Le simulazioni dei modelli climatici sono attualmente la principale fonte d’informazione sul clima del futuro. Oltre alle incertezze di scenario e all’errore dei modelli, la variabilità interna è una delle maggiori fonti d’incertezza, andando a ostacolare le previsioni dei futuri cambiamenti.
In uno studio pubblicato di recente sulla rivista Journal of Climate, un gruppo di ricercatori (tra loro, i ricercatori M. Zampieri, E. Scoccimarro, S. Gualdi della Divisione CSP – Climate Simulation and Prediction del CMCC) ha testato un metodo per determinare la più corta finestra temporale necessaria per rappresentare la variabilità interna delle precipitazioni, per condizioni climatiche stabili. Il metodo è stato applicato a livello globale a una simulazione di 200 anni del clima dell’era pre-industriale del modello accoppiato di circolazione generale CMCC -CM.

L’abstract dell’articolo:
Climate model simulations are currently the main tool to provide information about possible future climates. Apart from scenario uncertainties and model error, internal variability is a major source of uncertainty, complicating predictions of future changes. Here, a suit of statistical tests is proposed to determine the shortest time window necessary to capture the internal precipitation variability in a stationary climate. The length of this shortest window thus expresses internal variability in terms of years. The method is applied globally to daily precipitation in a 200-year pre-industrial climate simulation with the CMCC-CM coupled general circulation model. The 2-sample Cramér-von Mises test is used to assess differences in precipitation distribution, the Walker test accounts for multiple testing at grid cell level and field significance is determined by calculating the Bejamini-Hochberg false-discovery-rate. Results for the investigated simulation show that internal variability of daily precipitation is regionally and seasonally dependent and that regions requiring long time windows do not necessarily coincide with areas with large standard deviation. The estimated timescales are longer over sea than over land, in the tropics than in mid-latitudes and in the transitional seasons than in winter and summer. For many land grid cells, 30 seasons suffice to capture the internal variability of daily precipitation. There exist regions, however, where even 50 years do not suffice to sample the internal variability. The results show that diagnosing daily precipitation change at different times based on fixed global snapshots of one climate simulation might not be a robust detection method.

Leggi la versione integrale dell’articolo:
Schindler A., Toreti A., Zampieri M., Scoccimarro E., Gualdi S., Fukutome S., Xoplaki E., Luterbacher J.
On the internal variability of simulated daily precipitation
2015, Journal of Climate, DOI: 10.1175/JCLI-D-14-00745.1