Practical and Intrinsic Predictability of Multiscale Weather and Convectively Coupled Equatorial Waves during the Active Phase of an MJO

Through a series of convection-permitting regional-scale ensembles based on the Weather Research and Forecasting model, this study investigates the predictability of multiscale weather and convectively-coupled tropical waves during the active phase of a Madden-Julian Oscillation (MJO) event over the Indian Ocean. It is found that the practical predictability limit, estimated by the spread of the ensemble perturbed with realistic initial and boundary uncertainties, is as much as 8 days for horizontal winds, temperature and moisture for scales larger than 2000 km which include equatorial Rossby, Kelvin, inertia-gravity and mixed Rossby-gravity waves. The practical predictability limit decreases rapidly as scale decrease with a predictable timescale less than 1 day for horizontal scales smaller than 200 km. Through further experiments using minute initial and boundary perturbations an order of magnitude smaller than current realistic uncertainties, the intrinsic predictability limit for tropical weather at larger scales (>2000 km) may be achievable beyond 2 weeks but the limit is likely still less than 3 days for the small scales (<200 km).