Tuesday, 14 May 2002
Calibration of PQPF Forecasts Based on the NCEP Global Ensemble
Ensemble based Probabilistic Quantitative Precipitation
Forecasts (PQPF) have been operationally created on a daily basis at NCEP
since 1997. The PQPF forecasts are based on the relative frequency of ensemble
members indicating a particular event. Since neither the numerical weather
prediction model nor the ensemble generation technique is perfect, these
probability values are not necessarily reliable. To ameliorate the problem a
calibration algorithm to reduce the bias in the first moment of the ensemble
based probability distribution has been developed and applied on an
experimental basis. The calibration is based on the comparison of the
frequency distributions of observed and forecast precipitation amounts over a
2-3 month preceding period, over the continental US. verification results
indicate that the reliability and skill of the calibrated forecasts is
significantly enhanced. Further improvement is expected from a second
adjustment where the second moment of the ensemble is calibrated. It is
expected that the PQPF calibration procedure will be operationally
implemented, providing reliable and more skillful PQPF forecasts to the
hydrologic and other user communities.
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