Intrinsic versus Practical Predictability of Multi-Scale Weather and Convectively-Coupled Tropical Waves during the Active Phase of an MJO

Through a series of convection-permitting ensemble simulations, this study investigates the practical and intrinsic predictability of multiscale weather and convectively-coupled tropical waves during the active phase of the October 2011 MJO event over the Indian Ocean. The regional-scale convection-permitting ensembles based on the Weather Research and Forecasting model reveal variable-, scale- and flow-dependent predictability for tropical weather under the moist MJO envelope. It is found that the limit of practical predictability, estimated by the spread of the ensemble perturbed with realistic initial and boundary uncertainties, is about 7~10 days for horizontal winds, temperature and moisture at large scales (>2000 km) which include equatorial Kelvin waves and mixed Rossby-gravity waves. At medium scales (200~2000 km), the rapid error growth associated with convectively-coupled inertia-gravity waves may limit the practical predictability limit to about 3 days. At small scales (<200 km), diurnal and semi-diurnal variations of moist convection and smaller-scale internal waves will further reduce the practical limit to less than 2 days. Estimated with ensembles using minute initial and boundary perturbations an order of magnitude of smaller than current realistic uncertainty, the intrinsic limit of predictability can be extended beyond 2 weeks for large scales, but still limited to less than 3 days for the small scale. The vertical motion and precipitation display shorter practical and intrinsic predictability limit than wind, temperature and moisture, because these two variables have more variability at smaller scales.