Predictability of weather and climate

Abstract:

Monsoon variability on intraseasonal and interannual time-scales is analysed using data from five 10-year European Centre for Medium-Range Weather Forecasts Atmospheric Model Intercomparison Project integrations, which differ only in their initial conditions. The results show that monsoon fluctuations within a season and within different years have a common dominant mode of variability. The spatial pattern of the common dominant mode in precipitation has a pronounced zonal structure, with one band of anomalous rainfall extending from 20°N to 5°N, covering most of the land areas, with the other band, of opposite sign, lying between 5°N and 10°S, mostly over the Indian Ocean. This mode therefore describes both the active/break monsoon spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between the continental and the oceanic regime and the principal pattern of interannual variability of monsoon rainfall. In the observations the oscillations between active and break monsoon spells have similar behaviour, although the model is deficient in representing the rainfall variability over India. On the intraseasonal time-scale the transition between the two regimes seems to have a chaotic nature. In addition the probability density function of the principal mode is bimodal for the years in which this mode is particularly dominant. These two results indicate a possible similarity with the Lorenz 3-component chaotic model. Northward-propagating convective regions, simulated by the model, are not clearly associated with the phase transitions of the TCZ regime. The timing of the monsoon onset appears to be modulated by the phase of the El Niño/Southern Oscillation during the preceding season, consistent with observational studies. The results suggest that the dominant mode may also represent some components of the observed monsoon variability. The interannual fluctuations of the dominant mode exhibit only a weak level of reproducibility compared with the relatively large predictability of a broad-scale monsoon wind-shear index.

Abstract:

The combined use of the European Centre for Medium-Range Weather Forecasts high-resolution, at T213 spectral triangular truncation and with 31 vertical levels (T213L31, operational model and ensemble prediction system (EPS), during cases of intense Mediterranean storms, is studied. In particular, it is discussed how EPS products can be used to provide a measure of confidence in the high-resolution precipitation forecast. Three case studies (two extreme events plus one false alarm case) are analyzed. For the first two cases, the EPS probability values for precipitation occurrence supported the medium-range T213L31 prediction, which proved to be successful. By contrast, for the third case, the high-resolution forecast suggested heavy rainfall over northern Italy but was not supported by the EPS. The T213L31 prediction for this case was poor. EPS forecasts of extreme weather events are necessarily compromised by the moderate resolution of the T63L19 model (a version of the operational high-resolution T213L31 model, at T63 spectral triangular truncation and with 19 levels) used to generate the ensembles. In future studies, ensembles will be made using at least T106L31 resolution combined with an increase in ensemble size.