength of a Common Goal" in 2016, emphasizing the essential role ensemble forecasting will play in the organization's future operations.