8 Ensembles, Forecasts and
Predictability
ANTONIO NAV ARRA
Istituto Nazionale di Geofisica, Rome, ltaly
8.1 Introduction
The case of weather forecasts represents a mature example of prediction of a
complete environmental system. The achievement of such an objective has
required the overcoming ofseveral rather formidable obstacles, some ofthem technical and organizational, but most of them conceptual and scientific, whose successful treatment has constructed the success story we know now.
The deployment of an adequate observational network, its maintenance and
upgrading, the development of detailed numerical models and the availability of
powerful super computers, are examples of some of the technical issues that have
allowed progress. On the other hand, the understanding of the baroclinic lifecycles; the discovery of the interlocking areas of the planet, between middle latitudes and the tropics, between entrance and exit regions ofthe jet streams; the role
of time-scale interactions, that has shown how the synoptic scale is strongly interacting with the monthly and seasonal time scales; the understanding of the role of
diabatic processes, including most thermodynamical processes and land interactions, have allowed continuos progress and expanded the scope ofthe weather forecasts from the original limited application of aviation forecasts, i.e. upper air
predictions, to predictions of an expanded weather system that today include the
entire atmosphere, the soil, the upper ocean and can even include chemical constituents of the atmosphere as in the case of the experimental ozone predictions system.
The tremendous expansion of weather forecasts, supported by the increasing
computing capabilities, terrific data assimilation techniques and the development
of more accurate and detailed models, is opening the way that to what we can refer
todayas "environmental predictions", namely attempts to predict chemical and biological parameters not traditionally included in weather forecasts. The expansion of
the predictive capabilities has been limited only by a fundamental properties of the
meteorological equation, an extreme sensitivity to initial condition, sometimes
known as the "butterfly problem", or more scientifically as the fact that the equations describing the atmospheric motion are characterized by chaos.
The word chaos has been commonly used to describe this situation, but it is
somewhat of a misnomer. According to the dictionary, chaos is a "condition in
which things are out oftheir normal or proper places or relationships", though the
net result may be unpleasant, is difficult to think ofthe atmosphere-ocean-Iand system as "out of the its normal order", it is in fact following definite equations and
Predictability
ANTONIO NAV ARRA
Istituto Nazionale di Geofisica, Rome, ltaly
8.1 Introduction
The case of weather forecasts represents a mature example of prediction of a
complete environmental system. The achievement of such an objective has
required the overcoming ofseveral rather formidable obstacles, some ofthem technical and organizational, but most of them conceptual and scientific, whose successful treatment has constructed the success story we know now.
The deployment of an adequate observational network, its maintenance and
upgrading, the development of detailed numerical models and the availability of
powerful super computers, are examples of some of the technical issues that have
allowed progress. On the other hand, the understanding of the baroclinic lifecycles; the discovery of the interlocking areas of the planet, between middle latitudes and the tropics, between entrance and exit regions ofthe jet streams; the role
of time-scale interactions, that has shown how the synoptic scale is strongly interacting with the monthly and seasonal time scales; the understanding of the role of
diabatic processes, including most thermodynamical processes and land interactions, have allowed continuos progress and expanded the scope ofthe weather forecasts from the original limited application of aviation forecasts, i.e. upper air
predictions, to predictions of an expanded weather system that today include the
entire atmosphere, the soil, the upper ocean and can even include chemical constituents of the atmosphere as in the case of the experimental ozone predictions system.
The tremendous expansion of weather forecasts, supported by the increasing
computing capabilities, terrific data assimilation techniques and the development
of more accurate and detailed models, is opening the way that to what we can refer
todayas "environmental predictions", namely attempts to predict chemical and biological parameters not traditionally included in weather forecasts. The expansion of
the predictive capabilities has been limited only by a fundamental properties of the
meteorological equation, an extreme sensitivity to initial condition, sometimes
known as the "butterfly problem", or more scientifically as the fact that the equations describing the atmospheric motion are characterized by chaos.
The word chaos has been commonly used to describe this situation, but it is
somewhat of a misnomer. According to the dictionary, chaos is a "condition in
which things are out oftheir normal or proper places or relationships", though the
net result may be unpleasant, is difficult to think ofthe atmosphere-ocean-Iand system as "out of the its normal order", it is in fact following definite equations and