MAKING THE BEST USE OF THE ENSEMBLE LONG-RANGE FORECAST
PRODUCTS
A.Muraviev
Russian Hydrometeorological Center
9-13 B.Predtechensky per., 123242, Moscow, Russia
Abstract
The practice of meteorological long-range forecasts puts special emphasis on the a
priori assessment of the informative value of the first several days of the model outcomes.
The a priori assessment used in the framework of the complex dynamic- statistical long-range
forecast scheme relies on different techniques: model trajectory divergence, energy spectra of
the first planetary wave numbers, baroclinic instability evolution, and anomaly evolution in
the second prognostic 5-day interval, etc.
The latter has proved to be the most informative of all. The technique combines both
the modeled and real time series within the Perfect Prog ideology. Non-overlapping
threesomes of field values at each point are the target time series that are to be forecast. Two
proceeding months are included in the dependent sample to set the appropriate regression
equation modeling the ARMA(2,0) process. The first two threesomes of the hydrodynamic
forecast are put into the regression equation to obtain final anomaly assessment for the third
threesome. The anomalous value is a value viewed in relation to the mean and the standard
deviation of the objective analysis data.
The global spectral T41L15 GCM with a “full” physics package is used. Numerical
runs (5 trajectories in one) are performed every 10 days, with equaling the last integration to
the calendar month. The used data refer to every successive 12 hours. The scheme products
include potential heights (500, 850 and 1000 hPa), sea level pressure, temperature at 850 mb
height level, Earth’s surface temperature and precipitation at observation stations of the
former USSR. The final product comprises averaging, filtering and interpretation of the
variables averaged for the 10, 20 and 30-day intervals. The averaged fields and station values
are obtained as 1) one model trajectory averaging, 2) five trajectories averaging, 3) averaging
with spectral filtering of the fields in the second and the third prognostic decade, 4) regression
equation solving based on the first 5 or 10 prognostic days, 4) the “climate”, and, finally, 5)
persistence. To obtain the “climate” the NCEP Gridded Data Archive (Version III) is used.
Numerical experiments were performed on CRAY Y-MP, for the period April 1999 –
June 2000 (45 scheme runs, 225 integrations) for the Northern Hemisphere and its large scaleregions (three Natural Synoptic Regions, Europe, Asia, the northern Pacific, Atlantic, and
America). To obtain the most reliable forecast product a choice is made from different
forecast techniques (without resorting to the user’s loss matrix). The choice may be preset or
free and critically depends on the prognostic variable and the average interval. A
corresponding table of preferred selections is presented on the basis of the RMSSS estimates.
In some cases the gain may be of about 20%. A detailed study of the 500 mb potential
forecast compared to the “climate” on the basis of RMS is studied in detail. It shows that
climate and dynamic-statistical averaged 20-day products may be discriminated with the
help of the anomaly evolution in the second prognostic pentad, while the discrimination of
statistics and dynamics may by obtained with the help of the model trajectory divergence.
Significant results may be gained for regional scales, but hardly for the Northern Hemisphere
on the whole.