MPO 674 Lecture 2

1/20/15

Timeline (continued from Class 1)

• 1960s: Lorenz papers: finite limit of predictability?• 1966: First primitive equations model (6 layers)• 1971: First regional system (limited fine mesh

model)• 1978: Optimal Interpolation• 1980: Global Spectral Model• 1991: 3d-Var introduced at NCEP (Parrish and

Derber 1992)• 1993: First ensemble forecast systems at NCEP and

ECMWF

Ensemble Forecasts

• Epstein (1969), Leith (1974) suggested that instead of performing “deterministic” forecasts, stochastic forecasts providing an estimate of the skill of the prediction should be made.

• Several model forecasts with perturbations in the initial conditions or in the models themselves (will review perturbation methods later in the course)

Ensemble Forecasts

• Goals:– To provide an ensemble average that is more

accurate than individual forecasts, especially beyond the first few days. Components of forecast that are less predictable tend to be averaged out.

– To provide forecasters with an estimation of the reliability of the forecast

– Data assimilation– Adaptive Observations– Sensitivity Analysis

Ensemble Forecasts

• Can extend forecasts beyond Lorenz’s 2-week limit of weather predictability

• ENSO should be predictable a year or more in advance, since slowly varying surface forcing (from SST and land surface) should produce atmospheric anomalies that are longer lasting and more predictable than weather patterns

• Cane et al. (1986): first experiments

Ensemble Spaghetti Diagrams

108 h ECMWF Ensemble Forecast of pre-Karl, init. 0000 UTC 10 Sept 2010

CIRC

THICK

MSLPProbability

(TC at 108 h) = 68%

CIRC (x 10-5 s-1)TH

ICK

(m)

Ensemble prediction skill

Timeline (since 1993)

• 1993: Nonhydrostatic mesoscale models: MM5, CAPS, RAMS etc.

• 1997: ECMWF introduced 4d-Var operationally, most other centers (except NCEP) followed

• 2005: Canadian Meteorological Center introduced Ensemble Kalman Filter (EnKF) into operations

• 2012: NCEP introduced Hybrid 3d-Var / EnKF• 2015 (Jan 14): NCEP GFS 13 km resolution

Current and Future

• Detailed short-range forecasts (severe weather, rain and snow bands etc)

• Sophisticated, flow-dependent, continuous DA• Adaptive observing systems• Improving medium- and long-range forecasts,

primarily through ensembles• Fully coupled systems (atmosphere-ocean-

wave-land-ice-hydrology)• Public guidance: air pollution, UV radiation,

flooding levels, local winds, fires etc.