Forecast Error Growth across Multi-scales during the Madden-Julian Oscillation

The Madden-Julian Oscillation (MJO) has been increasingly recognized as an important source of potential predictability for the tropics and extratropics. The boreal 2004-05 winter is used as a case study to conduct predictability experiments with the Weather Research and Forecasting (WRF) model. That winter season was characterized by an MJO event during 18 December-20 January 2004-05, weak El Niño, strong North Atlantic Oscillation and extremely wet conditions over the contiguous United States (CONUS). The specific issues investigated are: 1) the growth of forecast errors in the tropics relative to the extratropics, 2) the propagation of forecast errors from the tropics to the extratropics, 3) forecast error growth on spatial scales associated with MJO and non-MJO variability and 4) the relative importance of MJO and non-MJO tropical variability on the potential predictability of precipitation over the CONUS.

Root-mean square errors in forecasts of normalized eddy kinetic energy (EKE) (200-hPa) are used to evaluate potential predictability. The results show that errors in initial conditions in the tropics grow appreciably faster than in the extratropics. Potential predictability extends out to about 4 days in the tropics (18-20N) and 9 days in the extratropics. Forecast errors in the tropics also quickly propagate to the extratropics, as demonstrated by experiments in which initial conditions are only perturbed in the tropics. Forecast errors in NEKE (200-hPa) on scales directly related to the MJO grow slower than in non-MJO variability. For this particular case study, potential predictability of precipitation extends to about 1-5 days over most of the CONUS but to longer leads (7-12 days) over regions with orographic precipitation in California. The results also show that errors in initial conditions on small scales relative to the large-scale characteristics of the MJO can quickly grow, propagate to the extratropics and degrade forecast skill of precipitation over the CONUS.

This presentation will also show results for other MJO events using global forecasts from the Subseasonal-to-Seasonal Prediction (S2S) Program. The main goal of the Subseasonal-to-Seasonal Prediction (S2S) research program is to explore sources of potential predictability on lead times beyond two weeks. The presentation will discuss how forecast errors in EKE vary across multi-scales during different phases of the MJO life cycle.